Bacterial Fruit Blotch on Muskmelon and Watermelon

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

There have been a couple of positive confirmations of bacterial fruit blotch over the past two weeks in the region. Scout for the appearance of small water soaked lesions on watermelon or muskmelon leaves. However, be aware that plants can harbor this pathogen and not show symptoms until harvest.

Some research-based information that we have on bacterial fruit blotch (BFB) is that it will only spread from infected transplants to plants during flowering and shortly afterward. In addition, spread of BFB will occur most rapidly under warm, humid conditions and during rainfall or overhead irrigation. When the bacterium is deposited on the watermelon flower, it can penetrate through stomates and infect fruit. The infections that cause fruit loss can only take place during flowering and fruit development before wax deposition (wax seals the stomates). That means that the yield damaging infections occur only during flowering and for about 3 weeks afterward. Although infections occur early in the season, fruit symptoms often do not develop until harvest.

Chemical treatments (i.e. copper) to protect the crop should be applied before and during flowering, and for three weeks afterward. If subsequent harvests are anticipated, those fruit will also need to be protected. Actigard also can be beneficial if applied prior to flowering (see the Mid-Atlantic Commercial Vegetable Recommendations for additional information). I recommend that the workers work in uninfected fields first and any suspect fields last in the day. They should shower and wash their clothes and shoes before going into another field the next morning.

If the pathogen is introduced into the field it will survive for a short time as debris on plastic. If plants are removed and clean transplants are placed in those holes, infection could occur. The soil can remain infested for two years. However, be aware that if volunteer plants are present in the field in the intervening year, the pathogen may survive longer.

A frequent question that I get is how to clean a greenhouse after a BFB outbreak. Discard trays, wash surfaces with a greenhouse cleaner that indicates that it is a bactericide (for example: quaternary ammonium compounds such as Green-Shield®, Physan 20®, and KleenGrow™; hydrogen peroxide & peroxyacetic acid such as Sanidate®; hydrogen dioxide such as ZeroTol® 2.0, Oxidate® 2.0; or chlorine bleach) etc.

Another question is if equipment can carry the pathogen from one field to another. The biggest risk is taking debris across a field on wheels that crush the foliage. To reduce field to field spread, remove any debris stuck on the truck or equipment and wash the truck with soap and water. If you are concerned about the wheels, they can be washed with the greenhouse sanitizers described above.