Field Evaluation of Some Models Estimating the Seasonal Pattern of Airborne Ascospores of Venturia inaequalis

Abstract

A 6‐year study was carried out to evaluate the accuracy of some models in estimating airborne ascospores of Venturia inaequalis. The proportion of the season’s ascospores trapped on each discharge event was compared with the proportion of mature ascospores, estimated by the New Hampshire model or by some related models. The models differed from each other in the degree‐day cumulation, accounting or not for the leaf litter wetness caused by rainfall or by deposition of atmospheric humidity. The New Hampshire model did not fit spore trappings well: 59% of the actual values fell outside the range of the estimates, and 83% of them were overestimates. The wide discrepancy between reality and estimates resulted from the effect of dryness: when many consecutive rainless days occurred, the proportion of ascospores trapped was constantly lower than the model estimates, due to a slowed spore maturation. The effect of dryness was evident during the greater part of the ascospore maturity season, irrespective of the proportion of the season’s ascospores that had just matured when the dry period began. Models accounting for leaf litter wetness significantly improved estimates. Therefore, in the Po Valley, the accuracy of the New Hampshire model can be improved by accumulating degree‐days only when leaf litter is wet.

A‐scab (Apple‐scab), a simulation model for estimating risk of Venturia inaequalis primary infections*

“…The dynamic of ascospore maturation is calculated indirectly, by determining the proportion of seasonal ascospores that can potentially become airborne on each ith day (PAT). The model uses three equations to define PAT and its 99% confidence bands (Rossi et al ., 2000):…”

A model simulating deposition of Venturia inaequalis ascospores on apple trees*

“…LAI progressively increases during the primary inoculum season. Usually, the period of ascospore release begins at green tip and lasts for less than 2 months (Rossi et al ., 1999). During such a period, canopies develop very rapidly.…”

Estimating the dynamics of airborne ascospores of Venturia inaequalis*

“…The seasonal pattern of discharged ascospores was estimated by the model elaborated by Rossi et al (2000), which fits the relationship between PAT (the average cumulative proportion of the season's ascospores trapped from the air on each ith day), or PAT1 and PAT2 (99% confidence limits for PAT) and DDCLw (the degree-day accumulation on the ith day) by a logistic model:…”

“…In particular, the development stage of overwintering pseudothecia was estimated using the model elaborated by James & Sutton (1982a), modified by Mancini et aZ. (1984), whereas the seasonal pattern of airborne ascospores was estimated by the model of Rossi et al (2000). The outputs of the two models were used to define, in each year, the length of five periods with different risks during the ascospore season.…”