Preventing Spread of Bacterial Fruit Blotch in Watermelon Transplants

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Now that transplant production is in full swing, it is timely to review what we do and don’t know, about bacterial fruit blotch (BFB) of watermelon. BFB of watermelon is caused by the bacterium Acidovorax avenae subsp. citrulli. Warm, humid conditions in greenhouse transplant houses are highly favorable for the spread of disease and the development of BFB symptoms. Although the disease spreads quickly in the transplant houses, it often is not noticeable in the field until shortly before harvest. There, BFB is damaging because it causes large olive green to brown water-soaked lesions on fruit, making them unmarketable.

Symptoms of BFB on seedlings are water-soaked areas of the lower surface of the cotyledons and inconspicuous lesions on leaves. In the image below, the leaves were incubated and the lesions have progressed along the veins and are obvious. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die. The pathogen also causes disease on muskmelon or cantaloupe, honeydew, and on squash and pumpkin.

There are many steps that can lower the risk of development and spread of BFB on watermelon in the transplant house. All seed in a commercial greenhouse should have been tested and found to have “no evidence” of the pathogen. Don’t grow experimental lots that were not tested in a commercial house. Remember too, that testing, and “no evidence” does not guarantee that BFB will not develop, it is one of many steps to reduce the risk of disease. Inspect seedlings beginning at cotyledon expansion and at frequent intervals afterward. If BFB is suspected, send plants to the University of Maryland or University of Delaware diagnostic lab, or the Lower Eastern Shore Research and Education Center, for identification. Destroy all trays with symptomatic plants and those within a five foot radius. Remove adjoining trays to a separate – isolated – area for observation. Monitor these isolated seedlings daily and destroy trays where symptoms develop. All plants in the greenhouse should be sprayed with copper such as Kocide or Nordox. The applications should continue until the plants are shipped or transplanted to the field.

All the greenhouse surfaces should be sterilized prior to the production cycle. A solution of one part of bleach to nine parts of water, or Greenshield or Physan can be used on implements or benches. Don’t reuse trays.

Additional good practices for greenhouse transplant producers are:
· Workers should wash their hands and use a shoe bath when entering the greenhouse to work.
· Minimize the number of people that enter the greenhouse.
· Eliminate all weeds in and around the house.
· Maintain low humidity in the greenhouse.
· Water plants at their base and avoid splash between plants.
· Keep greenhouse flaps closed if it is windy.
· Segregate seedlots and separate them from each other with a vertical plastic sheet to avoid spread by splash or in aerosols.

Bacterial fruit blotch symptoms after incubation. Note the range of symptoms from small lesions on the true leaves to advanced lesions on the cotyledons.

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