Physiological and environmental factors associated with the disease expression of nectria galligena bres. in apples
Creators: name: Lolas, Mauricio
Publisher: name: Imperial College London
Year: 2001

description: Nectria-canker of apple, caused by the fungus Nectria galligena Bres. (anamorph Cylindrocarpon heteronemum (Berk. And Broome) Wollenw.) produces serious losses to both young trees and stored fruit but the severity of the disease is highly variable between orchards and regions. This thesis describes experiments which were undertaken to identify mechanisms underlying the predisposing factors in Nectria canker expression, and thus to provide information on control strategies. A three-years study was carried out to determine if trees which became infected with N. galligena during propagation in the nursery expressed symptoms of canker as a result of water stress after lifting from nursery due of 13, 29, 43 and 56 days in cold store before planting. The experiment was subsequently extended to compare the susceptibility of these trees subjected to inoculum applied in the second year. The effect of nitrogen fertiliser on susceptibility was also tested. At the time of planting all of the trees appeared to be healthy and free of any symptom of infection. However in May, it was evident that some terminal buds on some of the trees held 29 days or more in store were dying. In the ensuing weeks the affected areas extended down the shoots resulting in obvious die-back. A number of fungal species were isolated from samples taken from these shoots, predominantly Aureobasidium sp. However N. galligena was not detected by direct isolation and diagnostic PCR tests also proved negative. Furthermore, no symptoms indicative of a natural infection by N. galligena were detected during the second growing season. The percentage of leaf-scars which became infected following inoculation with 50 and 100 conidia increased significantly with increasing time in store prior to planting. At concentrations greater than 500 conidia per leaf scar the effect of the delayed planting treatment was apparent only for trees held for more than 29 days. Therefore, it was demonstrated that even two years after planting trees which had been subjected to severe water stress in store were more susceptible to infection by artificial inoculation. The calculated EC50 values indicated that in the absence of nitrogen supplements 2-4 times as many conidia were required to infect leaf scars on trees planted ABSTRACT immediately on receipt as those on trees held in cold store 43 days or more. The application of N-fertiliser in the spring and early summer generally increased the percentage of leaf scars which became infected following inoculation at the next autumn leaf-fall. These differences were significant (P&lt;0.05) for N. galligena isolates at the lowest inoculum dose but not at concentrations of conidia greater than 100 per scar. Experiments made to determine the distance that conidia could be carried within the transpiration stream of shoots of apple at different stages of development within the 1997 season indicated that the results varied widely with respect to the isolate of N. galligena used in the test. In each of the four months that the experiment was repeated, N. galligena isolate W74 was recovered significantly higher up the shoots of all cultivars than isolate W69, which was not detected in Bramley's Seedling at any time. The greatest distance of upward transport of W74 was recorded for Gala (18.4 cm) whereas the corresponding distance for Bramley's Seedling was less than half of that (8.8 cm). Approximately 94% of 178 xylem vessels of Bramley's Seedling had a crosssectional area of 400 |im^ or less, by contrast 75% of 172 vessels of Gala were 600 pm^ or greater. Therefore, the relative diameter of the xylem vessels could be a factor determining colonisation by this pathogen. The EC50 test enabled cultivars to be ranked in order of their known field resistance; Bramley &gt; Cox's Orange Pippin &gt; Discovery &gt; Gala. The EC50 for Bramley was lOX greater than for the other cultivars. The same test indicated that the mean EC50 value for leaf scar infection for isolates obtained from fruit rots was approximately three times greater than for isolates obtained from stem cankers. Tests for their ability to rot ftuit revealed that few stem isolates could initiate progressive rots, but most fruit isolates caused extensive rotting. Significant negative correlations (f&lt;0.001) were found between the volume of fruit tissue rotted and protease production for N. galligena isolates of UK and Chile. Protease elicits the antifungal compound benzoic acid in fruit, and the possibility that low protease producing types are selected by fruit infection from a segregating population of the pathogen in orchards is discussed.