Soil, Crop and Pest Management

Late Blight and Early Blight Management in Potato

Robert Wick and Jeffrey Lerner, Dept. of Microbiology and Ruth Hazzard, Dept. of Entomology

There are two major diseases of potato in this region, late blight and early blight. It is important to understand that these are very different diseases, and they respond differently to fungicides. Early blight is caused by the fungus Alternaria solani. This fungus also causes early blight of tomato. Despite the name of the disease, early blight occurs later in the season, not early. To add to the confusion, late blight, caused by Phytophthora infestans usually occurs early in the season, although it can occur whenever environmental conditions allow. Late blight also occurs on tomato.

The Vegetable IPM newsletter provides weekly disease forecasting reports for both early and late blight. These forecasts are based on data collected by specialized weather monitoring equipment at the South Deerfield Research Farm and at a farm in Acton, MA. The same forecasting systems, BLITECAST and WISDOM, are used throughout the major potato growing regions of the country.

Disease forecasting enables growers to eliminate early fungicide applications, when they are less likely to be needed. Forecasts let growers know when it is time to begin fungicide applications, and what is the most appropriate spray interval to achieve effective control. Shorter intervals are recommended when the risk of early or late blight conditions is high, and longer spray intervals when the risk is low.

Management of early blight (Alternaria solani)

When possible, rotate away from potato and tomato, or grow on land that has not had potato or tomato for two years. Make sure plants have adequate nitrogen. Plants under nutrient or other stress will be more susceptible to early blight.

Chlorothalonil (Bravo), azoxystrobin (Quadris), maneb and mancozeb will all control Alternaria. Quadris is a new fungicide that has exceptionally good activity against Alternaria. To prevent resistance development, Quadris should be used in rotation with Bravo.

Early blight forecasting

Early blight is favored by warm temperatures and long periods of high relative humidity. Research has shown that fungicide sprays applied early in the growing season, before the crop starts to flower, provide little benefit. Later in the season, as the crop matures, potatoes become more susceptible, early blight progresses more quickly, and fungicides become more critical.

The WISDOM disease forecasting system was developed at the University of Wisconsin to forecast the conditions which promote early blight. In this system, crop maturity is measured by Physiological Days (P-days), based on the temperature accumulation since crop emergence. In most cases, fungicides for early blight are recommended at 300 P-Days. In a non-rotated field, a lower P-Day threshold of 200 or 250 is used. UMass provides P-Day data for two typical potato emergence dates.

The WISDOM forecasting report includes:

Management of late blight (Phytophthora infestans)

Late blight is one of the most destructive and fast-moving diseases known. It is similar in many respects to blue mold of tobacco, which is also very destructive.

The fungicides used for early blight have some protective activity against Phytophthora but cannot be relied on to provide control if an outbreak occurs. If late blight is reported within one half mile of your field, begin applications of Acrobat, Curzate or Tatoo. Acrobat and Curzate now have full registration for use in potato. Tatoo is a Section 18 fungicide, and it is important to fill out the necessary reports when it is used. Tatoo can be used on a 14 day schedule, alternating with a protectant fungicide for control of early blight. Curzate should be combined with a full rate of mancozeb to give additional protection against early blight and prevent resistance development. Acrobat already has mancozeb as one of its components.

Areas of the field with significant disease should be disked under.

Proper management of cull potatoes is a critical part of reducing the risk of late blight. Any plant growth developing from infected tubers in cull piles is a potential source of spores. Burying culls, preferably before the year’s crop is planted, reduces the risk of late blight outbreaks in the surrounding area.

Rapid identification and control of late blight outbreaks is important. Growers who suspect late blight is present should contact the University of Massachusetts Plant Disease Diagnostic Clinic, 413-545-1045.

Late blight forecasting

Late blight is favored by frequent or long periods of leaf wetness from rain or dew, with temperatures in the range of 45 F to 80 F. The weather station measures rainfall, periods when humidity is over 90%, and temperature. These conditions are converted to a daily Severity Value which indicates the likelihood of late blight developing. Late blight is predicted 7 to 14 days after 18 severity values have accumulated since crop emergence.

The BLITECAST forecasting report tells:

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