Watermelon Foliar Diseases

Anthony P. Keinath

Research and Extension Vegetable Pathologist, Clemson University

Depending on the weather, several foliar diseases can be found on watermelon in South Carolina. Because anthracnose and Cercospora leaf spot are less common than gummy stem blight, see the chart below to compare diagnostic symptoms of these three diseases. Gummy stem blight has the largest leaf spots and Cercospora has the smallest, whereas anthracnose leaf spots are intermediate. Dry conditions favor powdery mildew, which is described separately below.

Anthracnose of watermelon Cercospora leaf spot of watermelon

Anthracnose of watermelon

Cercospora leaf spot of watermelon

Symptom	Gummy stem blight	Anthracnose	Cercospora leaf spot
Size (diameter) of individual leaf spots	1/2 to 1 inch	1/4 inch	1/8 inch
Color of leaf spots	Dark brown, sometimes with rings inside	Dark brown, center often cracked	Tan or gray centers with dark brown edges and yellow haloes
Shape of leaf spots	Oval or round, usually at edges of leaves	Angular or irregular	Round
Vine spots	Reddish-brown or bleached sections on main vines	1 to 2 in. long, tan centers with reddish- brown edges	Rare
Stem canker	May be present	None	None
Fruit rot	Dark, sunken spot at blossom end, not common	Sunken spots on belly, 1/2 to 2 inches wide, with salmon- pink or orange spores	None
Relative severity	Severe	Moderate	Unimportant

Gummy stem blight and anthracnose can be controlled with the protectant fungicides mancozeb, maneb, or chlorothalonil, or with newer fungicides that are both protectant and systemic (eradicant). Cercospora leaf spot may cause some loss of foliage, but this disease is not serious enough to require fungicides. Details about fungicides are available in the Southeastern U.S. Vegetable Crop Handbook or the Watermelon Spray Guide.

Powdery mildew can infect leaves when the relative humidity is below 50% but disease increases as the relative humidity increases. Powdery mildew spores will not germinate when leaves are wet. The optimum temperature for infection is 81°F. (range 50 to 90°F.). Powdery mildew reproduces and spreads rapidly after a few leaves are infected.

Symptoms of powdery mildew are unlike those listed above for the other three diseases. Symptoms on watermelon also differ from symptoms on other cucurbits, since watermelon is the least susceptible cucurbit. White spots found on the upper leaf surface are the easiest to recognize, although the spots are not as powdery as they are on other cucurbits. If the fungus has infected the lower leaf surface, leaf spots will be yellow and round or irregular on the upper surface with some white powdery
sporulation on the under surface, although fungal growth is not as extensive as on other cucurbits. In an advanced stage, leaves turn pale yellow, bronze, or brown and look papery thin and dry. In this case, sporulation may be seen on the lower leaf surface.

Scout fields to decide if fungicides are needed to control powdery mildew. Check a total of 45 older leaves (leaves on the bottom halves of plants) on the top and bottom surfaces for white, powdery spots. At least 10 plants total should be checked. If any spots are found, begin fungicide sprays. Details about fungicides are available in the Southeastern U.S. Vegetable Crop Handbook or the Watermelon Spray Guide. Mancozeb products do not control powdery mildew.

For additional information about how to control watermelon diseases, contact Dr. Anthony P. Keinath.

This page was created on June 14, 1999. It was last modified on February 25, 2004.