Late blight in tomatoes, potatoes

Late blight is a destructive disease of potato and tomato in all growing regions of the world. This disease first gained notoriety 150 years ago as the cause of the “Potato Famine of Ireland and Northern Europe." It was last a major issue in California after the El Niño of winter 1998-1999. Then late blight appeared in many early planted tomato fields and potato fields in Kern County.

Late blight is a very explosive disease that can appear suddenly and move through a field or area very quickly. Cool, wet conditions are ideal for late blight to develop. With what appears to be another wet El Niño rainy season tomato and potato growers need to again be on the lookout for this disease.

Preventing the disease from occurring is the first step in managing late blight. The goals of a good cultural control program are to prevent introduction of inoculum, reduce inoculum buildup, reduce infection rate, and create conditions unfavorable for disease development. Growers can achieve these goals by incorporating several different techniques into their farming operation.

For transplanting tomatoes it is important to make sure that the initial source of disease is not infected transplants. Check transplants before planting and refuse plants that show signs or symptoms of late blight. Early signs of late blight on transplants can be difficult to identify and may need confirmation by a qualified expert.

Other potential sources of inoculum are potato cull piles and volunteer potatoes and tomatoes. Although not common in most parts of the San Joaquin Valley, infected tubers from potato cull piles can produce a tremendous amount of air borne spores that move by wind to shower onto nearby fields. Volunteer potatoes and tomato plants are another important source of the pathogen. These volunteer plants can be an overwintering sources for the fungus. Eliminating any near-by potato cull piles and destroying volunteer tomato and potato plants helps limit the initial source of inoculum in a region.

Regularly scouting tomato fields is important for early detection. Early detection of late blight will allow appropriate action to be taken quickly before the disease can spread to other parts of the field and having an overwhelming amount of spores blowing out into an even wider area. Diagnostic kits are another tool that can be used for early detection of the disease. Kits are available which help quickly confirm or refute whether a questionable lesion is caused by the late blight fungus. Regular field scouting and diagnostic kits are methods of early detection so appropriate action can be taken quickly.

Spot killing infected plants when the disease first appears will slow the spread of spores to other parts of the field. Plants can be quickly destroyed by burning or with the use of a fast acting herbicide. This method of cultural control will only be effective when blight first appears in a field or region. Once late blight is established in an area then the likelihood of influencing the amount of spores in that area becomes negligible.

Changing the climatic environment around the plant that is less conducive to late blight can also help reduce late blight severity. Late blight spreads and develops when conditions in the canopy are moist and humid. Sprinkler irrigation creates an ideal environment by keeping the canopy wet for long periods of time. If possible, avoid sprinkler irrigation after stand establishment. Excessive nitrogen is a factor because it promotes large dense canopies which prevent air movement for drying of leaves. Dense canopies also prevent fungicides from penetrating down into the lower leaves and stems of plants. Fertilizer management can be used to a grower’s advantage by making sure that the plant canopy is not unnecessarily inviting to this fungus.

Excessive nitrogen also increases the susceptibility of tomato and potato plants to infection. The late blight fungus prefers lush, young, actively growing tissue over stressed, senescing tissue. Excessive nitrogen will promote lush vegetative growth and delays maturity, which increases the chance of infection and prolongs the period that the crop is susceptible to late blight infection. Lastly there are effective fungicides that can minimize the impact of late blight. Broad spectrum fungicides (maneb, mancozeb, chlorthalonil) should be used preventively on a seven to 14 day schedule when conditions are favorable for late blight to occur. If late blight does appear in a field then other, more late blight specific fungicides can be used. Refer to the UC IPM Web site ( for specific details on the recommended use of these products. Good coverage of the plant canopy is really crucial to controlling late blight with fungicides.

TAGS: Vegetables
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