Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
The cool nighttime temperatures and overcast and rainy days are holding back growth of watermelon and cantaloupe transplants that have been planted in the field. Although I’ve received several watermelon transplants for disease diagnosis, most of the damage on the samples is weather related – not disease. However, continue to carefully examine transplant lots going to the field.
Gummy stem blight remains the most common seedling disease in our production region. Look for infected necrotic cotyledons and water-soaked lesions extending from the cotyledons into the stem. As the disease advances, stem lesions will become tan and small pycnidia can be seen with a hand lens.
Gummy stem blight in tray. Note the necrotic cotyledons, tan lesion on stem and the tiny black spots, which are the fruiting bodies of the pathogen.
I have seen one case of suspect angular leaf spot (ALS) on cantaloupe this spring. This bacterial disease occurred in Delmarva’s greenhouses in 2003. Symptoms are small dark brown irregular lesions on cotyledons or leaves. Lesions may have a chlorotic halo and may appear “shiny” (due to bacteria on the lesion surface). ALS is favored by cool, wet weather. 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. Past experience with the strains that have appeared here on Delmarva is that conditions after transplanting to the field do not favor ALS disease development. However, it is important to have the disease identified. If ALS is confirmed in the field applications of fixed copper plus mancozeb will minimize spread of disease. Also, avoid working fields when foliage is wet.
Angular leaf spot. Note the angular tan appearance of lesions, and the “shine” on the cotyledons.