I have observed very few foliar diseases of cucurbits this season. However, I have had several worried phone calls about these diseases, so here is an update.
Alternaria leaf blight-this disease is caused by a fungus that survives in crop residue. It usually is not an important disease. However, Alternaria can cause brown lesions with a ring-like structure in them. This disease is more important on cantaloupe. I have not observed this disease in 2017.
Anthracnose-after some effort, I have been able to start this disease in my own plots. Look for jagged, brown lesions on leaves and stems and pitted lesions on fruit. Except for my own experiments, I have not observed this disease in 2017.
Gummy stem blight-this disease does not seem to occur as often as anthracnose. However, this disease showed up uninvited in my research plots. I have not observed this disease in commercial fields. The fungus that causes gummy stem blight can survive in crop residue.
Powdery mildew-many different plants can get powdery mildew. However, this disease is usually pretty specific for a plant family. The powdery mildew that causes powdery mildew of cucurbits does not affect other plant families. It is more common to observe powdery mildew on cantaloupe than on watermelon. I have observed this disease on cantaloupe in a greenhouse.
Downy mildew- this disease has to overwinter on green cucurbit tissue. For example, it has to overwinter on the Gulf Coast or in greenhouses up north. This disease has not been observed in Indiana so far this season. It has been observed in southern Michigan and southern Kentucky. Downy mildew is caused by a fungus-like organism, not related to the organisms that cause Alternaria leaf blight, anthracnose, gummy stem blight or powdery mildew.
Phytophthora blight is caused by a fungus-like organism related to the microbe that causes downy mildew. However, Phytophthora blight survives well in the soil in the absence of a host. And the host range is large. Specialized fungicides should be considered for management of downy mildew and Phytophthora blight. While downy mildew has not been observed in Indiana, many fungicides that are used to manage for Phytophthora blight will also be helpful for downy mildew. Check the labels.
Fusarium wilt of watermelon-This disease causes the plant to wilt. Initially, only a portion of the plant may wilt. This disease often occurs shortly after the watermelon plant begins to run. This year, I have also observed Fusarium wilt on mature plants. It is possible to add the fungicide Proline® through the drip, but such a treatment is unlikely to help plants that are already wilting.
Bacterial wilt-This disease causes a wilt of cantaloupe, cucumber and, occasionally, pumpkin. Bacterial wilt is caused by a bacterium that is spread by the feeding of the cucumber beetle. Although it is possible to slow the spread of the disease by controlling the beetle, such a treatment is unlikely to help wilted plants. Even after the beetle feeding stops, it is possible to observe wilt from feeding that occured earlier.