bluuebunny/crossref_metadata_2025 huggingface dataset:
Two different strategies for light utilization in photosynthesis in relation to growth and cold acclimation
Authors: Savitch L. V. , Leonardos E. D. , Krol M. , Jansson S. , Grodzinski B. , Huner N. P. A. , Öquist G.
Published: March 2003

Seedlings of Lodgepole pine (Pinus contorta L.) and winter wheat (Triticum aestivum L. cv. Monopol) were cold acclimated under controlled conditions to induce frost hardiness. Lodgepole pine responded to cold acclimation by partial inhibition of photosynthesis with an associated partial loss of photosystem II reaction centres, and a reduction in needle chlorophyll content. This was accompanied by a low daily carbon gain, and the development of a high and sustained capacity for non‐photochemical quenching of absorbed light. This sustained dissipation of absorbed light as heat correlated with an increased de‐epoxidation of the xanthophyll cycle pigments forming the quenching forms antheraxanthin and zeaxanthin. In addition, the PsbS protein known to bind chlorophyll and the xanthophyll cycle pigments increased strongly during cold acclimation of pine. In contrast, winter wheat maintained high photosynthetic rates, showed no loss of chlorophyll content per leaf area, and exhibited a high daily carbon gain and a minimal non‐photochemical quenching after cold acclimation. In accordance, cold acclimation of wheat neither increased the de‐epoxidation of the xanthophylls nor the content of the PsbS protein. These different responses of photosynthesis to cold acclimation are correlated with pine, reducing its need for assimilates when entering dormancy associated with termination of primary growth, whereas winter wheat maintains a high need for assimilates as it continues to grow and develop throughout the cold‐acclimation period. It appears that without evolving a sustained ability for controlled dissipation of absorbed light as heat throughout the winter, winter green conifers would not have managed to adapt and establish themselves so successfully in the cold climatic zones of the northern hemisphere.
A model of stomatal conductance to quantify the relationship between leaf transpiration, microclimate and soil water stress
Authors: Gao Q. , Zhao P. , Zeng X. , Cai X. , Shen W.
Published: March 2003

A model of stomatal conductance was developed to relate plant transpiration rate to photosynthetic active radiation (PAR), vapour pressure deficit and soil water potential. Parameters of the model include sensitivity of osmotic potential of guard cells to photosynthetic active radiation, elastic modulus of guard cell structure, soil‐to‐leaf conductance and osmotic potential of guard cells at zero PAR. The model was applied to field observations on three functional types that include 11 species in subtropical southern China. Non‐linear statistical regression was used to obtain parameters of the model. The result indicated that the model was capable of predicting stomatal conductance of all the 11 species and three functional types under wide ranges of environmental conditions. Major conclusions included that coniferous trees and shrubs were more tolerant for and resistant to soil water stress than broad‐leaf trees due to their lower osmotic potential, lignified guard cell walls, and sunken and suspended guard cell structure under subsidiary epidermal cells. Mid‐day depression in transpiration and photosynthesis of pines may be explained by decreased stomatal conductance under a large vapour pressure deficit. Stomatal conductance of pine trees was more strongly affected by vapour pressure deficit than that of other species because of their small soil‐to‐leaf conductance, which is explainable in terms of xylem tracheids in conifer trees. Tracheids transport water by means of small pit‐pairs in their side walls, and are much less efficient than the end‐perforated vessel members in broad‐leaf xylem systems. These conclusions remain hypothetical until direct measurements of these parameters are available.
Measurement of xylem sap amino acid concentrations in conjunction with whole tree transpiration estimates spring N remobilization by cherry (Prunus avium L.) trees
Authors: GRASSI G. , MILLARD P. , WENDLER R. , MINOTTA G. , TAGLIAVINI M.
Published: March 2003

Prunus avium trees were grown in sand culture for one vegetative season with contrasting N supplies, in order to precondition their N storage capacities. During the spring of the second year a constant amount of 15N was supplied to all the trees, and the recovery of unlabelled N in the new biomass production was used as a direct measure of N remobilization. Destructive harvests were taken during spring to determine the pattern of N remobilization and uptake. Measurements of both xylem sap amino acid profiles and whole tree transpiration rates were taken, to determine whether specific amino acids are translocated as a consequence of N remobilization and if remobilization can be quantified by calculating the flux of these amino acids in the xylem. Whereas remobilization started immediately after bud burst, N derived from uptake by root appeared in the leaves only 3 weeks later. The tree internal N status affected both the amount of N remobilization and its dynamics. The concentration of xylem sap amino acids peaked shortly after bud burst, concurrently with the period of fastest remobilization. Few amino acids and amides (Gln, Asn and Asp) were responsible for most of N translocated through the xylem; however, their relative concentration varied over spring, demonstrating that the transport of remobilized N occurred mainly with Gln whereas transport of N taken up from roots occurred mainly with Asn. Coupling measurements of amino acid N in the xylem sap with transpiration values was well correlated with the recovery of unlabelled N in the new biomass production. These results are discussed in relation to the possibility of measuring the spring remobilization of N in field‐grown trees by calculating the flux of N translocation in the xylem.
Comparative host suitability to Meloidogyne spp. and Pratylenchus vulnus in Myrobalan plum (Prunus cerasifera)
Authors: Stalin N. , Salesses G. , Pinochet J. , Minot J. C. , Voisin R. , Esmenjaud D.
Published: March 2003

Nine Myrobalan plum clones (Prunus cerasifera) obtained from in vitro propagation were tested for host suitability to four root‐knot nematode (RKN) isolates (Meloidogyne incognita (2), M. javanica (1), and M. hispanica (1)) and one root‐lesion nematode (RLN), Pratylenchus vulnus, isolate. The clones are characterized according to the Ma major resistance genes that control resistance to M. arenaria, M. incognita and M. javanica and are categorized into five resistant (R) and four host (H) clones to these RKN species. As expected, the clones ranged, regardless of the RKN species or isolate, into the same distinct R (P.2175, P.1079, P.2980, H.17 and H.21) and H (P.2794, H.18, H.23 and P.16.5) groups for RKN resistance. The P. vulnus tests showed that all the clones had higher final populations than initial populations (Pf : Pi > 1) and that Pf : Pi ratios were equivalent in each RKN resistance group (R or H). Consequently, resistant alleles of RKN Ma genes in Myrobalan plum have no major effect on RLN multiplication. Our data must be taken into account for breeding strategies aimed at incorporating multiple resistance to both RKN and RLN in Prunus spp.
Use of transgenic plants to study the role of ethylene and polygalacturonase during infection of tomato fruit by Colletotrichum gloeosporioides
Authors: Cooper W. , Bouzayen M. , Hamilton A. , Barry C. , Rossall S. , Grierson D.
Published: March 2003

The infection of tomato fruit by the postharvest pathogen Colletotrichum gloeosporioides did not proceed until the onset of ripening in response to ethylene. Compared with fruit from wild‐type plants, infection progressed more slowly in transgenic fruit in which ethylene biosynthesis and ripening had been inhibited by an ACC oxidase (ACO) antisense transgene. In contrast, transgenic fruit deficient in polygalacturonase developed lesions at the same rate as the wild‐type fruit. Ethylene biosynthesis increased rapidly in response to infection of ripe wild‐type and ACO antisense fruit but was 25 times greater in the former. Fruit from the ripening mutant ripening inhibitor (rin), which are normally very resistant to infection, became infected quickly when incubated in the presence of ethylene, whereas fruit incubated in the absence of ethylene remained healthy. The ACO 1 mRNA accumulated to detectable levels within 24 h of inoculation of unripe wild‐type fruit, prior to the development of visible symptoms, whereas there was no detectable expression in the inoculated ACO antisense fruit. ACO transcripts accumulated to maximum levels during the early stages of infection of ripe fruit, correlating with maximum ethylene biosynthesis. Northern analysis using gene‐specific probes for each of the three ACO genes indicated that ACO 1 was the main gene expressed in response to infection and that there was no detectable expression of ACO 2 and 3.
Differential detection and genetic relatedness of phytoplasmas in papaya
Authors: Gibb K. S. , Schneider B. , Padovan A. C.
Published: March 2003

The genetic relatedness of phytoplasmas associated with dieback (PDB), yellow crinkle (PYC) and mosaic (PM) diseases in papaya was studied by restriction fragment length polymorphism (RFLP) analysis of the 16S rRNA gene and 16S rRNA/23S rRNA spacer region (SR). RFLP and SR sequence comparisons indicated that PYC and PM phytoplasmas were identical and most closely related to members of the faba bean phyllody strain cluster. By comparison the PDB phytoplasma was most closely related to Phormium yellow leaf (PYL) phytoplasma from New Zealand and the Australian grapevine yellows (AGY) phytoplasma from Australia. These three phytoplasmas cluster with the stolbur and German grapevine yellows (VK) phytoplasmas within the aster yellows strain cluster. Primers based on the phytoplasma tuf gene, which amplify gene products from members of the AY strain cluster, also amplified a DNA product from the PDB phytoplasma but not from either the PYC or PM phytoplasmas. Primers deduced from the 16S rRNA/SR selectively amplified rDNA sequences from the PDB and AGY phytoplasmas but not from other members of the stolbur strain cluster. Similarly, primers designed from 16S rRNA/SR amplified rDNA from the PYC and PM phytoplasmas but not from the PDB phytoplasma. These primers may provide for more specific detection of these pathogens in epidemiological studies.
Application of a flowing solution culture technique to study the parasitic fitness of the nematode Radopholus similis on banana plantlets under two different nitrogen nutrient regimes
Authors: Declerck  , Laloux  , Sarah  , Delvaux
Published: March 2003

A flowing solution culture technique was used to study the effects of N nutrition on banana root infestation by the burrowing nematode Radopholus similis. Paired groups of banana plants were grown in separate compartments containing coarse sand (2–5 mm diameter). Input nutrient solutions had fixed compositions and identical total N concentrations. The two compartments differed in their NH4+/total N concentrations, one receiving 0% NH4+/total N (100% NO3−/total N) and the other 60% NH4+/total N (40% NO3−/total N). The compartments were connected by a inverted T‐shaped tube containing fine sand (200–250 μm diameter) through which nematodes were inoculated. The pH and N concentrations of the output solution were determined at regular intervals. The pH of the output solution differed markedly between the two compartments, with values of 6.8 and 3.6 in the 0% and 60% NH4+ compartments, respectively. In the compartment receiving 0% NH4+, the number of nematodes in the roots and the growth of banana plants was significantly higher than in the compartment receiving 60% NH4+. Even though they were more infested by nematodes, banana plants receiving 100% NO3− developed better than the less infested plants grown in the medium containing 40% NO3−.
Phytoplasma transmission by in vitro graft inoculation as a basis for a preliminary screening method for resistance in fruit trees
Authors: Jarausch  , Lansac  , Bliot  , Dosba
Published: March 2003

In vitro grafting was tested as a technique for inoculating Prunus rootstock Prunus marianna GF 8‐1 with European stone fruit yellows (ESFY) phytoplasmas and apple rootstock Malus pumila MM106 with apple proliferation (AP) phytoplasmas. In vitro shoot cultures of ESFY‐infected Prunus marianna GF 8‐1 and AP‐infected Malus pumila MM106 were used as graft inoculum to transmit the phytoplasmas to the respective healthy rootstock. Phytoplasma transmission was assessed after a graft contact of 1, 2 or 3 months. Healthy autografts were used as controls to monitor parameters of in vitro grafting. Successful graft union formation ranged from 58 to 79% irrespective of the plant species and the sanitary state of the graft. Pathogen‐specific polymerase chain reaction (PCR) was used to test the inoculated rootstocks for the presence of ESFY and AP phytoplasmas, respectively. The rate of ESFY phytoplasma transmission in successful Prunus‐grafts increased from 69 to 94% with the time of contact. AP phytoplasma transmission to Malus occurred in 80 to 97% of successful grafts. To our knowledge this is the first report of phytoplasma transmission by grafting in vitro. The results provide a good basis for the establishment of a preliminary in vitro screening method for phytoplasma resistance in Prunus and Malus.
Screening for resistance to Diaporthe toxica in lupins by estimation of phomopsins and glucoseamine in individual plants
Authors: Shankar  , Cowling  , Sweetingham  , Than  , Edgar  , Michalewicz
Published: March 2003

Immunological and biochemical assays were developed for screening for resistance to Diaporthe toxica in individual plants of narrow‐leafed lupins (Lupinus angustifolius). The former was an enzyme‐linked immunosorbent assay (ELISA) for measuring phomopsin mycotoxins and the latter gave an estimation of glucoseamine in infected stem pieces. Stems of L. angustifolius seedlings were inoculated with conidia from D. toxica cultures and, as expected with this latent disease, remained symptomless for 21 days after inoculation. At this time, phomopsins were measured in excised stems that had been incubated for 6 or 8 days to allow mycelial growth from latent infection structures, thereby increasing the phomopsins to detectable levels in individual plants. The estimation of glucoseamine was carried out on the same stems that had been assayed for phomopsins. The method was based on the alkaline deacetylation of chitin to chitosan, the glucoseamine residues of which are de‐aminated with nitrous acid, yielding an aldehyde which is determined colorimetrically. At six days after excision, both tests clearly distinguished the very resistant, resistant, intermediate and susceptible lines and they may be useful in large‐scale resistance screening in lupin breeding programmes. The ELISA of phomopsins is easier to use and would be particularly useful in the elimination of susceptible plants and those plants expressing intermediate levels of resistance during early generations of the breeding programme.
Effect of local inoculum on the spread of sweet potato virus disease: limited infection of susceptible cultivars following widespread cultivation of a resistant sweet potato cultivar
Authors: Aritua  , Legg  , Smit  , Gibson
Published: March 2003

A study compared the spread of sweet potato virus disease (SPVD) into crops of two moderately resistant and initially SPVD‐free sweet potato cultivars in northern and southern Mpigi, Uganda. Whiteflies, the vector of sweet potato chlorotic stunt crini virus (SPCSV), a component cause of SPVD, were similarly abundant in farmers' sweet potato fields around Namulonge in northern Mpigi, and Kanoni in southern Mpigi. However, mean incidence of SPVD in farmers' crops neighbouring the trials was higher at Kanoni (13.3%) than at Namulonge (2.8%). Furthermore, spread of SPVD into initially SPVD‐free sweet potato plots of two only moderately resistant cultivars was greater in plots at Kanoni than in plots at Namulonge. The SPVD‐resistant New Kawogo was the most common cultivar grown in farmers' fields at Namulonge and had few diseased plants, whereas susceptible cultivars with relatively high incidences of disease predominated at Kanoni. Final SPVD incidence in each trial was positively correlated with a measure combining the proximity and level of inoculum in surrounding fields. The study demonstrates the importance of local SPVD inoculum in determining the rate of spread of the disease into fields and implies that the widespread cultivation of a resistant variety limits infection of susceptible cultivars grown nearby.
PCR detection of Fusarium oxysporum f.sp. gladioli race 1, causal agent of Gladiolus yellows disease, from infected corms
Authors: De Haan  , Numansen  , Roebroeck  , Van Doorn
Published: March 2003

Fusarium oxysporum f.sp. gladioli (FOG) race 1 infects both large‐ and small‐flowered Gladiolus cultivars. Race 2 isolates infect only small‐flowered cultivars but can be present as epiphytes on large‐flowered plants. When 160 arbitrary 10‐mer oligonucleotide primers were tested on FOG by PCR to find RAPD markers specific for race 1, the RAPD primer G12 amplified two discriminating DNA fragments, AB (609 bp) and EF (1196 bp), in race 1 isolates only. Both fragments were cloned and sequenced. Two pairs of race 1‐specific primers for multiplex PCR were designed. Tests of 112 F. oxysporum isolates by PCR showed that, in almost all cases, race 1 isolates of vegetative compatibility group 0340 could be distinguished with these primers. Seven putative race 1 isolates did not react in multiplex PCR; hybridization studies with labelled AB and EF DNA fragments showed that these isolates belong to separate groups. A bioassay was developed to detect corms that were latently infected with FOG race 1. Gladiolus corms were homogenized and incubated for 5 days at 28°C in a semiselective medium to induce growth of Fusarium. Cultivated mycelium was isolated and subjected to the developed multiplex PCR after standard DNA isolation or disruption by microwave treatment.
Hydroxyproline‐rich glycoprotein accumulation in tobacco leaves protected against Erysiphe cichoracearum by potato virus Y infection
Authors: Raggi V.
Published: March 2003

Tobacco cv. Havana 425 acquired resistance to a compatible isolate of Erysiphe cichoracearum after infection by a strain of potato virus Y (PVYN) that causes veinal necrosis; another common strain (PVYO) that does not cause necrosis gave less protection. Hydroxyproline‐rich glycoproteins (HRGPs), believed to be involved in resistance, were determined by analysing hydroxyproline (Hyp) in purified cell walls. Hyp content increased significantly in PVYN‐protected leaves, compared with untreated controls, 2–4 days after necrotic lesion symptoms developed. No further increase in Hyp was noted in PVYN‐protected leaves after E. cichoracearum challenge. Hyp increases were significantly higher in protected leaves of plants showing symptoms on day 7 than on day 10. Infection with the PVYO strain caused significant decrease in Hyp content, compared with uninoculated controls. Inoculation of virus‐free plants with E. cichoracearum induced moderate and transitory Hyp increases on day 2 or 3, followed by a quick decrease associated with a weak response by the compatible host. It is suggested that HRGP accumulation induced by PVYN (but not by PVYO) causes changes in the host cell wall that result in resistance to E. cichoracearum.
Analyses of RAPD data for detection of host specialization in Sclerotinia homoeocarpa
Authors: Hsiang T. , Ma X.L. , Yang L. , Cook S.
Published: March 2003

Upgma analysis, principal component analysis, genetic diversity analysis and genetic distance analysis of RAPD data were used to assess the extent of host specialization in 50 isolates of S. homoeocarpa from five turfgrass hosts. In
upgma analysis and principal component analysis, the occurrence of host specialization was not readily apparent based on visual inspection. Genetic diversity analysis showed significant differentiation among isolates from different host species (GST = 0.34, P < 0.001). The strongest evidence for some degree of host specialization came from the statistical analysis of genetic distances among isolates. By grouping pairwise genetic distances between isolates based on their host species, and analysing for average distance within the same host species and among different host species, it was found that the average distance within species was less than among species (P < 0.0001). An analysis of molecular variance of the genetic distances among isolates found that 32.3% of the total variation was attributable to host species. It is concluded that these isolates of S. homoeocarpa showed a weak level of host specialization, which was not readily apparent by
upgma or principal component analyses, but was revealed by genetic diversity analysis and statistical analysis of genetic distances among isolates. Inoculation tests on different host species and tests using a greater number of isolates are required to confirm the extent of specialization.
Random amplified polymorphic DNA (RAPD) analysis of Phytophthora infestans isolates collected in Canada during 1994 to 1996
Authors: Mahuku G. , Peters R.D. , Platt H.W. , Daayf F.
Published: March 2003

Mating type, glucose‐6‐phosphate isomerase (Gpi) allozyme banding patterns, response to the fungicide metalaxyl and random amplified polymorphic DNA (RAPD) markers were used to characterize genetic variability among 141 Canadian isolates of Phytophthora infestans collected between 1994 and 1996. Multiple correspondence analysis of RAPD profiles separated isolates into 21 groups that were not correlated to groups defined by mating type, Gpi allozyme banding patterns or response to metalaxyl. Population subdivision analysis showed that 97% of the total genetic variation was found among individuals within populations, compared with 3% among populations. The average similarity coefficient among isolates was 80%. No significant differences in haplotypic diversity were observed among the years under study, but levels of genetic diversity among local populations of P. infestans were high (0.76). All classes of response to the fungicide metalaxyl were observed, with 55% of isolates displaying moderate levels of insensitivity. The high level of genetic diversity detected within populations indicates that migration and sexual recombination probably play important roles in the population biology of P. infestans in Canada.
Identification of potential vectors of the coconut lethal disease phytoplasma
Authors: Mpunami A. , Tymon A. , Jones P. , Dickinson M. J.
Published: March 2003

Lethal disease (LD) is a lethal yellowing‐type disease of coconuts associated with phytoplasmas in Tanzania, but the insect vector for it has not yet been identified. In this study, the auchenorrynchous insects in LD‐infected coconut fields were surveyed to determine potential vectors for the disease. No significant correlation was found between disease incidence and numbers of insects collected from the field, possibly reflecting the unknown incubation period for the disease. However, analysis of more than 5000 individual insects by the polymerase chain reaction (PCR), using LD‐specific primers derived from the phytoplasma 16S rRNA gene, revealed PCR products of the correct size from eight individuals of Diastrombus mkurangai and four of Meenoplus spp. When digested with restriction endonucleases, fragments of the same size as the LD phytoplasma were obtained. No PCR products were detected in any of the other insect species tested. These results implicate D. mkurangai and Meenoplus spp. as probable vectors of the LD phytoplasma.
Effect of humidity and temperature on conidial germination and appressorium development of two Philippine isolates of the mango anthracnose pathogen Colletotrichum gloeosporioides
Authors: Estrada A. B. , Dodd J. C. , Jeffries P.
Published: March 2003

A comparison of rates of germination and appressorium formation by an isolate of Colletotrichum gloeosporioides on mango leaves, fruit surfaces and cellophane membranes showed that behaviour was broadly similar on all three substrates. Frequency of appressorium formation was slightly higher on cellophane membranes, and both hyaline and melanized appressoria were formed. Only melanized appressoria were formed on mango surfaces. In vitro experiments on membranes showed comparative differences in physiological behaviour with temperature for two Philippine isolates of C. gloeosporioides. The most stimulatory temperature for production of appressoria differed in isolates I‐2 and I‐4 (25 and 20°C, respectively). At 30°C more appressoria became melanized than at lower temperatures, but the frequency of formation of penetration pegs was highest at 25°C. Conidia of C. gloeosporioides germinated on cellophane membranes at relative humidities as low as 95%, but the percentage of conidia germinating and forming appressoria increased as the RH approached 100%. Approximately 18% of conidia of C. gloeosporioides I‐2 held at 62 and 86% RH for 4 weeks retained viability, and some were capable of forming appressoria when placed at 100% RH. These results have implications for epidemiological models for disease control.
Disease development and enhanced susceptibility of wounded pepper roots to Phytophthora capsici
Authors: Adorada D. L. , Biles C. L. , Liddell C. M. , Fernández‐Pavı´a S. , Waugh K. O. , Waugh M. E.
Published: March 2003

Greenhouse experiments were conducted to determine disease progression of phytophthora root rot on nonwounded and wounded pepper plants (Capsicum annuum) and to determine whether susceptibility to Phytophthora capsici decreases with wound aging. Two isolates of P. capsici were used in this study, one less aggressive than the other. Trimming the roots immediately prior to inoculation with either isolate increased susceptibility significantly (P ≤ 0·05) compared with plant roots that were not trimmed. Both isolates caused a higher level of disease severity on disturbed/trimmed than on disturbed/nontrimmed roots. Disease also occurred earlier with the more aggressive isolate on both wounded and nonwounded roots. Disease severity was three to four times more severe on plants treated with the aggressive isolate (NM6011) than on those inoculated with the less aggressive isolate (NM6040), regardless of root treatment. In separate experiments, pepper roots were wounded and allowed to age for up to 5 days before inoculation. Resistance to P. capsici increased as the wounds aged, resulting in significantly (P ≤ 0·001) lower disease severity on plants with 3‐ and 5‐day‐old wounds than on those inoculated at the time of wounding and the nondisturbed/nontrimmed controls. Wounding of the roots followed by immediate inoculation with zoospores resulted in significantly higher levels of attachment than when roots were inoculated with zoospores 48 h after wounding. The 48‐h postwounding inoculation treatment showed the same amount of zoospore attachment as nonwounded roots. Increase in plant resistance correlated (P ≤ 0·01) with an increase in total peroxidase activity. Isoelectric focusing‐polyacrylamide gel electrophoresis (IEF‐PAGE) indicated increased band intensity of three acidic and one basic isozyme as wounds aged. These data suggest that wound repair plays a role in decreasing infection and resultant disease symptoms of pepper to P. capsici.
Effects of growth and virulence of associated blue‐stain fungi on host colonization behaviour of the pine shoot beetles Tomicus minor and T. piniperda
Authors: Solheim H. , Krokene P. , Långström B.
Published: March 2003

The pine shoot beetles Tomicus minor and T. piniperda are common in the Nordic countries. Of these, T. piniperda may attack and kill living but severely stressed trees, whereas T. minor has never been reported to be individually responsible for killing live trees. Both species are associated with blue‐stain fungi: T. minor with Ophiostoma canum and T. piniperda with Leptographium wingfieldii and Ophiostoma minus. The growth of these fungi was studied in phloem and sapwood of live Scots pine trees, on malt agar, and on malt agar under oxygen‐deficient conditions. Leptographium wingfieldii was more virulent (i.e. caused more extensive host symptoms) grew more quickly on malt agar, and was less affected by oxygen‐deficient growth conditions than either O. minus or O. canum. Ophiostoma canum was least virulent. In low‐density inoculations it induced lesions similar to those induced by sterile control inoculations; it grew very slowly on malt agar and stopped growing after ≈30 mm under oxygen‐deficient conditions. Ophiostoma minus was intermediate in all respects. The different virulence of the blue‐stain fungi associated with the two pine shoot beetles may explain the lower level of aggressiveness in T. minor.
A detached seedling leaf technique to study resistance to Mycosphaerella graminicola (anamorph Septoria tritici) in wheat
Authors: Arraiano L. S. , Brading P. A. , Brown J. K. M.
Published: March 2003

A detached seedling leaf technique was developed to screen for resistance to septoria tritici blotch of wheat and to detect specific interactions between cultivars and isolates. Wheat seedlings were inoculated with spore suspensions of Mycosphaerella graminicola. Detached primary leaves were then placed in a clear plastic box such that their cut ends were sandwiched between layers of agar containing benzimidazole, with a gap below the middle of the leaves. Mean levels of disease were affected by light and temperature, and also by the concentration of benzimidazole, such that higher concentrations resulted in less disease. Second leaves were more susceptible than seedling primary leaves. However, none of these factors affected ranking of disease among cultivars or cultivar‐by‐isolate interactions. Kavkaz–K4500 1.6.a.4, Synthetic 6x and Triticum macha showed specific susceptibility and resistance to different isolates. The detached leaf technique could be a useful complement to field trials and an alternative to whole seedling assays in assessing cultivar resistance and investigating the genetics of the host–pathogen interaction.
Differential interaction ofPhytophthora infestanson tubers of potato cultivars with different levels of blight resistance
Authors: Flier W. G. , Turkensteen L. J. , Van Den Bosch G. B. M. , Vereijken P. F. G. , Mulder A.
Published: March 2003

Differential interactions in tuber blight attack between potato cultivars andPhytophthora infestansisolates were studied using whole tuber and tuber slice assays. Tuber blight incidence and severity were studied in a whole tuber assay, whilst necrosis and mycelium coverage were evaluated in a tuber slice assay. The overall defence reaction of the potato cultivars tested varied considerably. Cultivars like Kartel and Producent showed resistant reactions, whilst Bintje and, to a lesser extent, Astarte reacted more susceptibly after inoculation with aggressive strains ofP. infestans. A highly significant cultivar by year interaction was observed when tuber blight incidence was evaluated in two successive years. Differential responses were revealed by changing ranked order of cultivars after exposure to aggressive isolates ofP. infestans. The results show that cultivar by isolate interactions existed for all components of tuber blight resistance studied. The quantitative nature of the observed resistance responses suggests the presence of quantitative trait loci governing resistance to tuber blight. The consequences of differential interactions in relation to the stability of tuber resistance are discussed.
Detection of the nondefoliating pathotype of Verticillium dahliae in infected olive plants by nested PCR
Authors: Mercado‐Blanco J. , Rodríguez‐Jurado D. , Pérez‐Artés E. , Jiménez‐Díaz R. M.
Published: March 2003

An increasing incidence and distribution of verticillium wilt has occurred in the last few years in newly established olive orchards in southern Spain. This spread of the disease may result from use of Verticillium dahliae‐infected planting material. The early in planta detection of the pathogen would aid the implementation of certification schemes for pathogen‐free planting material. In this work, a nested polymerase chain reaction (PCR) method was developed for the in planta detection of the nondefoliating (ND) V. dahliae pathotype, aimed especially at nursery‐produced olive plants. For this purpose, specific primers were designed from the sequence of a 1958‐bp random amplified polymorphic DNA (RAPD) marker of ND V. dahliae, and a procedure for the extraction of PCR‐quality total genomic DNA from infected root and stem tissues of young olive plants was tested and further optimized. Nested PCR assays detected ND V. dahliae in 4‐ to 14‐month‐old artificially infected plants of three olive cultivars. The ND‐specific PCR product was not amplified from total genomic DNA extracted from olive plants infected with the defoliating V. dahliae pathotype. Detection of the ND pathotype was effective from the very earliest moments following artificial inoculation of olive plants with a V. dahliae conidial suspension. Also, detection was achieved in inoculated, though symptomless, olive plants as well as in plants that were symptomatic but became symptomless by 217 days after inoculation.
Further evidence of the association of a phytoplasma and a virus with yellow leaf syndrome in sugarcane
Authors: Aljanabi S. M. , Parmessur Y. , Moutia Y. , Saumtally S. , Dookun A.
Published: March 2003

Using the polymerase chain reaction (PCR), reverse‐transcriptase–PCR (RT–PCR) and double‐antibody‐sandwich enzyme‐linked immunosorbent assay (DAS–ELISA), a phytoplasma (sugarcane yellows phytoplasma, ScYP) and a virus (Sugarcane yellow leaf virus, ScYLV) were detected in sugarcane with yellow leaf syndrome (YLS) in Mauritius. Samples were collected from clones undergoing quarantine, in a variety‐collection plot and in commercial fields. A 1·25 kb DNA fragment encoding the phytoplasma 16S rRNA was consistently amplified by nested PCR. Of 134 samples with and without symptoms derived from 113 varieties, 111 were infected by either ScYLV or ScYP. The phytoplasma was detected in 63 samples by PCR. Restriction fragment‐length polymorphism (RFLP) analysis of the phytoplasma 16S rDNA amplified product indicated that sugarcane yellows phytoplasma group III, which is related to Western X phytoplasma, is present in Mauritius. ScYLV was detected by RT–PCR and ELISA. The virus was more widely distributed than the phytoplasma, and was found in 70 and 100 samples by ELISA and RT–PCR, respectively. There was a significant correlation between the presence of the phytoplasma and YLS symptoms, while such correlation was not significant for ScYLV detected by RT–PCR. ELISA was less sensitive than RT–PCR for detection of ScYLV. Forty‐one samples were coinfected with both microorganisms. Eighty‐five per cent of the samples displayed symptoms when ScYLV and SCYP coexisted, while 55 and 38% were observed when ScYP or ScYLV, respectively, was present alone. The results indicate that the presence of both organisms enhanced the syndrome.
Severity of root rot on tomato plants caused by Phytophthora nicotianae under nutrient‐ and light‐stress conditions
Authors: Grote D. , Claussen W.
Published: March 2003

Abiotic stress induced by three concentrations of nutrients in solution, and biotic stress caused by three inoculum densities of Phytophthora nicotianae, were imposed on tomato plants grown hydroponically under natural light in the glasshouse. The pathogen causes stem and root rot, thereby inducing water deficiency symptoms. The progress of the disease, as measured serologically by DAC–ELISA, was markedly accelerated by enhanced concentrations of nutrients. Significant differences in proline content of infected and noninfected plants were observed at a nutrient concentration equivalent to 5·0 dS m−1. Higher nutrient concentrations increased further proline concentration in leaves. Inoculation of tomatoes under two light regimes in a phytotron produced significantly more affected roots with increasing inoculum density, but disease was not affected by different light treatments. However, higher light intensity accelerated the appearance of symptoms and produced higher proline contents in tomato leaves. It is suggested that the proline content of tomato leaves is a suitable marker for stress induced by both abiotic and biotic factors.
Identification of different isolates of Cassava brown streak virus and development of a diagnostic test
Authors: Monger W. A. , Seal S. , Cotton S. , Foster G. D.
Published: March 2003

An RT‐PCR based detection method for Cassava brown streak virus (CBSV)‐infected cassava has been developed. The RT‐PCR detection method described includes RNA extraction methods for cassava leaves, a distinct primer set for the virus and RT‐PCR conditions. The primers were designed to the virus coat protein gene and generate a virus‐specific product of 231 bp from infected cassava. The test can detect the virus in the new growth of cassava sticks before any disease symptoms are visible. This test was used successfully with infected cassava from both Tanzania and Mozambique. Three isolates from Tanzania were found to exhibit different symptoms on the secondary host plants Nicotiana benthamiana and N. tabacum SR1. They have nucleotide sequence variation within the coat protein region of up to 8% and amino acid differences of up to 6%.
Evolution of gene‐for‐gene systems in metapopulations: the effect of spatial scale of host and pathogen dispersal
Authors: Thrall P. H. , Burdon J. J.
Published: March 2003

The concept of gene‐for‐gene coevolution is a major model for research on disease resistance in crop plants. However, few theoretical or empirical studies have examined such systems in natural situations, and as a consequence, there is little knowledge of how spatial effects are likely to influence the evolution of host resistance and pathogen virulence in gene‐for‐gene interactions. In this work, a simulation approach was used to investigate the epidemiological and genetic consequences of varying host and pathogen dispersal in metapopulation situations. The results demonstrate clear impacts of dispersal distance on the total number of host and pathogen genotypes that are maintained, as well as on genetic variation at individual host resistance and pathogen virulence loci. Several other important results also emerged from this study. In contrast to the predictions of many earlier nonspatial models, so‐called ‘super‐races’ of pathogens do not always evolve and dominate, indicating that it is not necessary to assume costs of resistance or virulence to maintain high levels of polymorphism in biologically realistic situations. The rate of evolution of both resistance and virulence depend on the scale of dispersal, with greater mixing (as a function of dispersal scale) resulting in a faster approach to a dynamic endpoint. The model in this paper also predicts that, despite the greater total genotypic diversity of pathogens across the metapopulation, variation in host resistance will generally be greater than variation in pathogen virulence within local populations.
Effects of timing of Leptosphaeria maculans ascospore release and fungicide regime on phoma leaf spot and phoma stem canker development on winter oilseed rape (Brassica napus) in southern England
Authors: West J. S. , Fitt B. D. L. , Leech P. K. , Biddulph J. E. , Huang Y.‐J. , Balesdent M.‐H.
Published: March 2003

At Rothamsted during 1997/98, 1998/99 and 1999/2000 winter oilseed rape growing seasons, numbers of air‐borne ascospores of Leptosphaeria maculans were often > 4 m−3 from autumn (September/October) to spring (April/May), while few or no ascospores were detected during the summer. Mature pseudothecia were generally not observed on debris of the previous crop until September. One‐year‐old debris (harvested in July 1998) had 95% discharged and 5% mature pseudothecia in August 1999, but by 15 September new pseudothecia (of which 30% were mature) were observed and the first increase in air‐borne ascospores (> 4 m−3) occurred. Phoma leaf spotting appeared in untreated field plots 14–25 days after the first increase in air‐borne ascospores in autumn. The fungicide mixture difenoconazole plus carbendazim decreased the incidence of new leaf lesions for 1 month after application in autumn and for 2 months in mid‐winter. When L. maculans was isolated from infected leaves, the growth rate of isolates from leaves to which fungicide was applied was less than that of those from untreated leaves. Foliar applications of fungicide to field plots in the autumn and winter not only decreased the incidence of crown cankers but also reduced the rate of canker development on stem bases in the spring and early summer (when severity of crown cankers increased linearly with time). In untreated crops, when phoma leaf spots appeared early in the autumn, crown cankers developed early in the spring but only became severe enough before harvest to reduce yield greatly in 1997/98. Yield loss was associated with crown cankers that girdled more than half of the stem by harvest (mean severity > 3 on a 0–5 scale). Infections of new leaves produced after stem extension, from January onwards, led to phoma stem lesion development above the crown. In the three seasons, phoma stem lesions became moderately severe (> 2) by harvest only in untreated plots in 1997/98.
Phenotypic and genotypic structure of Phytophthora infestans populations on potato and tomato in France and Switzerland
Authors: Knapova G. , Gisi U.
Published: March 2003

A total of 134 isolates of Phytophthora infestans were collected from potato and 42 from tomato fields in Switzerland and France in 1996 and 1997, and compared with isolates from other countries. The structure of the populations was analysed phenotypically and genotypically, and associated to geographical, seasonal and host origin. Phenotype characteristics of the isolates included mating type; sensitivity to phenylamide fungicides; virulence on potato differentials; and pathogenic fitness. Genotypes were assessed for mitochondrial DNA haplotype with restriction fragment length polymorphism‐polymerase chain reaction (RFLP‐PCR) as well as amplified fragment length polymorphism (AFLP) and simple sequence repeats or microsatellites (SSR). The majority (96%) of isolates originating from potato were the A1 mating type, whereas half the isolates collected from tomato were A2 mating type. Isolates with sensitive, intermediate and resistant responses to the phenylamide fungicide metalaxyl‐M were detected in the populations. Isolates from potato represented races with highly complex virulence spectra, whereas those from tomato belonged to simple races. The pathogenic fitness of isolates was highest on the host of origin, and was significantly reduced for isolates from potato on tomato. One of the four haplotypes, Ia, dominated the population (93% of isolates). Among isolates collected from potato, 15 different SSR genotypes were detected of which two, A‐03 and A‐06, dominated the population. From tomato, 11 SSR genotypes were found of which four, A‐03, B‐03, D‐03 and F‐01, formed the major proportion of the population. Every ninth and fourth isolate from potato and tomato, respectively, represented a different SSR genotype. Four genotypes were isolated from both hosts, whereas 11 and seven genotypes, respectively, originated exclusively from either potato or tomato. The SSR genotype D‐02, represented by the ‘old’ Ib haplotype, was still detected in a few isolates in the current population, and in older reference isolates from different countries. The SSR genotype was not associated with mating type or sensitivity to phenylamide fungicides. A total of 40 AFLP genotypes were distinguished among the isolates, every second isolate representing another genotype. The diverse phenotypic and genotypic structure of the current field populations in Europe suggests that they may have evolved from local processes including sexual recombination, host preference and selection rather than through long‐distance migration.
Potential of sexual reproduction among host‐adapted populations of Phytophthora infestans sensu lato in Ecuador
Authors: Oliva R. F. , Erselius L. J. , Adler N. E. , Forbes G. A.
Published: March 2003

To determine the potential of sexual reproduction among host‐adapted populations of Phytophthora infestans sensu lato in Ecuador, 13 A1 isolates belonging to clonal lineages US‐1, EC‐1 and EC‐3, and 11 A2 isolates belonging to the clonal lineage EC‐2, were paired on agar plates to induce crossing. In the first experiment, six A1 isolates (three US‐1, two EC‐1 and one EC‐3) were each crossed with three A2 isolates (total = 18 crosses). Matings involving isolates of the EC‐1 lineage produced more oospores of healthy appearance than did matings with isolates of US‐1 or EC‐3. In the second experiment, the oospores of 35 crosses (21 EC‐1 × EC‐2; 10 US‐1 × EC‐2; four EC‐3 × EC‐2) were dispersed on water agar to assess oospore germination. Overall, germination percentages were low. Only one cross produced enough progeny for evaluation. Twenty‐three single‐oospore offspring were isolated and evaluated for mating type; electrophoretic patterns of glucose‐6‐phosphate isomerase (Gpi) and peptidase (Pep) alloenzyme loci; mitochondrial DNA haplotype; and genomic DNA fingerprint. Multilocus genotype data indicated that all 23 isolates resulted from meiotic recombination. Four progeny with homothallic phenotype appeared to be unstable heterokaryons. Markers at several loci segregated according to simple Mendelian expectations for a diploid organism, but the ratios of three RFLP loci and the Pep locus were not consistent with Mendelian expectations. All progeny were nonpathogenic on hosts of the parental genotypes. Reduced mating success and reduced pathogenic fitness of progeny appear to be postmating mechanisms of reproductive isolation in populations of P. infestans sensu lato in Ecuador.
Factors affecting mummification and sporulation of pome fruit infected by Monilinia fructigena in Dutch orchards
Authors: Van Leeuwen G. C. M. , Holb I. J. , Jeger M. J.
Published: March 2003

A 2‐year field experiment (1997–98, 1998–99) was conducted to study mummification and subsequent sporulation in spring of apple (cvs James Grieve, Golden Delicious) and pear (cv. Conference) fruits infected by Monilinia fructigena. Most mummified fruits were found in James Grieve and Conference, whereas in late‐infected Golden Delicious, fruits were still soft when examined in April. In the first year, these late‐infected fruits had a significantly higher sporulation intensity per sporulating fruit (P = 0·05) compared with Golden Delicious fruits infected 9 and 5 weeks before harvest maturity, which were partly mummified. It was concluded that early‐ and late‐infected fruits contributed to primary inoculum in the next season. In a postinfection regime of 25°C and 65–75% relative humidity under controlled conditions, the number of Conference fruits sporulating decreased rapidly, and after 12 weeks’ incubation sporulation had completely ceased. After 8 weeks’ incubation, sporulation intensity in the postinfection regime at 10°C was significantly higher than that at 20 and 25°C in a first experiment with inoculated unripe fruit (P = 0·05). Results of a second experiment with ripe fruit were less clear. These results are discussed in relation to orchard disease management.