Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
This week I have received three watermelon samples that appear to be angular leaf spot. I wrote about this disease on cucumber and squash in the May 31 WCU because it had occurred on zucchini. Angular leaf spot is fairly common on cucumber, squash, and muskmelon, however it is uncommon on watermelon in the field. When I have seen it in Maryland or Delaware in the past, it has been in greenhouses. Past experience with the strains that have appeared here on Delmarva is that conditions after transplanting to the field, which are typically hot and dry, usually do not favor angular leaf spot development. However conditions this year differ. The current cool wet weather is highly favorable for angular leaf spot.
Symptoms of angular leaf spot are a chlorotic halo and may appear “shiny” (due to bacteria on the lesion surface). Small irregular lesions expand and become angular. On watermelons the borders are chlorotic. Older lesions may turn brown, dry and tear to produce a tattered appearance.
The disease may be seedborne. In addition it has a wide host range and can also survive as an epiphyte on several weeds. The pathogen spreads from plant to plant in splashing rain, irrigation, or mechanically (such as on hands, windblown sand, or equipment). There are several bacteria (Pseudomonas viridiflava, P. syringae pv. lachrymans, and possibly others) that cause similar symptoms and vary in their ability to cause damage.
It is important to have the disease identified. The symptoms look similar to anthracnose. However the fungicides used to manage the two diseases are different. If angular leaf spot is confirmed in the field, applications of fixed copper plus mancozeb will minimize spread. Also avoid working field when foliage is wet.
Upper and lower surface of leaf with symptoms of angular leaf spot.
Angular leaf spot. Note the angular tan appearance of lesions, and the “shine” on the cotyledons.