Photo 1. The large black spots are typical of gummy stem blight, Didymella bryoniae, on the leaves. Notice the concentration of the spots at the margins of the leaf where water stays for longer. Some of the spots have joined together.
Photo 2. This is typical of the defoliation that occurs with gummy stem blight infection, making it a serious disease. Leaves go yellow, collapse and die when they have only a few spots. The older leaves die first.
Photo 3. Gummy stem blight infection,Didymella bryoniae, on a seedling. It is just possible to see the black dots that contain the spores in the centre of the spot. Infection of seedlings in the nursery is a major threat to watermelon production as it means the fungus is taken to the field and early infection and spread is guaranteed.
Gummy stem blight
Stagonosporopsis cucurbitacearum; (previously, Didymella bryoniae). Also known by the asexual state, Phoma cucurbitacearum or Ascochyta cucumis. The later is commonly found on plants in the field producing minute oval spores in round black structures in the leaf called "pycnidia" that are just visible to the naked eye.
Worldwide. Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. The fungus is recorded on cucurbits from American Samoa, Australia, Fiji, Federated States of Micronesia, French Polynesia, New Caledonia, New Zealand, Palau, Samoa, Solomon Islands, Tonga, and Vanuatu.
Watermelon, cantaloupe melon and cucumber.
Spots occur on the leaves, grow rapidly and the leaves blacken, shrivel and die (Photos 1&2). It happens so quickly that it is called a 'blight'. Spots occur on the vines, and they can release a gummy (sticky) substance - hence the name of the disease. Infection of the fruit is uncommon.
The loss of leaves, and the damage to the vines, means that the fruits are starved of food, and do not develop properly.
The fungus produces small, black, round structures inside the leaf spots; they can just be seen with the naked eye, although they are clearer with a hand lens. These are tiny 'sacs' that contain millions of spores.
Fruit rots develop if leaves are severely infected. Small over to round spots occur with a greasy green colour and these turn dark brown to black as they enlarge. The fruiting bodies may develop in the spots.
When they are ready, and when it rains, the baskets burst open and the spores ooze out. Wind and rain carry the spores to other leaves of the same plant, to other plants or to plants in other gardens. When the spores land on watermelon leaves, they germinate like a seed. The fungus enters the leaf and grows inside. As it grows, it kills the leaf, forming the round, brown spots. The spots grow and join together. Then more baskets of spores develop, and the cycle starts again (Diagram).
If the disease is not controlled when first seen, it spreads very quickly, especially during days of heavy and continuous rain. It is common for whole crops to be ruined by gummy stem blight.
In the nursery, look for large brown spots on the leaves (Photos 3&4). In the field, look for the rapid development of black spots and blotches that are typical of this disease, especially during wet weather.
Note, farmers sometimes mistake the disease for damage caused by caterpillars of a moth, Diaphania indica. These roll the leaves with silk threads and eat the parts between the veins (see Fact Sheet No. 33).
In Solomon Islands, variety trials with farmers found that the disease was less on New Dragon.
Use one of the following fungicides: chlorothalonil, mancozeb or copper oxychloride. Spray every 7-10 days, depending on the weather.
AUTHORS Helen Tsatsia & Grahame Jackson
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
This fact sheet is a part of the app Pacific Pests and Pathogens
The mobile application is available from the Google Play Store and Apple iTunes.