bluuebunny/crossref_metadata_2025 huggingface dataset:
Vegetative incompatibility in Diaporthe ambigua
Authors: SMIT W. A. , WINGFIELD B. D. , WINGFIELD M. J.
Published: November 2003

Vegetative compatibility of strains of Diaporthe ambigua has not previously been examined. Single ascospore and single ascus strains, originating from individual apple, pear and plum rootstocks, were paired on freshly prepared oatmeal agar to determine if vegetative incompatibility could be detected in D. ambigua. Barrage reactions were evident as black lines along the zone of mycelial contact between expanding colonies (vegetative incompatibility reaction). Strains from cankers within an area were of numerous vegetative compatibility groups (VCGs). Strains from adjacent rootstocks usually differed in VCG. D. ambigua has the ability to outcross, and does so, despite its apparent homothallic nature. DsRNA‐containing strains of D. ambigua developed a broad, clear zone when paired with a dsRNA‐containing strain from a different VCG.
Management of root‐knot disease of tomato by the application of fly ash in soil
Authors: KHAN M. R. , KHAN M. W. , SINGH K.
Published: November 2003

The effects of fly ash at different concentrations (0, 10, 20, 30 … 100% vol./vol. in soil) on plant growth and yield were investigated in tomato plants infected or noninfected with root‐knot nematode, Meloidogyne incognita (2000 juveniles per plant) in clay pots. An increase in fly ash concentration in the soil correspondingly increased the availability of carbonates, bicarbonates, sulphate, chlorides, B, P, K, Ca, Mg, Mn, Cu and Zn in the soil. The porosity, water‐holding capacity, pH, conductivity and cation exchange capacity also increased progressively in the fly ash amended soil. Ash application enhanced plant growth, leaf pigment concentrations, fruit production, weight of fruit/plant and mean fruit weight of both nematode‐infected and noninfected tomato plants, being maximum in the soil containing 50 or 60% fly ash. The yield enhancements were 93.6 (infected plants) and 84% (noninfected plants) at 50 and 60% fly ash levels, respectively. Fly ash treatments adversely affected root invasion by juveniles, disease intensity and reproduction of the nematodes. A gradual increase in the ash concentration in soil caused a corresponding decrease in the numbers of invading juveniles, galls and egg masses per root system and eggs/egg mass, being lowest at 100% fly ash i.e. 52, 16, 10 and 81 against 289, 137, 131 and 238 (control), respectively. Linear regression suggested 40% fly ash as the most economic level, enhancing yield of infected plants by 96% and suppressing the nematode disease and reproduction by 63 and 76%, respectively.
The incidence of Fusarium moniliforme on maize from Central America, Africa and Asia during 1992–1995
Authors: MACDONALD M. V. , CHAPMAN R.
Published: November 2003

Samples of maize seed were obtained from countries in Central America, Africa and Asia and assessed for fungal infection. Fusarium spp. were the largest single group of fungi present, and from these Fusarium moniliforme was the species most frequently isolated. Other fungi, including Stenocarpella (Diplodia) maydis, S. macrospora and Acremonium strictum, were also present in significant numbers. Isolates of F. moniliforme were characterized for mating populations, using RAPDs, and a number of isolates, taken at random from those assigned to specific mating groups, were also confirmed by crossing. Isolates were also characterized for fusaric acid production and significant differences in fusaric acid production were detected between isolates from different countries and regions within countries. A detailed analysis of isolates from one country, Kenya, was undertaken. The importance of the pathogens is discussed in relation to human, animal and seed health and quarantine regulations, and plant breeding objectives.
Viruses in the northern potato‐producing regions of Saudi Arabia
Authors: AL‐SHAHWAN I. M. , ABDALLA O. A. , AL‐SALEH M. A.
Published: November 2003

Agar double diffusion tests and, later, ELISAs were used to detect viruses associated with potato in 242 samples collected in 16 trips to Tabuk and Hail, northern regions of Saudi Arabia, in four consecutive growing seasons (autumn 1989, spring and autumn 1990 and spring 1991). Eleven different viruses were detected in Tabuk and 12 in Hail. The viruses detected in Tabuk were alfalfa mosaic (AMV), cucumber mosaic (CMV), tobacco mosaic (TMV), potato leaf roll (PLRV), tomato spotted wilt (TSWV), tobacco ringspot (TRSV) and potato A, M, S, X and Y. The same viruses, plus potato yellow dwarf (PYDV), were detected in Hail. AMV was most frequently and CMV least frequently detected in Tabuk, whereas in Hail the most and least common were PVA and PLRV respectively.
Involvement of resistance induction by Penicillum oxalicum in the biocontrol of tomato wilt
Authors: DE CAL A. , PASCUAL S. , MELGAREJO P.
Published: November 2003

Penicillium oxalicum, a biocontrol agent for Fusarium oxysporum f.sp. lycopersici, was tested for its ability to induce resistance against tomato wilt. P. oxalicum and F. oxysporum f.sp. lycopersici were placed at separate sites on tomato plants or in soil, avoiding a direct interaction between the fungi. P. oxalicum induced resistance as expressed by a reduction in disease severity, area under disease progress curve and stunting induced by the pathogen. P. oxalicum colonized the tomato rhizosphere during the experiments but it was not detected inside stems, demonstrating that P. oxalicum and Fusarium oxysporum f.sp. lycopersici remained spatially separated. Biological control was observed both in sensitive and ‘resistant’ cultivars, indicating the role of a general resistance mechanism. In both cultivars P. oxalicum treatment alone did not produce disease symptoms. Therefore P. oxalicum could be a suitable biocontrol agent in cases of cultivar resistance failure. These results suggest that P. oxalicum can trigger defence mechanisms in the plant.
Interactions between the biocontrol agent Pseudomonas fluorescens CHA0 and Thielaviopsis basicola in tobacco roots observed by immunofluorescence microscopy
Authors: TROXLER J. , BERLING C.‐H. , MOE¨NNE‐LOCCOZ Y. , KEEL C. , DÉFAGO G.
Published: November 2003

Pseudomonas fluorescens CHA0 protects plants from damage caused by several soilborne fungi. In this work, immunofluorescence microscopy was used to investigate the colonization of tobacco roots by CHA0 and its physical relationship with the black root rot fungus Thielaviopsis basicola. The pseudomonad colonized the rhizoplane shortly after planting of tobacco seedlings in sterile soil microcosms, in which it had been introduced as soil inoculant. CHA0 was found between and inside cells in the epidermis and the cortex, as well as in the xylem vessels, within 4–7 days after planting of seedlings. The presence of CHA0 delayed the colonization of the interior of tobacco roots by T. basicola compared with the treatment in which only the fungus had been inoculated. Likewise, the pseudomonad reduced the extent of black root rot from 82% to 28%. However, CHA0 was seldom found in contact with the mycelium of T. basicola or in its vicinity, indicating that direct colonization of the mycelium of T. basicola by CHA0 was not required for protection of tobacco against black root rot. Overall, the results suggest that the interior of the root is a key site for implementation of the strain’s biocontrol activity against soilborne plant‐pathogenic fungi.
Influence of crop growth and structure on the risk of epidemics by Mycosphaerella graminicola (Septoria tritici) in winter wheat
Authors: LOVELL D.J. , PARKER S.R. , HUNTER T. , ROYLE D.J. , COKER R.R.
Published: November 2003

Generally, it is recognized that inocula of Septoria tritici present on the basal leaves of winter wheat crops are spread towards the top of the canopy by splashy rainfall. This mechanism of inoculum dispersal is commonly accepted to be a key limit on disease progression. Therefore, attempts to forecast epidemics of S. tritici often quantify rainfall by some means, but largely ignore measurement of pathogen and host variables. In the present study, we show that new wheat leaves emerge initially at a height below established leaves that can contain sporulating lesions of S. tritici. This presents the possibility of horizontal inoculum transfer, even without splashy rainfall. The extent and duration of overlap between emergent and established leaves was found to differ considerably with cultivar and sowing date. Nitrogen application had little effect on overlap, because differences in crop phenology, e.g. leaf area and nodal length, were relative. However, estimates of raindrop penetration to the base of crop canopies suggested that vertical movement of inoculum is affected by nitrogen application. Crops receiving more nitrogen are denser, and therefore less rainfall reaches the base of the canopy. The interactions between crop and pathogen development are discussed with reference to the implications for predicting disease risk. In particular, cultivar traits that promote disease escape are quantified.
Electrophoretic analysis of natural populations of Leptosphaeria maculans directly from leaf lesions
Authors: Brun H. , Levivier S. , Eber F. , Renard M. , Chèvre A. M.
Published: November 2003

On oilseed rape, 207 leaf lesions attributed to Leptosphaeria maculans were classified as typical or atypical. Starch gel electrophoresis of glucose phosphate isomerase (GPI) performed on extracts of 229 leaf lesions comprising the 207 with L. maculans symptoms and 22 with Pseudocercosporella capsellae symptoms, yielded four different electrophoretic patterns of alloenzymes designated ET1 to ET4. In addition to ET1 and ET2, characteristic respectively of A‐ (highly virulent) and B‐ (weakly virulent) group isolates of L. maculans, the previously undescribed ET3 allozyme was recovered from a few typical and atypical L. maculans leaf lesions. The fastest ET4 allozyme was specific to P. capsellae. All but two typical leaf lesions produced the ET1 allozyme, whereas atypical lesions produced one of the three L. maculans allozymes. Occasionally a mixture of two allozymes was recovered from a same‐leaf lesion. GPI electrophoresis performed directly on leaf lesions proved a useful and reliable method to identify L. maculans, and to differentiate between L. maculans and P. capsellae. This method of discrimination enabled deductions, from 377 leaf lesions analysed, about the structure of L. maculans populations on different oilseed rape varieties.
Effect of plant genotype, virus isolate and temperature on the expression of the potato tuber necrotic ringspot disease (PTNRD)
Authors: LE ROMANCER M. , NEDELLEC M.
Published: November 2003

The present study shows that a large range of potato cultivars (29/33 tested), widely grown in the world, are susceptible to potato tuber necrotic ringspot disease caused by potato virus Y. The three factors studied in this work, which proved to influence the level of tuber necrosis reaction, were, first, the plant genotype, since varietal behaviour exhibited large differences; second, the virus genotype, since variations of virulence occurred between the four isolates tested; and third, the environmental conditions, as shown by the different rates of tuber necrosis obtained under contrasting conditions of temperature as much during the growing period as during storage. Three of the cultivars tested, Spunta, Maris Piper and Thalassa, failed to produce necrotic tubers, although infected with a virulent tuber‐necrosing isolate. This result, following observations on the inheritance of the tuber necrosis trait, suggests that at least a major dominant gene controls this reaction in non‐sensitive cultivars. On the other hand, the extreme resistance genes (Ry) provide a good resistance to virus infection, thus, preventing tuber necrosis under field conditions.
Pathogenicity of fungi associated with winter loss and injury in white lupin
Authors: BATEMAN G. L.
Published: November 2003

Fungi isolated from diseased white lupin plants were tested for pathogenicity in standardized glasshouse tests to assess their potential as causes of plant death in winter in autumn‐sown crops. Pleiochaeta setosa and, to a slightly lesser extent, Fusarium avenaceum caused lesions or plant death when inoculated onto hypocotyls of lupin seedlings. Disease was more severe when the hypocotyls were wounded before inoculation. F. avenaceum also caused injury when inoculated onto roots; wounding the roots had no effect. Some isolates of F. solani also caused injury, especially after root inoculation, but isolates of F. oxysporum and Cylindrocarpon destructans were only slightly or non‐pathogenic. Isolates of Fusarium and C. destructans, but not of P. setosa, differed in pathogenicity. There were differences in susceptibility to disease among three lupin cultivars.
Biochemical and pathogenic differences between Kenyan and Brazilian isolates of pseudomonas syringae pv. garcae
Authors: Kairu G. M.
Published: November 2003

Isolates of Pseudomonas syringae pv. garcae from Kenya and Brazil differed in pathogenic and biochemical characters. In inoculations on Coffea arabica var. SL28 from Kenya, only the Kenyan isolates were virulent. The Kenyan isolates were not bacteriocin producers while the Brazilian isolates were active producers comparable to P. s. syringae from lilac (Syringa vulgaris). Pigment production separated the two types of P. s. garcae isolates distinctly. The Kenyan isolates produced the UV fluorescent yellow‐green siderophore while the Brazilian isolates produced a nonfluorescent brown diffusible pigment on King’s B medium. API‐20NE diagnostic kits were largely ineffective in distinguishing between biochemical reactions of P. s. garcae isolates from Kenya and Brazil or between these and P. s. syringae. Syringomycin activity on lemon and Geotrichum candidum distinguished P. s. syringae from P. s. garcae isolates. It is concluded that P. s. garcae (as represented by the seven cultures from the National Collection of Plant Pathogenic Bacteria, Harpenden, UK) exists in at least two strains, the Kenyan isolates comprising one strain while the Brazilian isolates comprise one or more distinct strains.
The influence of verticillium wilt epidemics on cotton yield in southern Spain
Authors: BEJARANO‐ALCÁZAR J. , BLANCO‐LÓPEZ M. A. , MELERO‐VARA J. M. , JIMÉNEZ‐DÍAZ R. M.
Published: November 2003

Experiments were conducted in the Guadalquivir Valley of Andalucía, southern Spain, in 1986 and 1987, using field plots naturally infested with different inoculum densities of the defoliating and nondefoliating pathotypes of Verticillium dahliae to determine the influence of verticillium wilt epidemics on yield of cotton cultivar Coker 310. The total number of bolls, the number of open bolls, and seed cotton yield were related to the growth stage of plants at first appearance of foliar symptoms, and to inoculum density and virulence of the V. dahliae pathotype prevailing in the soil. For the three yield components, the greatest reduction was observed in plants showing symptoms before opening of first flowers (about 650 degree‐days after sowing). Yield increased with delay in the development of foliar symptoms during the crop season, and the effect of the wilt epidemics on yield was small or nil for plants that developed symptoms after opening of the first bolls (1400–1500 degree‐days after sowing). A multiple regression equation was derived that related yield reduction to the physiological time accumulated from the time of sowing until the appearance of foliar symptoms and to the rate of disease intensity increase over physiological time. This multiple point model explained about 70% of the variation in cotton yield loss due to verticillium wilt.
Glucosinolates and disease resistance in oilseed rape (Brassica napus ssp. oleifera)
Authors: GIAMOUSTARIS A. , MITHEN R.
Published: November 2003

The hypothesis that enhancing the level of glucosinolates in leaves of oilseed rape would increase resistance to two fungal pathogens was tested. Thirty‐three Brassica napus lines with variable leaf glucosinolate contents were assessed for susceptibility to infection by Leptosphaeria maculans and Alternaria spp. under field conditions. The data clearly showed that there was not a simple positive relation between glucosinolate content and resistance, and that the hypothesis can be refuted. The level of Alternaria infection of both leaves and pods was positively correlated with glucosinolate content, while for L. maculans there was no significant relationship between the two variables. It is suggested that these pathogens have become specialists on glucosinolate‐containing taxa in an analogous way to insect herbivores.
Nitrogen‐induced changes in colony density and spore production of Erysiphe graminis f.sp. hordei on seedlings of six spring barley cultivars
Authors: JENSEN B. , MUNK L.
Published: November 2003

The influence of increasing nitrogen supply (30, 60, 120 and 240 mg N per pot) on susceptibility was studied on seedlings of six cultivars of spring barley inoculated with virulent isolates of powdery mildew. The colony density (CD) measured as colonies per cm2 was significantly increased with increasing application of nitrogen on all cultivars, and a significant interaction was found between N and cultivar. The different reactions of the cultivars could not be ascribed to lack of N uptake. In general, increasing N application enhanced the sporulation capacity of colonies (CSC) irrespective of increased CD and the cumulative production of spores per cm2 leaf (CSCM) increased strongly with N application in all cultivars. No interaction between N and cultivar was found for the latter component. The increase in CSCM closely corresponded with the increase in N content and fresh weight of uninoculated leaves. No interaction between N treatment and powdery mildew isolates was found for infection efficiency and spore production per colony, when tested on one cultivar. The N‐induced changes in infection and sporulation can explain the main part of the increasing effect of N fertilization on powdery mildew development in the field. The results indicate that it may be possible to breed for or select barley cultivars with low N impact on powdery mildew development.
Suppression of Diaporthe phaseolorum f. sp. meridionalis in soybean stems by Chaetomium globosum
Authors: PEREIRA J. , DHINGRA O. D.
Published: November 2003

The effect of two isolates of Chaetomium globosum (CgA‐1, a producer and CgNA, a non‐producer of antibiotic in vitro) on the sporulation of pycnidia and survival of Diaporthe phaseolorum f. sp. meridionalis (Dpm) in soybean stems was determined under conditions of intermittent wetting and drying. Dpm‐colonized stems placed on the surface of field soil or of quartz gravel were exposed to a 12‐ or 24‐h wet period followed by a 48‐h drying period until a cumulative wetness duration of 120, 240 or 480 h was attained. Both isolates reduced the pycnidia sporulation index to zero within 120 h of cumulative wetness duration if the wet period was 24 h. With a 12‐h wet period the sporulation index decreased gradually, to between 4 and 8% after 480 h of cumulative wetness. In general, on the soil surface and with a 12‐h wet period, CgA‐1 reduced sporulation more efficiently than CgNA whereas with a 24‐h wet period there was no difference between the isolates. Recovery of Dpm from C. globosum‐treated stems decreased gradually with increase in wet period and cumulative wetness duration. The recovery rate from stems treated with isolate CgA‐1 was always significantly less than that from stems treated with CgNA. In glasshouse experiments, disease incidence and the proportion of the stem length colonized by Dpm at the V6 growth stage was reduced significantly when soybean seeds treated with ascospores of C. globosum were sown in Dpm‐infested soil. Incorporation of bran inoculum of either of the isolates in the upper soil layer dramatically reduced disease incidence. Isolate CgA‐1 reduced the disease more efficiently than CgNA.
Preliminary investigation of the causal agent(s) of a disease causing leaf curl of tobacco in South Africa
Authors: Paximadis M. , Dusterwald G. , Duyver L. , Rey M. E. C.
Published: November 2003

A high incidence (86%) of potyvirus infection was noted in tobacco plants exhibiting a form of leaf curl in South Africa. Despite leaf curl being reported in the literature to be of geminiviral aetiology, no geminiviruses were detected. Furthermore, no other virus particles were detected by virus purification, TEM and serology. Twelve species of dsRNA were consistently isolated from these tobacco plants, but were absent from other forms of leaf curl‐affected and healthy tobacco. Aphid and mechanical inoculation demonstrated that the purified potyvirus(es) did not cause leaf curl symptoms, but rather mild mottle and mosaic symptoms in tobacco. Partial characterization of the potyvirus preparation showed a possible relationship to a South African strain of potato virus Y. Because potyvirus‐inoculated plants did not manifest leaf curl symptoms, and because leaf curl symptoms were noted in some plants not infected with a potyvirus, it was concluded that the potyvirus is not involved in the leaf curl aetiology, but causes a latent infection, the symptoms of which are masked. The pattern of the dsRNA banding, induction of enations and lack of mechanical and seed transmission are common to plant reoviruses. The possibility of a phytoreovirus involvement in this form of leaf curl is currently being investigated. The results from this study suggest that tobacco leaf curl disease worldwide, with regard to geminiviruses, be re‐evaluated.
Weak selection by field sprays for flutriafol resistance in Septoria tritici
Authors: PIJLS C. F. N. , SHAW M. W.
Published: November 2003

Change in sensitivity of populations of Septoria tritici resulting from single sprays of the sterol demethylation‐inhibiting fungicide flutriafol was measured. Field trials were conducted over 3 years at two sites separated by about 15 km, on cvs Mercia and Riband at each site, in plots at least 10 × 12 m. Treatments included the full recommended rate and a reduced dose of flutriafol and a mixture of flutriafol with chlorothalanil. Sprays were applied at GS37; samples were taken just before spraying and as soon as lesions appeared on leaf 2, which had had no visible disease at the time of spraying. Epidemiological evidence suggests that sprays acted both curatively and as protectants. Significant shifts in sensitivity occurred after spraying in both water‐sprayed and fungicide‐sprayed plots; all plots became less sensitive. Disease severity after spraying clearly decreased with increasing fungicide concentration and with the use of a mixture of flutriafol and chorothalanil. However, after correction for the shifts in the water‐sprayed plots, no dose produced a significant change in population sensitivity level, although sample sizes were large. There was no trend in sensitivity in the population over the 3 years of the experiment.
Effects of the springtail Folsomia fimetaria and the nematode Aphelenchus avenae on Rhizoctonia solani stem infection of potato at temperatures of 10 and 15°C
Authors: LOOTSMA M. , SCHOLTE K.
Published: November 2003

Effects of mycophagous soil mesofauna on Rhizoctonia solani infection of potato stems were investigated in two experiments carried out in growth chambers at 10 and 15°C. The springtail Folsomia fimetaria and the nematode Aphelenchus avenae were selected as organisms suppressive to R. solani. Reduction in Rhizoctonia stem infection by mycophagous soil mesofauna was equally effective at 10 and 15°C. F. fimetaria tended to be more effective than A. avenae, but the best control was obtained when both organisms were present in the soil at high densities.
Effect of soil moisture content on the suppression of Rhizoctonia stem canker on potato by the nematode Aphelenchus avenae and the springtail Folsomia fimetaria
Authors: LOOTSMA M. , SCHOLTE K.
Published: November 2003

The effect of soil moisture content on the suppression of Rhizoctonia stem canker on potato by mycophagous soil animals was studied in growth chambers. Three soil moisture levels were established in two bioassays, in which potato sprouts grew through a 15‐cm soil layer inoculated with sclerotia of Rhizoctonia solani (AG‐3). In one experiment two levels of R. solani inoculum were applied. The effect on plant disease of mycophagous soil fauna was assessed by adding the springtail Folsomia fimetaria and/or the nematode Aphelenchus avenae to the soil. In the absence of mycophagous organisms, Rhizoctonia disease severity on potato stems was highest in dry soil. A. avenae and F. fimetaria reduced Rhizoctonia stem canker when applied at populations found in the field. They were effective over a broad range of soil moistures. The stimulatory effect of dry soil conditions on Rhizoctonia stem canker was counteracted by a greater efficacy of the mycophagous soil fauna under these conditions. Mild drought stress did not seem to be a limiting factor in the biological control of stem canker by these two organisms.
The effect of leaf rust (Puccinia hordei) on yield response in barley (Hordeum vulgare L.) crops with different yield potentials
Authors: WHELAN H. G. , GAUNT R. E. , SCOTT W. R.
Published: November 2003

The effect of leaf rust was investigated on field‐grown crops, in two consecutive seasons, each at a single location, using a non‐replicated design. The experiments involved three crops and 10 distinct epidemics in the 1987/8 season, and four crops with seven epidemics in the 1988/9 season. The epidemics were concurrent in each season in subplots within each crop, and the yield range was from 2.2–9.0 t ha−1 in nil‐diseased plots. Variation in the experiments, assessed as the percentage coefficient of variation, was in the same range as similar, but replicated, experiments with fewer treatment combinations. Early sowing and high agronomic inputs were associated with greater leaf areas, green area durations and yields than in comparable crops sown later or with lower inputs. Disease intensity was described both by per cent disease severity and by the effect on green area. Disease reduced yield mostly via reduced number of fertile ears (mean 31% for all crops) and grain weight (21% mean reduction). Reductions in these components were greater in the 1987/8 season than in the 1988/9 season, but there was no consistent effect associated with yield potential. The number of grains per ear was affected much less (2.5% mean for all crops) than the other components. There was little evidence for compensation between yield components.
Effect of temperature of production of Botrytis allii conidia on their pathogenicity to harvested white onion bulbs
Authors: BERTOLINI P. , TIAN S. P.
Published: November 2003

Botrytis allii colonies incubated at low temperatures have been reported to produce larger conidia that germinate faster and give rise to longer germ‐tubes than those grown at room temperature. The present study compared the effect of conidia produced at 20°C and at 0 and –2°C on their pathogenicity to artificially inoculated white onion bulbs, and the effect of conidial concentration (5×103 and 5×104 conidia/mL) on disease incidence, lesion area, incubation and latent period during storage at 20, 5 and 0°C. At all storage temperatures and periods tested conidia produced at −2°C caused a higher disease incidence and larger areas of rot than those produced at higher temperatures. When the conidial production temperature was raised to 20°C, the duration of incubation on the bulbs inoculated with 5×104 conidia/mL was more than doubled during storage at 0°C, tripled at 5°C, and took 50% longer at 20°C. The incubation period was not significantly affected by conidial concentration at 20°C, and only slightly at 5 and 0°C, but at low temperatures the latent period was longer because of the delay induced in sporulation. These data are consistent with the packers' opinion that cross‐infection of spring onions by long‐term refrigerated onions in grading lines caused earlier and heavier rotting.
The repression and stimulation of growth of Erysiphe sp. on Rhododendron by fungicidal compounds
Authors: KENYON D. M. , DIXON G. R. , HELFER S.
Published: November 2003

Erysiphe sp. is a causal agent of powdery mildew on Rhododendron. A novel in vivo method permitting the screening of fungicides on woody plants is described. Eight fungicides were evaluated for activity against Erysiphe sp. using Rhododendron ponticum microplantlets grown in vitro. Pathogen development changed with both the type of fungicidal compound and the concentrations applied. The most active materials were fenpropidin and penconazole, which showed high activity at the lowest concentrations. Six of the compounds performed more effectively than a mixture of bupirimate + triforine (Nimrod T), the standard recommendation for control of this pathogen on Rhododendron. All fungicides affected the sporulation of Erysiphe sp., with propiconazole, pyrazophos and triadimenol causing a significant increase in sporulation at the lowest concentrations. At higher concentrations, sporulation was significantly reduced by all treatments. No phytotoxic effects were detected with any fungicide at any concentration. The growth of plantlets in most treatments showed no significant difference from the untreated controls. The results of the study are discussed in relation to strategies for control and the epidemiology of Erysiphe sp. infecting Rhododendron.
Analytical models of weed biocontrol with sterilizing fungi: the consequences of differences in weed and pathogen life‐histories
Authors: SMITH M. C. , HOLT J.
Published: November 2003

A model of the population dynamics of healthy weed plants, weed seeds in the soil, pathogen‐infected weed plants and pathogen spores in the soil, was devised to investigate interactions that are important for the success of biocontrol with pathogens that prevent seed set. Three particular features of the host‐pathogen interaction were examined in detail: the form of the density dependent relationship which determined seed and spore production, the host life stage at which infection could occur, and the relative competitive abilities of healthy and infected host plants. It was found that, when both weed and pathogen coexist, the equilibrium abundance of the weed in the presence of the pathogen was independent of the form of the relationship between plant density and seed or spore production. However, the form of this relationship did affect estimated equilibrium densities in the absence of biocontrol, and also affected the parametrizations under which both host and pathogen could coexist. Parameters derived from experiments with isolated host plants may therefore be sufficient to assess the biocontrol potential of new pathogens along with knowledge of densities achievable when the weed is uncontrolled. The form of the relationship used to control seed and spore production also had a marked influence on the range of parameter values over which the pathogen could persist. Other control measures were represented by changes to appropriate parameter values, e.g. weeding was represented by a change in weed death rate. With one exception, the use of additional control measures was not antagonistic to biocontrol. Often, however, the combined effect was less than additive, and the existence of synergy (where the combined effects are more than additive) was critically dependent on the form of the relationship of the rate of seed production per plant with density and the efficiency of the other control measures.
Development of a powder formulation of Pseudomonas fluorescens for control of rice blast
Authors: VIDHYASEKARAN P. , RABINDRAN R. , MUTHAMILAN M. , NAYAR K. , RAJAPPAN K. , SUBRAMANIAN N. , VASUMATHI K.
Published: November 2003

Pseudomonas fluorescens strain Pf1, inhibitory to the growth of the rice blast pathogen Pyricularia oryzae in vitro, was developed as a talc‐based powder formulation. When rice seeds were treated with this formulation, the bacteria spread to roots, stems and leaves of the plants and protected against leaf infection by P. oryzae. When applied as a foliar spray, the bacteria survived on the leaves. The powder formulation controlled leaf blast under greenhouse conditions. In tests as a seed treatment and foliar spray in four field trials it effectively controlled the disease and increased grain yield.
The effect of solar heating of soil on natural and inoculated agrobacteria
Authors: RAIO A. , ZOINA A. , MOORE L. W.
Published: November 2003

Solarization trials were carried out over 3 years and in two countries to control crown gall disease on fruit trees and eliminate Agrobacterium. In 1992, agrobacteria in naturally infested soils of two Italian nurseries were monitored before and after solarization. Agrobacteria populations decreased by 99% and 92% after the treatment; however, crown gall incidence did not decrease. In 1993 and 1994 solarization was tested in Oregon in fields artificially infested with two marked strains of A. tumefaciens. In sandy loam soil, the target bacteria were eliminated in 4 weeks, while in silty clay soil the populations were markedly reduced after 2 months of treatment. Crown gall incidence on cherry rootstocks transplanted to the field at the end of 1993 was 3.7% in the sandy loam soil control plots, while no tumours were observed on plants from solarized plots. The use of solarization in combination with reduced doses of metham‐sodium was also evaluated.
The use of pectic enzymes in the characterization of Armillaria isolates from Africa
Authors: MWENJE E. , RIDE J. P.
Published: November 2003

Isozyme analysis has been used to compare 26 tropical African and 26 European Armillaria isolates from a wide range of geographic regions and host plants. Pectic enzymes from culture filtrates, especially pectin lyase (PL) and pectin methylesterase (PME), were particularly useful in grouping the isolates. Isolates from Africa previously classified as either A. heimii or A. mellea were compared with three Zimbabwean groups, previously distinguished by their morphological and biochemical properties. Group I isolates from Zimbabwe resembled A. heimii isolates, while groups II and III from Zimbabwe, and A. mellea from Kenya, formed distinct groups. The African A. mellea group was absent from isolates collected in Zimbabwe. Five European Armillaria species (A. tabescensA. melleaA. gallicaA. ostoyaeA. cepistipes) also showed species‐specific pectic enzyme patterns. Thirteen isolates of A. mellea sensu stricto collected from four European countries showed almost identical PL and PME patterns, but these patterns were quite distinct from those of isolates from Africa previously referred to as A. mellea, indicating that this species is not identical to A. mellea sensu stricto. These observations confirm the potential use of pectic enzymes in grouping Armillaria species.
Effect of wind on the dispersal of oospores of Peronosclerospora sorghi from sorghum
Authors: BOCK C. H. , JEGER M. J. , FITT B. D. L. , SHERINGTON J.
Published: November 2003

The effect of wind on the dispersal of oospores of Peronosclerospora sorghi, cause of sorghum downy mildew (SDM) is described. The oospores are produced within the leaves of aging, systemically infected sorghum plants. These leaves typically undergo shredding, releasing oospores into the air. Oospores are produced in large numbers (6.1 × 103 cm−2 of systemically infected leaf) and an estimate of the settling velocity of single oospores (0.0437 m s−1) of P. sorghi indicated their suitability for wind dispersal. In wind tunnel studies wind speeds as low as 2 m s−1 dispersed up to 665 oospores per m3 air from a group of leaves previously exposed to wind and displaying symptoms of leaf shredding. The number of oospores dispersed increased exponentially with increasing wind speed. At 6 m s−1, up to 12 890 oospores per m3 air were dispersed. Gusts increased oospore dispersal. A constant wind speed of 3 m s−1 dispersed a mean of 416 oospores per m3. When gusts were applied the mean was 15 592 oospores per m3. In field experiments in Zimbabwe, oospores were sampled downwind from infected plants in the field and at a height of 3.8 m above ground level immediately downwind of an infected crop. These data indicate that wind could play a major role in the dispersal of oospores from infected plants in areas where SDM infects sorghum, perhaps dispersing oospores over long distances.
Genomic and phenotypic characterization of the bacterium causing blight of kiwifruit in New Zealand
Authors: YOUNG J. M. , GARDAN L. , REN X.‐Z. , HU F.‐P.
Published: November 2003

Strains of the pathogen causing bacterial blight of kiwifruit in New Zealand, previously identified as Pseudomonas viridiflava, were examined using phenotypic and genotypic methods. Percentage DNA–DNA reassociation values for strains of the pathogen, with the type strains representing P. viridiflava and P. savastanoi, and representative strains within P. syringae, were obtained using the S1 endonuclease method. Strains of the pathogen were most similar to the type strain of P. savastanoi. This similarity was supported by examination of the ΔTm between representative strains. It is concluded that the pathogen can be considered as a member of the P. savastanoi genomic species. The pathogen from kiwifruit in New Zealand was also differentiated in genomic terms from P. syringae pv. actinidiae. Strains of the kiwifruit pathogen compared using the Biomerieux API Biotype 100 system exhibited consistent determinative tests which distinguished the pathogen from P. viridiflava and P. syringae pv. actinidiae. The origins of the pathogen in New Zealand are discussed.
Control of loose smut (Ustilago nuda and U. tritici) infections in barley and wheat plants by foliar application of triadimefon
Authors: JONES P.
Published: November 2003

Foliar application of the broad‐spectrum systemic fungicide triadimefon to loose smut‐infected plants of wheat and barley produced a marked curative effect. Triadimefon was particularly effective when applied at the beginning of and during stem extension. In addition to reducing percentage plant infection, foliar application of this fungicide (particularly prior to flag leaf emergence) resulted in an increased proportion of infected ears which were only partially smutted. The curative effect of these early fungicide applications resulted in significant yield increases, as a consequence of increased grain number per m2. Application of triadimefon to crops at ear emergence had no significant effect on percentage plant infection but resulted in lower levels of re‐infection following artificial or natural inoculation of healthy ears, apparently as a result of a protectant effect of fungicide residues on or in the ovaries.
Efficacy of resistance inducers, free‐radical scavengers and an antagonistic strain of Pseudomonas fluorescens for control of Rhizoctonia solani AG‐4 in bean and cucumber
Authors: KATARIA H. R. , WILMSMEIER B. , BUCHENAUER H.
Published: November 2003

Sixteen chemicals from different groups of known resistance inducers were tested as a soil drench in a humosoil : sand mix infested with an isolate of Rhizoctonia solani AG‐4. Among these, 5‐nitrosalicylic acid (2 mM aqueous solution), o‐acetylsalicylic acid (2 mM), 2,6‐dichloroisonicotinic acid (0.25 mM), 2‐aminoisobutyric acid (2 mM) and lichenin (2 mM) controlled pre‐emergence damping‐off and post‐emergence seedling mortality of bean cv. Dufrix, but gave little or weak and variable disease control in cucumber cv. Delikatess. Two‐component mixtures of these five chemicals controlled the disease effectively in bean, mixtures containing 2‐aminoisobutyric acid being the most effective, with seedling stands of up to 94% in infested soil. As none of these five chemicals (≤ 2 mM) reduced mycelial growth of R. solani on PDA plates, induced resistance in seedling tissues is implied in their disease control. Control was augmented in both hosts when any one of the five inducers was applied in a mixture with a cell suspension (1 × 109 cells mL−1) of an antagonistic fluorescent strain of Pseudomonas fluorescens; these mixtures showed additive effects and provided much better disease control in bean than in cucumber. The free‐radical scavengers (antioxidants) ascorbic acid, benzoic acid, gluconic acid lactone and thiourea inhibited R. solani growth in vitro and efficiently controlled both pre‐emergence damping‐off and post‐emergence seedling mortality of bean; ascorbic acid and benzoic acid also offered significant protection in cucumber. However, disease control values were reduced substantially when inducers were applied with antioxidants, because of strong antagonistic interactions in most of these mixtures. Probable mechanisms of disease control by 5‐nitrosalicylic acid, o‐acetylsalicylic acid, 2,6‐dichloroisonicotinic acid, 2‐aminoisobutyric acid and lichenin and their interactions with P. fluorescens and antioxidants are discussed.
New genes for resistance to downy mildew (Peronospora parasitica) in oilseed rape (Brassica napus ssp. oleifera)
Authors: NASHAAT N. I. , HERAN A. , MITCHELL S. E. , AWASTHI R. P.
Published: November 2003

The response of cotyledons of oilseed rape (Brassica napus ssp. oleifera) accessions to infection by isolates of Peronospora parasitica under controlled conditions was assessed on a 0–7 scale (disease reaction). In interactions scored 0–3, 4–5 and 6–7, the host was considered resistant, partially resistant and susceptible, respectively. Accession RES‐26, selected from the spring oilseed rape cultivar Janetzkis, was partially resistant to isolate R1 and resistant to isolate P003 of P. parasitica, which distinguishes it from three previously described differential response groups (‘A’, ‘B’ and ‘C’) of accessions in B. napus. The resistance of RES‐26 to isolate P003 seemed to be conditioned by a single, partially dominant gene and the resistance of RES‐02, which belongs to group ‘A’ (resistant to R1 and P003), by two independent partially dominant genes. The gene for resistance to P003 in RES‐26 is either closely linked, allelic or identical to one of the two genes for resistance in RES‐02. Resistance of RES‐02 to R1 is conditioned by a single, incompletely dominant gene. The genes for resistance to isolates R1 and P003 in RES‐02 are either closely linked, allelic or identical. The cotyledonary leaves of each seedling responded independently when inoculated simultaneously each with a different isolate of the pathogen.
Biological control of Botrytis cinerea on chickpea seed with Trichoderma spp. and Gliocladium roseum: indigenous versus non‐indigenous isolates
Authors: BURGESS D. R. , KEANE P. J.
Published: November 2003

Establishment of chickpea seedlings from seed inoculated with Botrytis cinerea was increased under controlled environmental conditions by treatment of seed with Gliocladium roseum or Trichoderma virens. An isolate of G. roseum (GrH) from Horsham in the chickpea‐growing region of western Victoria outperformed T. virens in three soil types, including one to which T. virens was indigenous. There was no effect of soil moisture on seedling establishment from seed treated with GrH, but recovery of G. roseum from seed in continuously wet soil was lower than from wet/dry soil. A factorial assay with three isolates of G. roseum and the soils from which they were isolated showed no interaction between isolate and soil type and no difference between isolates for establishment or recovery. In a field trial at Horsham sown in winter (July), seed treatment with GrH increased establishment from 26.0% (control) to 37.5%. Biocontrol by GrH was ineffective in a winter‐sown field trial at Bundoora, but all three isolates increased establishment from 47.5% (control) to up to 87.5% in a trial at the same location sown in spring (October). The commercial implications of these observations are discussed.
Leaf blotch severity on spring barley infected by isolates of Rhynchosporium secalis under different temperature and humidity regimes
Authors: SALAMATI S. , MAGNUS H. A.
Published: November 2003

The infection efficiency and severity of leaf blotch on spring barley inoculated with three pathotypes of Rhynchosporium secalis from central Norway were studied under different temperature and humidity regimes. Seedlings of the cultivar Arve were subjected to two constant temperatures, 13° or 18°C. Dry periods of 8 h or longer before or after a wet period of 4 h, carried out in the first 48 h postinoculation, reduced disease severity assessed 16 days after inoculation. The effect of dry periods of up to 24 h was nullified when plants were subjected to high humidity for 48 h after the dry treatment. The disease developed most rapidly when the wet period was 48 h and the temperature 18°C. At or near the optimum temperature for R. secalis (18°C), leaf wetness duration as short as 2 h resulted in considerable disease. Isolates reacted differently to temperature. The most aggressive isolate caused severe disease irrespective of temperature (56–70% of the leaf area infected); however, disease severity caused by the least aggressive isolate was significantly higher at the optimum temperature compared with a lower temperature (13°C). This information can facilitate evaluation of weather data in relation to predicting leaf blotch for advisory purposes.
Biological control of Sclerotinia sclerotiorum by film‐coating Coniothyrium minitans on to sunflower seed and sclerotia
Authors: McQUILKEN M. P. , BUDGE S. P. , WHIPPS J. M.
Published: November 2003

Conidia of the biological control agent, Coniothyrium minitans, were applied to uninfected and S. sclerotiorum‐infected sunflower seeds and to sclerotia of two isolates of S. sclerotiorum by polymer film‐coating, using a fluidised‐bed seed treater. Rates of 5 × 106 and 7–14 × 106 conidia per g of seed and sclerotia were obtained, respectively. The activity of C. minitans was compared to treatments with the fungicides thiram + fenpropimorph and iprodione. In agar plate tests, C. minitans increased seed germination and decreased recovery of S. sclerotiorum from S. sclerotiorum‐infected seeds. However, in seedling tests carried out in a potting compost‐soil mix with S. sclerotiorum‐infected seeds, C. minitans failed to increase survival of seedlings. A high and low rate of thiram + fenpropimorph and a high rate of iprodione film‐coated onto S. sclerotiorum‐infected seeds gave consistently better results than C. minitans in these tests. The film‐coating process slightly decreased the germination of C. minitans conidia recovered from sunflower seeds and sclerotia of S. sclerotiorum, but after storage for 1 year at 10°C C. minitans grew from more than 97% of seed and sclerotia, and the number of colony forming‐units per g seed or sclerotia had declined by less than 1 log10 unit. In agar plate tests, all fungicide treatments were more effective than C. minitans in delaying myceligenic germination of film‐coated sclerotia, although for sclerotia of isolate JN1, all film‐coatings were effective in decreasing the weight of sclerotia subsequently produced. With isolate SB, only the C. minitans treatment reduced subsequent sclerotial production. In glasshouse and field pot bioassays with sclerotia buried in soil for 20 and 34 weeks, respectively, C. minitans completely suppressed apothecial production and no sclerotia were recovered from the C. minitans treatment pots. C. minitans spread to infect sclerotia in other treatment pots. In these bioassays, both fungicides were less effective than C. minitans, even though apothecial production was almost completely inhibited and less than 10% of sclerotia were recovered.