Forecasting Ascospore Dose of Venturia inaequalis in Commercial Apple Orchards

Abstract

Factors affecting potential ascospore dose (PAD) of Venturia inaequalis, were studied in commercial apple orchards from 1981 to 1984. PAD was the product of the numbers of scab lesions per square meter of leaf tissue on terminal shoots at leaf fall, pseudothecia per lesion, asci per pseudothecium, ascospores per ascus, and the proportion of the orchard floor covered by leaf litter at bud break. The number of lesions per square meter of leaf tissue was estimated from the numbers of lesions counted and terminal shoots examined, and the estimated leaf area per shoot. Most lesions on overwintered leaves bore aborted (rather than mature) pseudothecia; however, even when the incidence of infected leaves approached 0%, approximately 3% of the lesions produced mature pseudothecia. Neither the numbers of mature pseudothecia per fertile lesion nor asci per pseudothecium differed significantly (P = 0.05) on McIntosh, Cortland, or Delicious leaves, hence they were set at 21.6 and 122, respectively, in calculations of PAD. Most leaf decay occurred during winter, and leaf litter density decreased by only 16% from bud break to petal fall. The number of lesions per square meter of leaf tissue at leaf fall was the major determinant of the variance of PAD in the predictive model. PAD ranged from 13 to 44,544 ascospores/m2/yr when terminal leaf infection was 0.04 and 9.32%, respectively, in autumn of the previous year. Time shifts in disease progress curves were computed based on infection rates and differences in PAD. Delays of 2-22 days suggested that spray programs for apple scab could be delayed in orchards with low amounts of overwintering inoculum. The significnce of PAD to predictions of ascospore maturity, spore trapping, and the use of postharvest eradicant treatments for management of apple scab were also discussed.