Cucumber leaf spot
Worldwide, especially in the tropics. It is recorded from Fiji, Samoa, Solomon Islands, and Tonga.
Common on a wide range of crops, including, beans and other legumes, capsicum, cucumber and other cucurbits, lettuce, papaya, tomato. Ornamentals and weeds are also hosts. The fungus causes an important disease of rubber in Asia and Africa.
Symptoms & Life Cycle
Numerous cream-coloured round spots, up to 4 mm diameter; often, irregular shaped or angular, bordered by veins (Photos 1-4). The leaves dry out and fall prematurely. Fruits of cucumber are not affected. The fungus is a secondary invader of rotting fruits of papaya and tomato.
The spores are spread by wind-blown rain, and if windy wet weather continues for a few days, spread is fast and plants lose their leaves quickly.
The source of the fungus is from other infected crops, the remains of the previous crop and, perhaps, other host species. The fungus is very common on papaya leaves causing angular, light brown or grey spots, 2 mm diameter, sometimes surrounded by a yellow margin; the centres of the spots often fall out producing a “shot-hole” effect.
A fungus causes this disease. Where rainfall is high and large numbers of small spots occur on the leaves, they dry out, fall down, the plants die prematurely, and yields are seriously affected (Photo 2). The fungus also causes a leaf spot on papaya, and a common and serious disease on tomato called target spot ( see Fact sheet no. 163 ).
Detection & Inspection
Look for round, but often angular, cream spots on the leaves, bounded by the veins.
Cultural control is important. The following should be done:
- Do not plant new crops next to older ones that have the disease.
- Plant as far as possible from papaya, especially if leaves have small angular spots, with “shot-hole” symptoms.
- Check all seedlings in the nursery, and throw away any with leaf spots.
- Keep plots free from weeds, as some may be hosts of the fungus.
- Grow cucumbers on a trellis or vertical wires to increase air circulation around them. If the leaves dry quickly, there is less time for the spores to germinate and infect.
- Do not work in the cucumbers plots when leaves are wet, especially if some plants have signs of leaf spot. Touching the plants can help to spread the disease in water droplets.
- Collect and burn as much of the crop as possible when harvest is complete.
- Practice crop rotation, leaving 3 years before replanting cucumber on the same land.
There are no reports of varieties with resistance to leaf spot.
Warm wet conditions favour the disease such that fungicides are needed to give adequate control. The products to use are chlorothalonil, copper oxychloride or mancozeb. Treatment should start when the first spots are seen and continue at 10-14-day intervals until 3-4 weeks before last harvest. It is important to spray both sides of the leaves.
AUTHOR Grahame Jackson
Photos 2&3 Konrad Englberger, Pohnpei, Federated States of Micronesia. Photo 4 Kohler F, Pellegrin F, Jackson G, McKenzie E (1997) Diseases of cultivated crops in Pacific Island countries . South Pacific Commission. Pirie Printers Pty Limited, Canberra, Australia.
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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Common Name Cucumber leaf spot Scientific Name Distribution Worldwide, especially in the tropics. It is recorded from Fiji, Samoa, Solomon Islands, and Tonga. Hosts Common on a
Leaf Spot Prevention In Cucumbers
Members of the extensive cucurbit family, cucumbers are a staple in many vegetable gardens. The plants are prolific producers and gardeners have dozens of varieties to choose from when planning their vegetable patch. Because cucurbits are susceptible to a wide variety of diseases and pests, breeders work constantly to develop cultivars with resistance to as many of these problems as possible. No one variety resists all diseases, so gardeners need to do as much as they can to protect their cucumber plants.
Caused by Pseudomonas lachrymans, angular leaf spot causes water-soaked, angular leaf spots that may appear greasy and have yellow halos. The spots do not cross main veins. Initially green, the spots may turn brown and dry up, developing tattered-looking holes in the centers. The disease also infects the stems and fruit of the cucumber plant. To prevent angular leaf spot, plant seeds or seedlings labeled as resistant varieties. Buy only certified disease-free seeds. Don’t use high-nitrogen fertilizer, avoid overhead irrigation and don’t do gardening chores while the cucumber plants are wet. Rotate crops and plant cucumbers away from other cucurbit species. After harvest, plow the cucumber plants under as deeply as you can. Remove all surface plant debris and destroy it away from the garden.
Bacterial Leaf Spot
Leaf spots caused by other bacterial infections can infect the foliage and the fruit, causing the cucumbers to rot on the plant. The spots are very small, distinguishing them from angular leaf spot, and spots are not confined by the veins on the cucumber plant’s leaves. Bacterial leaf spots cause lesions on the cucumber which can penetrate the cucumber rind into the flesh of the fruit. Plant resistant varieties and practice prevention as for angular leaf spot disease.
Cucumber Mosaic Virus
Yellow spots and streaks on cucumber leaves, yellow leaf veins, ring spots or patterns of different shades of green are all symptoms of cucumber mosaic virus. This disease can be spread by insects such as aphids, via weeds and by failure to disinfect gardening tools and your hands after contact with an infected plant. Cucumber mosaic virus cannot be diagnosed from symptoms alone. Since there are no chemical treatments or preventives for cucumber mosaic virus, remove infected plants and destroy them away from the garden. Grow cucumbers from seed rather than seedlings. Keep the garden weed-free and disinfect gardening tools between cuts and between plants to avoid spreading the disease. Wash hands thoroughly after handling an infected cucumber plant. If you wear gloves while gardening, change or wash the gloves before handling healthy plants.
Fungal Leaf Spots
Myriad fungal infections can attack cucumber plants, including alternaria, anthracnose, downy and powdery mildews, gunny stem blight and phytophthora, septoria and ulocladium. Most produce tan or brown spots, sometimes with yellow halos. The spots often appear sunken or wet, and the centers may fall out, leaving the cucumber plant’s foliage looking ragged. Downy and powdery mildew produce white spots. These leaf spot infections may spread to the stems and the fruit of the cucumber plant, limiting or ruining the harvest. Proper diagnosis of the infection is crucial to treating or preventing it. Many cooperative extension service offices and university horticulture departments can diagnose leaf infections, sometimes for free. To prevent fungal infections, plant resistant cucumber varieties, if available. Rotate crops in the vegetable garden each year. Make sure the cucumber plants have good air circulation and are not crowded together. Remove severely infected plants and destroy them away from the garden. Avoid overhead watering. If the infection is light, you may be able to prune away the diseased area and prevent the infection from spreading. Always disinfect pruning tools between pruning cuts and in between each plant.
Leaf Spot Prevention In Cucumbers. Members of the extensive cucurbit family, cucumbers are a staple in many vegetable gardens. The plants are prolific producers and gardeners have dozens of varieties to choose from when planning their vegetable patch. Because cucurbits are susceptible to a wide variety of diseases and …