In the past couple of weeks we have seen a few cases of watermelon seedlings infected with gummy stem blight. This early detection is concerning, as transplant infections can serve as the initial source of inoculum for field epidemics later in the season. Growers should scout seedlings for gummy stem blight symptoms to prevent introducing the pathogen to fields.
Gummy stem blight (GSB), caused by the fungus Stagonosporopsis cucurbitacearum, affects the leaves, stems, and fruits of all cucurbits. Watermelons are particularly susceptible to this disease. Symptoms in transplants may include water-soaked or dark brown lesions on stems, which can become sunken and necrotic as the disease progresses. A distinguishing feature of gummy stem blight is the presence of sticky, amber-colored sap exuding from infected stem tissues. Leaf lesions may also be present, often circular with tan centers and dark margins. Concentric rings may develop in the leaf spots, giving them a target-like appearance (Fig. 1 and 2). These symptoms can develop rapidly under warm, humid conditions typical of transplant greenhouses and hardening areas (Fig. 3).
When transplants are already infected prior to planting, they can introduce the pathogen into the field environment, especially if they are symptomatic but not discarded. Spores produced on infected transplants can be spread by splashing water, equipment, or workers during transplanting and irrigation. This can lead to early and severe field outbreaks, even in areas where rotation and sanitation practices are otherwise well managed.
Growers and transplant producers should inspect watermelon transplants carefully before field setting. If overhead irrigation is used during transplant production, ensure watering is completed early in the day to minimize leaf wetness duration, as this encourages disease growth. Remove and destroy symptomatic plants as much as is practical. Do not attempt to salvage symptomatic plants, as they pose a high risk of pathogen dissemination.
For transplant producers, greenhouse sanitation is essential. Disinfect trays between uses, avoid reuse of potting mix, and maintain adequate airflow to reduce leaf wetness duration. Fungicide programs may be necessary in transplant facilities where infection is suspected or confirmed.
If transplants are already in the field, early detection through scouting and timely fungicide applications will be critical to prevent secondary spread. In dry weather, a less aggressive spray program can be used.
Fungicides listed in our factsheet have shown efficacy against gummy stem blight and are recommended as part of a rotation strategy to minimize resistance development. Samples can be submitted to the NC State Plant Disease and Insect Clinic for confirmation of suspected gummy stem blight infections in transplants.
For further assistance with diagnosis, management options, or fungicide scheduling, please contact your local Extension agent.
