Purdue University 
department of Botany and Plant Pathology Lilly Hall of Life Sciences
Plant Disease Control
BP-8-2 May 1974

MAJOR BLIGHTS OF MELONS AND CUCUMBERS IN INDIANA

Walter R. Stevenson, Donald H. Scott, Paul C. Pecknold Extension Plant Pathologists

Growing conditions in Indiana are often ideal for one or more of five distinct blights of cantaloupes, watermelons and cucumbers. Any of these five blights may lie found in Indiana during a portion of the season or in some cases throughout the season, when seasonal climatic conditions include warm, humid weather with excessive rainfall and high relative humidity.
Good control of these blights can generally be achieved through repeated applications of a suitable fungicide. A sixth disease, bacterial wilt, depends upon cucumber beetles for its spread, and control of this disease is accomplished by applications of insecticides.

Alternaria Leaf Spot (Rust) caused by the fungus Alternaria cucumerina is commonly observed on melons and cucumbers grown in sandy or low organic soils deficient in nitrogen. High humidity and frequent rainfall favor the development of this foliar disease. Small tan, often watersoaked spots first appear on older or crown leaves sear the center of the hill. Under warm, humid conditions, spots appear on younger leaves at the tips of vines. Spots enlarge rapidly to form dark brown round to irregular lesions 1/8 inch or more in diameter.
Lesions may coalesce, killing large areas of leaf tissue. Severely infected leaves curl, dry, and lall prematurely. Vines may be completely defoliated at harvest. Melons ripen more rapidly and are smaller in size and of lower quality when plants are defoliated. Fruit infection results in circular sunken lesions which are often covered with a dark olive green or black mold.

Angular Leaf Spot caused by the bacterium Pseudomonas lachrymans can severely injure slicing and pickling cucumbers during periods of frequent rainfall. Although most serious on cucumbers, Zuccini squash, and honeydew melon, angular leaf spot will infect most other cucurbits. First symptoms of infection are small angular water-soaked lesions on infected leaves. The lesions are generally angular because of confinement between small leaf veins. Lesions are tan on their upper surfaces and gummy or shiny underneath. In the early morning when dew is still present, tear shaped droplets of bacterial ooze are commonly observed on the underside of infected leaves. Infected leaf tissue may dry and tear away from healthy tissue giving the leaf a tattered appearance.

Circular water-soaked lesions may be

Cooperative Extension Work in Agriculture and Home Economics State of Indiana Purdue University and U. S. Department of
Agriculture Cooperating H. G Diesslin Director West Lafayette Ind. issued in furtherance of the Acts of May 8 and June 30, 1914.
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