Gummy Stem Blight In Cucurbits | Fungal Disease & Management | Farming Inspirations

Gummy stem blight (GSB) is a major disease affecting many cucurbits crops which includes watermelon, cucumber, pumpkin, squash, muskmelon and other melons. The fungus Didymella bryoniae is the causal organism for this disease. The disease is also referred as black rot due to its characteristic appearance on infected fruits, which can cause significant production losses when conditions are ideal for the spread of this disease.
The disease can affect all above-ground parts of the plant at any growth stage from seedlings to mature vines with fruit. Infection and symptoms can occur on all plant parts except roots.
Symptoms on seedlings occur as light to dark brown spots on the cotyledons. Infection of the cotyledons or hypocotyl causes a water-soaked, brown discolouration of the tissues, followed by tissue desiccation and collapse. Seedlings die rapidly after infection of either the hypocotyl or cotyledons.
On older plants, leaf symptoms appear as small, circular tan spots, sometimes surrounded by a yellow halo. Under favourable conditions, leaf lesions may enlarge rapidly and become irregular in shape. When the lesions come together, the entire leaf may become blighted. The spots dry, become cracked and may tear, giving leaves a tattered appearance. Infection often begins at the leaf margins.
Stem infections consist of brown, oblong, water-soaked lesions. Main stem lesions enlarge and slowly girdle the main stem resulting in a brown canker that produces a characteristic red or brown gummy fluid giving the disease its name. When infections became severe, it leads to complete wilting & dieback of vines.
Symptoms on fruits occur only if the vines are severely infected with gummy stem blight. Infected fruits turn into black rot which show small water-soaked spots that advance into large brown spots, sometimes also exhibiting the brown gummy secretion as found in stems. Hence, this disease is also known as black rot.
Didymella bryoniae is an ascomycete fungus that can survive on seeds, weeds and plant debris from previously infected cucurbits.The fungal fruiting bodies contain large numbers of asexual (pycnidia giving rise to conidia) and sexual (perithecia giving rise to ascospores) spores that spread through wind and splashing water.
A sequence of management plans must be initiated to control GSB/black rot disease.
Seed: One source of GSB inoculum is the seed. As seeds can be infested without expressing symptoms, seeds from healthy fruits should be preferred. In alternate cases, seed treatment is necessary in which disinfectants as liquid solutions are more effective than dry dust treatments.
Transplants: GSB is common at the seedling stage and displays one or more of the characteristic symptoms like (necrotic areas on the leaf margin, water-soaked regions on the stem, gummy ooze from the stem). Growers should regularly inspect transplant seedlings in the greenhouses. Whenever possible, avoid using healthy-looking seedlings from trays with infected plants.
Organic debris: Another source of disease spread is organic debris from previous cucurbit crops. As soon as a cucurbit crop is harvested, the decaying debris from that crop should be properly removed particularly the crops infected with GSB inoculum.
Crop rotation: Fields should not be cultivated with cucurbits routinely, and rotation with a non-cucurbit crop is important. A 2-3 year rotation with non-hosts is an effective way to reduce incidence of GSB.
Alternate hosts: Certain weeds serve as the alternate hosts to GSB and thereby field should be maintained clean & weed free throughout the crop season. Wild citrons, balsam pear, or volunteer cucurbits are other sources of inoculum which serve as hosts for the disease and should be eradicated before planting cucurbits.
Scouting: Routine scouting of the fields helps in timely application of fungicides that can prevent major crop losses.
Fungicide application: Generally, fungicides should always be applied in a preventive manner. If weather conditions are conducive for the occurrence and rapid spread of GSB, fungicide application should start during the early stages of plant growth. For management of GSB, both contact & systemic fungicides can be recommended.
Integrated pest management practices which includes cultural and chemical measures, are often recommended for comprehensive disease control.
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