Bacterial Fruit Blotch Epidemiology

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

I have continued to receive questions about bacterial fruit blotch (BFB), including how it spreads through a transplant house and the production field during the growing season. To understand BFB it is important to understand a little about its’ epidemiology.

Infected transplants are the most common source of BFB inoculum in Delmarva fields. (However, it can overwinter on debris and on infected volunteer plants). The reason that transplants remain a major source of inoculum is that 100% detection of infested seed is not possible. Many steps are taken by seed companies and transplant growers to avoid infestation, detect infection, and eliminate the disease. However, currently we don’t have the technology to accomplish this.

The environmental conditions in watermelon transplant production houses are highly conducive to disease development and spread of BFB. High temperatures, high humidity, overhead irrigation and high plant populations favor BFB and result in rapid symptom development. As a result, detection of BFB in transplant production is common.

BFB spreads from plant to plant on hands or equipment, in splashing water (irrigation or rain), or in aerosols. Once it lands on a plant it enters (infects) through wounds or stomates.

In commercial fields, 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.

Low humidity in watermelon fields prevents the development of both foliar and fruit symptoms. In fact infested seed can be produced from completely symptomless plants. Simply put, infected plants can appear symptomless. (This is one reason why, during seed production, infections cannot be completely eliminated based on symptoms).