Pest Of The Month: Bacterial Rind Necrosis Of Watermelon
Typical rind necrosis is characterized by a light brown, dry, and hard discoloration interspersed with lighter areas. These areas enlarge and may merge to form extensive necrotic areas. The disease develops in the rind and rarely extends into the flesh of infected melons in the field. Occasionally, the affected area is limited to the vascular bundles, but generally the discoloration spreads, sometimes affecting the entire rind. The disease occurs sporadically and is thought to be caused by bacteria that are naturally present in fruit, but the reasons for symptom development are not understood.
The cause of rind necrosis is not completely understood at this point. Results from several experiments to determine a causal agent were inconclusive. Bacterial infection has been implicated in several studies that have suggested an Erwinia species to be the causal organism. In other experiments, the diversity of bacterial flora isolated from healthy and diseased fruit was similar, except that enterobacteria were isolated more frequently from diseased than from healthy fruit. Rind necrosis occurred at inoculation sites following the injection of Erwinia, Pseudomonas, Enterobacter, and Bacillus. Drought stress also is reported to predispose melons to rind necrosis.
Survival And Spread
The means of dissemination of the pathogen is not known, nor is the etiology of the disease well understood, but apparently infection is limited to the fruits. The disease was first reported in the continental U.S. in several areas of Texas in 1968. Since then, it has been reported in most major watermelon growing areas.
Watermelon bacterial rind necrosis appears to be genetically controlled, but its frequency is affected by an interaction with the environment. Variation among varieties in susceptibility was reported in Florida and in California. The most resistant cultivars in studies conducted in Florida over a three-year period during the 1970s were Sweet Princess and Jubilee, while the most susceptible were Klondike Blue Ribbon and Louisiana Queen.
For the most part, varieties in use today appear not to be as susceptible as those used in the 1970s, suggesting that watermelon breeders have been successful in selecting against the disease. Since usually there are no external symptoms, a small percentage of diseased melons could make the crop unmarketable, as diseased melons are not easily culled. Although, experienced pickers can often detect affected melons by the subtle knobbiness that is visible on the surface of affected melons.
As of presstime, there are no known control measures.