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Incidence, etiology and epidemiology of stonefruit dieback in the Okanagan Valley Cujec, Thomas Peter


A Cytospora species isolated from infected tissues and sporulating stromata on diseased trees caused typical dieback symptoms when inoculated into Prunus species and was identified as the primary cause of stonefruit dieback in the Okanagan. Based on the morphology of the stromata, spore dimensions, and colony growth and color on malt extract agar, the fungus was identified as C. leucostoma (Sacc). After including the number of trees removed during the winter of 1985-86 and 1986-87 because of Cytospora sp., an average of 14.8% of the trees in 17 stonefruit orchards were affected by dieback from September 1985 to September 1987. The incidence of Cytospora sp. in the individual blocks ranged from 3.0-56.9%. In 11 of the 17 orchards surveyed in 1986 and resurveyed in 1987, dieback symptoms were evident on trees which had been symptomless in 1986. The percent of newly infected trees in these 11 blocks ranged from 0.4-8.8% and averaged 2.9%. The majority of sporulating Cytospora sp. infections were found on the scaffold limbs (69%) or trunks (28%) of infected trees. Pruning wounds (65%), rather than winter injury (25%), were the major infection courts. Fall and spring inoculations of a spore suspension (10³ spores/ml) of either a peach isolate (P8-19) to peach, or a cherry isolate (C9-23) to cherry revealed that intraspecies spread of the disease can occur at any time of the year. Although spring spore inoculations of the peach isolate to cherry or the cherry isolate to peach resulted in significantly (P = 0.05) more infections than the control treatments, identical fall inoculations did not. This suggests that spread of Cytospora sp. between cherry and peach is most likely to occur in the spring. The effect of temperature on spore germination and mycelial growth of Cytospora sp. in vitro was isolate-dependent. The minimum lag period for Cytospora sp. spore germination occurred at 27° C. Spores germinated at temperatures as low as 10° C, and remained viable even after exposure to -18° C for 1 week. The temperature optima for the in vitro growth of most stonefruit isolates in this study was 20-23° C. Viable Cytospora sp. spores were washed from infected trees (10⁵-10⁶ spores/ml) and adjacent healthy trees (10⁴ spores/ml) in mid-December and collected in funnel traps after the first rain the following spring (late April). Under Okanagan conditions, infection of fresh pruning wounds made in the spring can occur either by spores which overwintered on infected trees and were dispersed by spring rains, or by spores dispersed by fall rains to healthy trees on which they overwintered and infected following pruning. Benomyl (1 g a.i./L), dichlone (1 g a.i./L), flusilazole (0.01 g a.i./L) and ziram (5 g a.i./L) applied as water sprays did not significantly (P = 0.1) reduce the percent infection compared to the unprotected, inoculated controls. Of eight fungicide-pruning paste mixtures, only benomyl added to either Heal 'n' Seal or linseed oil significantly (P = 0.1) reduced the number of cankers which developed compared to the untreated control.

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