2 pm Fri, April 11: Leaves collected this morning from an abandoned orchard just north of the Hudson Valley Lab showed a wide variation of ascospore maturity with most pseudothecia of the dozen examined still showing little or no spore differentiation. However, two pseudothecia from one leaf had roughly 20% mature spores. No ascospores were released in the tower shoot. Based on these observations, I am confident that no scab spores will be released in rains tonight or tomorrow.
High temperatures in Highland for today through Monday of next week are predicted (by Accuweather) to be 65, 66, 70, and 74 with lows on Sunday and Monday nights predicted to be 52 and 50. Currently Accuweather is predicting rain will begin about 3 AM on Tuesday with a strong potential for 24 hr of leaf wetting. At noon today, I noted the first hint of green in buds of McIntosh, Jerseymac, and Ginger Gold in our research orchards at the Hudson Valley Lab.
If the weather forecasts are correct, we will almost certain be at full green-tip by the time rains begin on Tuesday of next week. I am fairly certain that the warm weather, possibly with a few drizzles, between now and Tuesday will allow enough additional spore maturation to ensure some spore release with the rains next Tuesday. Thus, I believe that by next Tuesday there may be a significant risk of scab infection for orchards that had carry-over inoculum from last year. That risk will be much higher if temperatures during the rains next Tuesday are in the mid-50’s rather than in the low-40’s as commonly occurs with early-season rains. Given the current forecast, the mean temperature for next Tuesday from 6 AM (when the first spore release might occur with the coming of daylight) until 8 pm that evening will be 56 F. After than the temperature will drop, with rains ending after midnight, but with a strong likelihood of continue wetting until at least 6 AM. The mean predicted temperature for that potential 24-hr wetting period is currently 50.6 F.
Mike Biltonen of Apple Leaf will be making another scab assessment on Monday morning, and I will be running my own test on Monday as well. Furthermore, the weather forecast may change to reduce the duration of rainfall and/or the predicted temperatures, either of which would significantly reduce the risks of a major scab infection period on Tuesday. Nevertheless, it would seem prudent to plan on having a fungicide applied before Tuesday morning in orchards that have carry-over scab from last year. Orchards that had no leaf or fruit scab last year should not be at risk from this first infection period. However, a warm rain with a wetting duration of 24 hours will provided a good test for the hypothesis that any given orchard was “clean” last year. We rarely get significant green-tip scab infections when temperature during the first infection period is in the 40’s, but I can recall several years when warm rains at green-tip allowed for massive infections in unsprayed blocks.
Last year the weather forecasts for the lower Hudson Valley for the period between green-tip and mid-bloom were consistently inaccurate and resulted in scab sprays being applied repeated for predicted wetting events that never developed into infection periods. Thus, the predicted green-tip wetting period for Tuesday may still fizzle, but those who need several days to cover orchards (or to clear brush before they can spray) have now been given fair warning!
