In the past couple of weeks we have seen a few watermelon seedlings affected with gummy stem blight. Growers are advised to scout their watermelons seedlings to make sure they are not presenting gummy stem blight symptoms and to avoid introducing the pathogen into fields by planting infected transplants.

Gummy stem blight is caused by the fungal pathogen Didymella bryoniae and affects the leaves, stems and fruits of all cucurbits. Watermelons are particularly susceptible to this disease. Seedlings infected with D. bryoniae will present water soaked lesions on the stems (Fig. 1) and necrotic, brown spots on the edges of leaves (Fig. 2). When infected tissue is inspected closely under a microscope, reproductive structures of the pathogen called pycnidia, will become apparent (Fig. 3). For a detailed description of the disease, its diagnosis, and control options, see our Gummy Stem Blight of Cucurbits fact sheet.

Fig. 1: water soaked lesions on crowns of watermelon seedlings infected with gummy stem blight (Photo: Shawn Butler, NCSU Plant Disease and Insect Clinic)

Fig. 2: leaves with necrotic, brown lesions on edges due to gummy stem blight infection (Photo: Shawn Butler, NCSU Plant Disease and Insect Clinic)

Fig. 3: close-up of a watermelon seedling stem infected with gummy stem blight seen under a microscope. Note the pycnidia (round, brown reproductive structures) on the surface. Pycnidia can be seen with a 10x handheld lens as small brown to black dots (Photo: Shawn Butler, NCSU Plant Disease and Insect Clinic)

The gummy stem blight pathogen can be seedborne, therefore, treating seeds prior to planting is recommended. The disease is favored by warm (61°F–75°F), wet conditions (4–10 hours of leaf wetness), and the pathogen is dispersed by water splashing (rain, overhead irrigation), thus, using drip irrigation will help contain infections. Once greenhouse transplants in a tray become infected, it is advised to destroy the affected tray and any adjacent trays, since the pathogen will have likely spread to neighboring trays due to irrigation splashing, even if transplants look healthy. Spores of the pathogen can survive on crop residue, so sanitation is key to prevent this disease in the greenhouse and a 2-year crop rotation may be needed for fields that have experienced severe infections.

The disease has been difficult to control in recent years due to the development of fungicide-resistant populations. Nonetheless, a fungicide trial conducted in North Carolina last year revealed some products that are effective in controlling gummy stem blight on conventional operations. While there are some products labeled for organic operations, no efficacy data has been published. For a complete report of results from our gummy stem blight trial please refer to our demonstration trials.

Products and rates assayed in this trial included:

Products were applied every 7 days. All treatments with the exception of Manzate Pro-Stick and Cabrio suppressed gummy stem blight when compared to the non-treated control (Fig. 4). Treatments labelled with the same letter are not statistically different, so all treatments not labelled with the letter a, which corresponds to the non-treated control, presented significantly less gummy stem blight.

Fig. 4: products for gummy stem blight control on watermelon

When planing your spray program, make sure you alternate fungicide groups of products to avoid generating fungicide-resistant strains. You can find some example products on our Gummy Stem Blight of Cucurbits fact sheet. Growers are encouraged to read the 2014 Watermelon Spray Guide developed by Dr. Anthony Keinath from Clemson University, and the Southeastern US Vegetable Crop Handbook for the latest fungicide recommendations.

If you think you have gummy stem blight in your cucurbits please contact your local Extension Agent and send photos and/or physical samples to the Plant Disease and Insect Clinic for confirmation.