Photo 2. Downy mildew, Bremia lactucae, on lettuce showing sporulating areas between the main veins.
Lettuce downy mildew
Bremia lactucae; there are several strains of the oomycete.
Worldwide. In temperate and sub-tropical countries. Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from Australia, New Caledonia, and New Zealand.
Lettuce and relatives, including globe artichoke, endive, chicory and ornamental and wild species in the daisy or sunflower (Asteraceae) family. The weeds, groundsel, (Senecio), sowthistle (Sonchus), and dandelion (Taraxacum) are wild hosts.
The disease is caused by an oomycete or water mould, not a fungus. Although downy mildews look like fungi, they are related to algae.
Light green to yellow, round to angular spots on the top of the leaf, merging and later turning brown, and sometimes becoming soft and slimy (Photo 1). On the lower side of the leaf, a white growth develops and this contains the spore masses that develop under cool and humid conditions (Photos 2&3). Thick-walled resting spores called "oospores" are formed, but their importance in the life cycle of lettuce downy mildew is uncertain.
Spread of the downy mildew is by spores; these are produced at night and in the morning released from their spore-bearing stalks and blown in the wind to plants nearby and over long distances. The spores do not live for long; for successful infection, leaves need to be wet for 3-4 hours.
Survival can occur as oospores in the soil and on weeds, but often it is by the transfer of spores from older to younger crops.
Lettuce downy mildew is an important disease capable of causing the rapid destruction of the plant. Damaged plants may require trimming at harvest, and they are susceptible to secondary rots in transport and storage.
Look for the yellowish angular spots on the top of the older leaves, and white fungal-like growth on the underside. Look for spots that turn brown as they age.
As this disease is not recorded in many countries, biosecurity authorities should consider the potential pathways for entry.
Varieties with resistance to downy mildew exist, but there are several strains.
Use fungicides that are recommended for Pythium and Phytophthora. Spray with systemic products, e.g., (i) metalaxyl (mixed with mancozeb or as alternating sprays to prevent build up of resistance to metalaxyl) or (ii) fosetyl-Al; or use protectant products, e.g., mancozeb, copper or chlorothalonil. To increase effect of fungicides, keep humidity and leaf wetness as low as possible (see above).
AUTHOR Grahame Jackson
Information (and Photo 1) from Diseases of vegetable crops in Australia (2010). Editors, Denis Persley, Tony Cooke, Susan House. CSIRO Publishing; and from CABI (2014) Bremia lactucae Crop Protection Compendium (http://www.cabi.org.cpc/). Photos 2&3 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.
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