Photo 2. Purple blotch, Alternaria porri, on leaves of shallots showing zoned spots. Spores are produced in large numbers on these spots.
Photo 4. Purple blotch caused by Alternaria porri, on onion. The large spots are oval, purple and up to 150 mm long.
Onion purple blotch
Worldwide. Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from Australia, Fiji, French Polynesia, New Caledonia, New Zealand, Palau, Papua New Guinea, Samoa, and Vunuatu
Onion, and also shallot, garlic and leek.
Spots on the leaves are at first small with white centres, but expand rapidly into oval, brown to purple blotches, several centimetres long (Photos 1-4). Light and dark rings appear on the blotches. If the blotches grow around the leaves, or merge, the parts above the blotch wilt, collapse and die (Photo 1). The leaf tips are commonly infected (Photo 5). The bulbs can also be infected at harvest; infection is usually at the neck where a watery yellow to red rot develops.
The disease is favoured by high humidity, and can also occur as a secondary infection on plants damaged by other pests, e.g., onion thrips.
The fungus survives on crop debris on or in the soil for at least a year. Spores of the fungus are produced in the brown to purple blotches (Photo 2) and are spread by wind, rain splash and, possibly, by insects. Heavy dews or rains favour the disease, as the leaves need to stay wet for 8 hours for the spores to infect. Older leaves are more susceptible, especially those damaged by the onion thrips. Spread on seed has been reported, but is probably rare.
A 'blotch' is a word that describes a large irregular mark; in this case the blotch occurs on the leaves. Onion blotch is a common disease, especially in hot humid countries. It is said to be the most common disease of onion in Samoa. It occurs throughout the year, but damage is greater during the drier season when heavy dews occur. Losses are more than 40% in Samoa, and are reported to be higher in other countries. In parts of the Philippines, the area planted to the red onion variety was reduced to half that usually planted in 2016 because of the disease. Losses may continue to develop post-harvest.
Look for the characteristics large brown to purple blotches. Look for tips of leaves that blacken and hang down, wither and die.
Cultural control is particularly important in the management of this disease:
Resistance has been reported in onion and also in garlic.
If chemical control is necessary, use:
AUTHOR Grahame Jackson
Information (and Photo 3) from Gerlach WWP (1988) Plant diseases of Western Samoa. Samoan German Crop Protection Project, Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) Gmbh, Germany; and from Diseases of vegetable crops in Australia (2010). Editors, Denis Persley, Tony Cooke, Susan House. CSIRO Publishing. Photo 2 Howard F. Schwartz, Colorado State University, Bugwood.org. Photos 3&5 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 4 Howard F. Schwartz, Colorado State University, 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.
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