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THE POTATO BLIGHTS AND THEIR CONTROL
IN INDIANA

By
R. W. Samson
Purdue University Agricultural Extension
Botany and Plant Pathology
Mimeograph No. PB 12	April, 1946
Two potato diseases occur in Indiana, requiring spraying or
dusting with a fungicide for their control. One is early blight, pri-
marily a leaf disease. The other is late blight, destructive to both
vines and tubers.

The Early Blight Disease
Early blight damages potatoes in Indiana more or less every
season. It develops over a wide range cf temperatures and the usual
humidities at night are high enough to permit infection. Rains and dews
may promote greater disease development. The symptoms are small, dark-
colored leaf spots, J to £. inches in size, sometimes marked with concen-
tric rings. These leaf spots become more numerous as the vines mature.

The Late Blight Disease
Potato late blight is more serious than early blight but does
not occur every year in Indiana, as most summers are too hot or too dry
or both. It develops with a vengence when it does occur, and has damag-
ed potatoes in at least 8 out of the past 27 seasons. - The outbreak in
1945 was the worst on record, and resulted from the abnormally high rain
fall and low temperatures throughout the season.
The disease appears on the leaves as large, purplish black
lesions, on which a white mildew develops in the presence of abundant
moisture. On tubers it appears as slightly sunken, dark-colored blotch-
es, extending into the flesh as a shallow, brownish or reddish decay.
When and where late blight may occur in Indiana: Abnormally
warm wet springs, such as that of 1945, may result in severe late blight
infection of very early potatoes in southern-Indiana. The same may be
true for May-planted potatoes throughout the State if below normal temp-
eratures and excessive rains occur in June and July. Late blight more
frequently occurs on late potatoes throughout the State, when September
and October, prior to frost, are cool and wet.

Severe vine infection may cut the yields of earlier potatoes,
but little tuber infection may take place because of generally higher
temperatures at time of maturity. Severe tuber infection may be expect-
ed in late potatoes, unless the disease is satisfactorily controlled by
spraying or dusting.

Recommended Control Measures
Blight-free seed potatoes: Late blight is commonly shipped in
with certified or other seed potatoes, or it may be present in Indiana
seed potatoes following blight years. It is impossible to detect and
sort out all blight infection from such potatoes, but careful sorting
will eliminate much of the disease and result in planting stock capable
of giving good stands.