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Effect of Fungicides, Application Timing, and Canker Removal on Incidence and Severity of Constriction Canker of Peach

The fungal plant pathogen Phomopsis amygdali, causal agent of constriction canker, initiates infections of peach twigs through petiole scars in fall and bud scale scars and flowers in spring. Fall fungicide treatments, consisting of seven to eight sprays during leaf abscission, reduced canker incidence by 45 to 63%. In contrast, spring applications consisting of four to five sprays from bud-break through bloom, provided only 10 to 28% control. When applied during both fall and spring, chlorothalonil (46 to 71% control) and captan (46 to 69% control) provided the lowest canker incidence and severity, followed by azoxystrobin (41% control) and myclobutanil (28 to 44% control). Removing cankers by pruning significantly reduced disease incidence by 42% 1 year but had no effect in another year. Possible causes for the inability of any treatment to achieve >90% disease control include: (i) none of the fungicides tested were highly effective against the pathogen, (ii) additional infections may have occurred outside the time period during which fungicides were applied, or (iii) use of single-tree plots surrounded by heavily infected nonsprayed trees. Also, the relationship between disease incidence and severity was quantified and observed to be the same for the two cultivars examined. When modeled using the power and quadratic functions, incidence explained 89% of the variation in severity.