3 PM, Mon, 14 April: My previous message on apple scab ascospore assessments contained some confusing language as it relates to the ascospore maturity model on the NEWA website. In that posting, I stated “the NEWA model is predicting actual spore discharge, not percentage of mature spores, which would be much higher.” That statement was incorrect because the NEWA model does, in fact, predict ascospore maturity even though the spore “maturity” predicted by NEWA is much different from spore “maturity” as assessed by squash mounts.
The NEWA model is based on information first published by Gadoury and MacHardy in the early 1980s and was derived and validated primarily by measuring spore discharge following various degree-day accumulations, although they also used some squash mount data. Many spores that appear morphologically mature in squash mounts fail to discharge in the next rain because they actually are not physiologically mature enough to be ejected (see Plant Disease 76:277-282 [1992]). In fact, at the Hudson Valley Lab we usually found that no significant spore discharge actually occurred under field conditions until our squash mount assessments showed about 12-15% of asci with morphologically mature spores.
To sum up, the percentage of mature ascospores predicted in the NEWA model is an estimate of mature spores likely to discharge in the next rain, which contrasts with the fact that most spores assessed as “morphologically mature” in early-season squash mounts are NOT yet mature enough to discharge. My short-cut approach to this longer explanation was simply to state that the NEWA model predicts discharge so as to avoid confusion with the concept of morphological maturity as depicted in squash mounts. I should have stated instead that “the NEWA model is predicting physiologically mature spores ready for discharge in the next rain, not the percentage of morphologically mature spores as assessed in squash mounts, which would be much higher.”
