Late blight of potatoes and tomatoes is a disease caused by a fungus-like oomycete pathogen Phytophthora infestans that was responsible for the Irish potato famine in the mid-nineteenth century. It can infect and destroy the leaves, stems, fruits, and tubers of potato and tomato plants.
High moisture and moderate temperatures are conditions that favor rapid production of the disease. Severe late blight epidemics occur when P. infestans grows and reproduces rapidly on the host crop. Sporangia disperse to healthy tissues via rain splash or on wind currents. Reproduction is asexual; each sporangium is an exact copy of the one that initiated the parent lesion, and each can initiate a new lesion.
When the environment is favorable, late blight symptoms will become visible in three or four days. On leaves of tomato or potato, late blight begins as pale-green or olive-green areas that quickly enlarge to become brown-black, water-soaked, and oily-looking. Stems can also exhibit dark-brown to black areas. If weather conditions are cool and wet, entire plants can collapse and die from late blight in seven to 10 days
Tomato fruits with late blight develop large, often sunken, golden- to chocolate-brown, firm spots with distinct rings. Potato tubers with the disease develop a reddish-brown discoloration under the skin and these areas may become sunken.
Affected leaf, stem, fruit or tuber tissue often eventually develops a white-gray, fuzzy look as the late blight organism begins to reproduce.
Disease life cycle
Phytophthora infestans requires a living host to survive between seasons. Usually it lives in infected potato tubers, which can survive in storage or the soil after harvest or anywhere potatoes might be discarded.
- Phytophthora infestans is usually dispersed aerially one to several miles from the overwintering site to living potato or tomato foliage via sporangia. Sporangia can germinate within a few hours after landing on potato or tomato foliage if free moisture (e.g., dew, rainfall, sprinkler irrigation, fog) is present. Germination takes place either indirectly via zoospores or directly via a germ tube that penetrates into foliage, stems, or fruit to initiate infections. Infections are visible as small lesions after three to four days. Necrotic areas on some lesions are only 1 to 2 mm in diameter
Lesions enlarge as the pathogen grows through the tissues, and the pathogen can sporulate from older lesions when the environment is favorable (leaf wetness and moderate temperature). Under dry conditions no sporulation occurs and the lesion has a brown dead center, surrounded by host tissue that has collapsed.
Disease development (growth and reproduction of the pathogen) is favored by moderate temperatures and wet conditions. Epidemics can be rapid and devastating because of the high reproductive potential of this pathogen. Individual lesions can produce 100,000 to 300,000 sporangia per day. Each sporangium is capable of initiating a new infection that will become visible within three to four days and produce sporangia within another day or two under optimal conditions.
Since, late blight is a community disease, for effective control integrated management MUST be adopted. Fungicides cannot be used alone for effective control of late blight, but must be used as one tool in an integrated management strategy.
Prevention and Control
The most important aspect of late blight management is to eliminate or reduce the initial source of the disease each season. Sources of disease include infected seed, cull piles and volunteers.
Obtain seed from farms with effective disease management practices. The use of certified seed is highly recommended.
Use proper cultural practices, including the following, as the first line of defense:
- Increase spacing of plants to reduce canopy density
- Promptly remove or destroy volunteer potatoes found in other crops grown in rotations or elsewhere
- Control weed hosts, such as hairy nightshade
- Use proper hilling to reduce infection in tubers
- Carefully manage irrigation to avoid increasing disease risk through prolonged periods of wetness
- Practise crop rotation
Scheduled Fungicide Programs
- Use fungicides as part of a preventative program. It is difficult to eradicate the disease with fungicides when it has already set in.
- Begin a fungicide program early in the season, always before disease develops, and continue through until harvest. Crops should be monitored throughout the growing season for late blight.
- Always follow fungicide label instructions for application rates, spray intervals and limitations on numbers of sprays.
- When the maximum number of applications of a fungicide is reached, switch to an unrelated alternate product with a different mode of action.
- When applying fungicides, complete coverage of the foliage (stems and leaves, top to bottom of canopy) with fungicide is necessary to enable disease prevention
Recommended fungicides to control late blight
- TRINITY GOLD 452 WP at 50g in 20L of water (Preventive and curative fungicide for used during extreme weather conditions)
- ABSOLUTE STAR 400 SC at 10ml in 20L of water. (Preventive and curative fungicide, good for resistance management)
- TOWER EXTREME 680 WG at 50g in 20L of water. (Preventive and curative fungicide)
- CADILAC 800 WP (Preventive fungicide)