Photo 1. Large brown spots of early blight, Alternaria solani, on tomato, showing characteristic rings or targets.
Photo 3. Characteristic rings or target structures on potato leaf caused by early leaf blight, Alternaria solani.
Photo 7. Spots of Alternaria solani on potato leaves leading to collapse and death as they expand and join together.
Early blight of tomato and potato, target spot
Worldwide. Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from Australia, Cook Islands, New Caledonia, Palau, Papua New Guinea, Solomon Islands, and Vanuatu.
Tomato, and many others in the same family, including potato, capsicum, eggplant, and some wild hosts in the tomato/potato (Solanum) family.
All parts of the plant, leaves, stems and fruit, are infected. Small, pinpoint to 6 mm, circular to angular brown spots occur on the older leaves, with yellow margins. As they grow, they develop dark brown rings, like a target, a characteristic of the disease (Photos 1&2, tomato & Photo 3, potato). The spots often merge, and the leaves dry up and fall. Loss of leaves can cause sunscald on the fruits.
On the stems, the spots are similar, except that they are darker and more elongated (Photo 4).
On the fruits, the spots are dark brown or black, sunken, extending over part or all of the fruit. The rots become covered in black spore masses of the fungus (Photo 5).
Spots also occur on the stems of seedlings as large brown spots near soil level, and may lead to stem break and death.
Spread over short distances occurs when spores are moved plant-to-plant in wind, rain and overhead irrigation water. Survival occurs in crop remains, and also on seed. Favourable conditions for the disease are warm, humid weather with heavy dews or rains.
Early blight of tomato is a serious disease requiring control measures, including fungicide applications. The disease occurs wherever tomato (and potato, Photos 6-8) is grown, and can cause severe defoliation, resulting in fewer, smaller fruit. Loss of yield is difficult to estimate, but probably at least 5%. Many millions of dollars are spent on fungicides to control the disease.
Look for the circular to angular brown spots on the leaves, checking to see if they have the characteristic target-like rings. Look to see more elongated spots on the stems. Look for fruits which produce dense black spore masses of the fungus.
Complete resistance does not exist, but there are varieties which show differences in tolerance to the disease.
Use protectant fungicides, mancozeb, chlorothalonil or copper products. Systemic products are available, e.g., strobilurins, although they are expensive and, if used too often, the fungus may develop resistance to them.
AUTHOR Grahame Jackson
Information from Diseases of vegetable crops in Australia(2010). Editors, Denis Persley, Tony Cooke, Susan House. CSIRO Publishing. Photos 1,4-6 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. Photo 2 (taken by Eric McKenzie), and used in this fact sheet, appeared previously in McKenzie E (2013) Alternaria solani PaDIL - http://www.padil.gov.au. Photos 3,7&8 Jacquie (Wright) Kami, formerly Plant Pathologist, Secretariat of the Pacific Community.
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.
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