Photo 1. Marginal rot of cucumber leaf caused by gummy stem blight, Didymella bryoniae, more advanced than in Photo 1.
Photo 2. Marginal rot with yellow margin caused by gummy stem blight, Didymella bryoniae, on cucumber.
Cucumner gummy stem blight, cucubit gummy stem blight, gummy stem blight of cucurbits. See also Watermelon gummy stem blight (Fact Sheet no. 07).
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 in temperate, sub-tropical and tropical countries: Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from American Samoa, Australia, Cook Islands, Federated States of Micronesia, Fiji, French Polynesia, New Caledonia, New Zealand, Palau, Solomon Islands, and Tonga.
Cucumber and other cucurbits, including melon, gourd, pumpkin, watermelon, as well as species of Luffa and Momordica.
Spots and areas of decay often occur at the tips and other places along the margins of the leaves (Photo 1). The decayed areas are light brown, sometimes with a yellow margin extending back into the leaf (Photo 2). Commonly, there is heavy leaf fall (Photo 3). On the stems, pale brown spots occur at wounds where leaves and fruits have been removed. Tiny black dots that are the spore-producing structures (also called fruiting bodies or 'pycnidia') cover these areas. A sticky golden liquid may be present, which is characteristic of the disease, and the reason for its name. If the spots grow around the stem, it dies. Spots on fruit are oval to round; at first, they have a greasy green appearance, later they merge, and develop into a brownish-black rot (hence the common name of black rot). Symptoms on the fruit can continue to develop in storage, when they become more obvious.
Survival is by the dormant cottony growth of the fungus and thick walled resting spores (called 'chlamydospores') inside the undecomposed remains of crops. They can survive for up to 2 years. Spread is by splashing water, wind-driven rain, and on fingers, knives and clothing during cultural operations. Development of the disease depends on how long the leaves, stems and fruits stay wet so that spores can germinate, infect, and for the fungus to continue to grow.
This is a common and serious disease of cucumber worldwide. It occurs in the field in the tropics and also in greenhouses in Europe and other temperate regions. The fungus infects all parts of the plant, leaves, stems, flowers and fruits, causing leaf fall, flower drop, fruit rots and wilts. Rots can occur on the fruits in storage or in transit to markets.
Look for infections at the leaf tips and other places along the leaf margin. At the leaf tip, the rots may be V shaped as they advance along the major veins. Look for infections on the stems, especially on the lower part of the vine; they may surround the stem and cause a wilt. Look for flower-end rots on the fruit.
No varieties have been bred with resistance, but see Fact Sheet no. 07 on watermelon gummy stem blight.
Use one of the following fungicides: chlorothalonil, mancozeb or copper oxychloride.
AUTHORS Grahame Jackson
Photos 2 Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org.
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
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