Prolonged Dry Weather Slows Disease Progression
Posted: September 2, 2015
In recent days, there have been very few new reports of cucurbit downy mildew. This is likely due in part to the weather as well as the close proximity of many fields to harvest. As we get closer to harvest, managing downy mildew becomes less important however, maintaining a powdery mildew program will be important to protect the handles of pumpkins and gourds used for fall decoration. Similarly, there have been no new reports of late blight either.
In some fields growers are reporting seeing small circular whitish lesions on the pumpkin fruit. Depending on the cultivar, they can be surrounded by dark margin or sometimes a more orangish-yellow to tan margin if the cultivar has a white rind. These could be the result infections that occurred from either bacterial leaf spot caused by Xanthomonas cucurbitae or angular leaf spot caused by Pseudomonas syringae pv. lachrymans earlier in the growing season. Both are foliar diseases and the fruit become infected when the bacteria are splash dispersed from the leaves onto the fruit or transferred during via passing equipment or people. The bacteria will colonize the lenticels on the fruit surface and under favorable conditions multiply and lead to the development of visible symptoms. The lesions can expand and exudates can ooze from the lesions and dry on the outside of the fruit. Secondary organisms can also infect and lead to soft rot.
Both pathogens are thought to be seedborne but the impact on disease development is not well understood. Efforts are being made at several universities to better understand the epidemiology of these bacterial diseases and to identify a potential seed treatment protocol since cucurbit seed is too sensitive to hot-water treat. A 2-year minimum crop rotation is recommended to allow the crop residue to thoroughly decompose. Applications of copper tank mixed with mancozeb beginning at fruit set through expansion will help reduce fruit symptoms, however thorough coverage of the leaves and fruit is necessary. Several university research trials have also demonstrated a reduction in fruit symptoms with the application of Actigard, a plant defense inducing product, in addition to copper prior to disease onset.