BLACK PIT

Alternaria alternata, fungus; foliar = brown spot

Infection:

Because historically A. alternata has been considered a minor problem, there is little known about this disease in potatoes. However, in recent years, alternata blight and now black pit has been reported to be a problem more and more in several States from Texas to North Dakota. Because of early blight, it has largely been ignored. The only information available describing the foliar and tuber symptoms come from research done in Israel in the 1980s. Infection of foliage is much like early blight. How tubers get infected is not quite known. Recent observations in Nebraska suggest leaching through the soil much as late blight. Pathologists in Colorado have suggested that the fungus can live in the soil as well. Direct contact with infected vines remains a possibility.

Symptoms:

Black pits appear on tuber. These are similar to pits by common scab but are deeper, narrower and blacker. Skin patches nearly identical to early blight can also be clearly seen after tubers are washed.

Control Practices:

The vine disease is favored by high temperatures, long dew periods and sandblasting. I usually detected on vines several days before early blight, but, once early blight is present, early blight takes over. Alternata blight has been reported to be sensitive to EBDCs. Canadian research has suggested that a single, August application of metalaxyl plus mancozeb may reduce the tuber disease but control of black pit is unknown. Whether black pit will spread in storage is unreported.

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