Potato Late Blight

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Late Blight Information


Late blight is one of the most important potato diseases in the world. It was the cause of the great Irish Potato Famine of the 1840's and it continues to be a challenge to control in production regions that have climatic conditions ideal for its development. The climatic conditions in the arid regions of the Western United States have usually not been very conducive to late blight development except in years when the spring temperatures are cool and rainfall is above normal. Recent identification of more virulent strains of the causal organism, Phytophthora infestans, may change that picture for arid production regions. In 1995 late blight caused significant economic losses in Malheur County, Oregon and parts of Idaho; areas that have seen only isolated patches of the disease in past years.


Late blight may appear on leaves, stems, and tubers. Leaf symptoms appear as pale green water soaked lesions. These lesions are usually at the tips or margins of the leaves, but not always. The lesions grow into brown or purplish-black lesions which sometimes have a yellow halo. The halo is not always present in late blight infections. During humid conditions a white growth of mycelium and spores can be seen on the underside of the leaves. Lesions also appear on the petioles and stems as black, greasy areas. Stem lesions may girdle the stem and kill the foliage above the lesion. Stem lesions were prevalent during the 1995 season in Malheur County and Southwestern Idaho.

A field severely infected with late blight will give off a characteristic foul odor caused by rapidly decaying potato tissue.

Disease Development

For late blight to occur, there are three conditions that must be present: 1. abundant inoculum (late blight spores), 2 a susceptible host (potatoes, tomatoes, or related specie), and 3. the environmental conditions favorable for late blight.

The pathogen survives from one year to the next on infected tubers. These can be cull tubers discarded as storage sheds are emptied for packing or processing, tubers left as volunteers in the field, chips from seed cutting operations or infected tubers planted as seed. All serve as sources of inoculum. Wind and air currents will move the spores over a wide area. Late blight spores have been known to travel over 40 miles under the right conditions. A small amount of inoculum can contaminate a large area very quickly. Because of the widespread infections in 1995, we are assuming inoculum will be present that are potential sources of inoculum for the 1996 growing season.

Environmental conditions must be conducive to disease development before the disease can develop. Humidity needs to be 90% or greater for spore development, conditions which can occur frequently inside the potato canopy. Temperatures below 780 F are necessary for the spores to develop. Free moisture must be present on the plant in order for the spores to germinate and infect a new plant. Infection requires cool days to keep evapotranspiration low and frequent rainfall or overhead irrigation or a combination of both to provide long periods of free moisture over a 3-5 day period of time. To recap, the following environmental conditions are necessary for late blight development:
HUMIDITY- 90% and higher
MOISTURE- free moisture for 8-12 hours

Management Guidelines

Scouting and Forecasting

Fields should be scouted at least two times each week. Particular attention to should be given to scouting during the two weeks after a cool rainy period. Oregon State University and University of Idaho will have scouts looking for late blight in susceptible fields in areas near the center pivots and those near rivers. When a late blight infection is reported in a production area, it is important that all fields in that area receive a protectant fungicide spray. Growers and fieldmen are encouraged to stay informed of current scouting reports and weather forecasts.

Weather forecasts, scouting reports and disease forecasting will be made available from the following sources.

Chemical Recommendations

Oregon State University Potato Late Blight IPM