BY ROB TRAWICK, JACKSON COUNTY HORTICULTURE AGENT
With the unusually warm winter we experienced this year coupled with what was a warm and wet spring for most of us, there have been a lot of diseases showing up in numbers larger than what we are normally accustomed to here in Northwest Florida. One in particular that has caught my attention and seems to have achieved a good foothold is gummy stem blight (GSB), a major fungal disease of watermelons, cucumbers, and other cucurbit family members. The fungus responsible for gummy stem blight, Didymella bryoniae, is most active during warm, wet periods, just like what was experienced this spring, and is contagious so it can spread across an entire field extremely rapidly when conditions are favorable. GSB will also damage stem tissues at all stages of development. There are many keys to controlling this disease, but being able to identify it is one of the most important. While the first signs sometimes vary depending on the species of plant, common symptoms are water spotting on foliage often turning into brown, irregularly shaped lesions containing concentric ring patterns, an oozing black or brown gummy bead of fluid emanating from the stem, and split lesions showing up on the stems, oftentimes girdling them. Ultimately the fruit produced will be rotten which is both unmistakable and inedible.
The first line of defense in preventing GSB is crop rotation. Cucurbits planted in the same areas as last year’s crops are going to be exposed to plant debris left over in the soil that will harbor the spores of this fungus. Use seed from a reputable company with a history of fungus free seed as the disease can manifest itself even on seedlings. Carefully inspect any you have purchased prior to planting. Any seedlings showing brown lesions and dry leaf edges should not be planted. However, even in our climate where we are extremely susceptible to warm, moist bloom conditions, these preventive steps often only go so far, therefore fungicides should be applied in a preventive manner. Chlorothalonil, mancozeb, and maneb are extremely effective when applied as a contact fungicide and materials such as tebuconazole, difenoconazole, and thiophanate-methyl are excellent systemic fungicides for control of GSB. You can find a great resource of currently labeled chemicals for use on cucurbits in the University of Florida EDIS publication, “Management of Gummy Stem Blight (Black Rot) on Cucurbits in Florida.”