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.” Today it is again causing widespread problems as new strains of the fungus have begun to appear in North America and Europe. With the development of these more aggressive strains, growers need to use all methods of control to their fullest potential.

Cultural control is a method that can greatly reduce the impact of late blight and should be part of every grower's disease management strategy. 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 transplant 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 can be overwintering sites and are another important source of the pathogen. Eliminating any near-by potato cull piles and destroying volunteer tomato and potato plants helps limit the initial source of inoculum in a region.

Early detection

Late blight is a very explosive disease that can appear suddenly and move through a field or area very quickly. Regularly scouting tomato fields is important for early detection. Early detection of late blight allows time for quick appropriate action before the disease has a chance to spread to other parts of the field or release an incredible amount of spores into an even wider area.

The use of late blight computer models may be beneficial as an early warning system of potential late blight development. The use of models helps time fungicide applications just before late blight actually appears and when repeat applications are needed, thus reducing the risk of crop loss and the number of fungicide applications.

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, computer prediction models, and diagnostic kits are all 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 so that it 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. Dense canopies also prevent fungicides from penetrating down into the lower leaves and stems of plants.

Excessive nitrogen is a negative factor because it promotes large dense canopies which prevent air movement for drying of leaves. 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 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.

Unfortunately, plant resistance is not currently available in any commercial tomato plants.

These are all examples of cultural control methods that help manage this destructive disease. Although it may not be possible to incorporate all of them, the more cultural control methods employed then the more effective overall disease management will be.