Angular Leaf Spot of Cucurbits

We recently received a pumpkin sample showing marginal chlorosis (yellowing) of the leaves. The combination of symptoms and the presence of the bacteria in the sample were indicative of a bacterial disease. Angular leaf spot is a common bacterial disease of pumpkins and other cucurbits that causes chlorosis and necrosis (death) of leaf tissue.

The bacterium that causes angular leaf spot, Pseudomonas syringae pv. lachrymans, can enter the leaf through the stomates (breathing pores), hydathodes (water-excreting pores on the edge of the leaf), or wounds in the leaf tissue. Symptoms of angular leaf spot usually first appear as water-soaked lesions bound by the veins of the leaf, so they are angular in shape. As they progress, the lesions may turn yellow and then brown and the leaf tissue may tear. Lesions may have yellow haloes. During wet weather, bacteria may ooze from the spots and later dry to a white residue.

Angular leaf spot is favored by rainy weather and temperatures of 75 to 82 degrees F. High nitrogen levels also favor the disease.

Angular leaf spot can be managed by using disease-free seed. Some resistant varieties of cucumber are available. Crop rotation helps to lower inoculum levels. Avoid handling plants when they are wet, and limit overhead irrigation. Copper compounds can be applied preventatively, before symptoms appear, and every 5-10 days during weather that favors disease. Insecticides may also be used to manage insect pests, which may cause wounds that serve as entry points for the bacteria.

Angular leaf spot on pumpkin
Pumpkin leaf affected by angular leaf spot. The causal bacterium has entered the leaf through the hydathodes (pores on the edge of the leaf), causing marginal yellowing. Photo by Dan Carlson.
Page References: 

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Horticulture and Home Pest News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 12, 2006. The information contained within may not be the most current and accurate depending on when it is accessed.