Cherry leaf spot and the need for postharvest fungicide applications in this early harvest season
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
With the cherry season behind us for 2010, growers have inquired about post-harvest management strategy for cherry leaf spot (CLS). In a typical year, we harvest tart cherries in mid-July to mid-August, and a post-harvest fungicide spray is applied within a week of harvest. The intent for this spray is to prevent early defoliation that can lead to reduction in tree winter hardiness, diminished fruit set the following year, and result in poor fruit quality in future seasons. These post-harvest applications are commonly sprayed mid- to late-August, which in most years is effective enough to prevent premature leaf loss in September. In the case of 2010, much of the tart cherry harvest was finished by early July, which leaves almost an extra month to manage for cherry leaf spot. The following guidelines should help growers when making their post-harvest cherry leaf spot management decisions.
First, all growers should have made the “typical” chlorothalonil application just after harvest. If the orchard was clean or fairly clean up until this point, this spray will keep the leaves protected until the first of August. Further fungicide applications will be warranted if conditions remain wet and warm. Long periods of warm, dry weather will keep the cherry leaf spot fungus in check.
Under cherry leaf spot-conducive conditions, a second post-harvest fungicide application in early August will further protect the leaves until mid-August, the traditional timing for the post-harvest spray. Again, if the orchards do not already show signs of cherry leaf spot, this second post-harvest application should protect foliage through to September, and because the cherry leaf spot fungus grows slowly, the pathogen will not have adequate time to move through its life cycle and result in premature defoliation. On the other hand, if an orchard is already showing signs of leaf drop at this time, a third fungicide application may be warranted at the end of August. Additionally, if conditions in August are wet and warm, even clean orchards may need another fungicide application. Because there are many formulations of chlorothalonil available, growers should check the label for the maximum allowable limit for the season.