Bacterial Rind Necrosis Of Watermelon
This article by Don Maynard appeared in
“Vegetarian” magazine, 98-05.
OLLOWHEART OF WATERMELON was discussed in the March “Vegetarian.” The focus of this month’s article is on another fruit defect -- bacterial rind necrosis, sometimes simply referred to as rind necrosis.
Rind necrosis was first reported in Hawaii. Typical rind necrosis is characterized by a light brown, dry, and hard discoloration interspersed with lighter areas. The disease develops in the rind and rarely extends into the flesh. Occasionally the affected area is limited to the vascular bundles, but generally the discoloration spreads, sometimes affecting the entire rind. The causal organism was believed to be an Erwinia.
The disease was first reported in the continental United States in several areas of Texas. Fruit from some fields were completely free of the disease, whereas in other fields infected fruit ranged from few to many. Bacterial rind necrosis was often more severe in fields where plants were heavily infested with watermelon mosaic virus. This led to speculation that a common vector may facilitate the spread of both diseases. The causal organism again was narrowed down to an Erwinia species.
Rind necrosis is a serious disease of watermelon that regularly occurs in California’s Imperial Valley. Results from several experiments to determine a causal agent were inconclusive.
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 injection of Erwinia, Pseudomonas, Enterobacter, and Bacillus.
Variation among varieties in susceptibility was reported in Florida and in California. Over a three-year evaluation period in Florida, ‘Sweet Princess’ and ‘Jubilee’ were most tolerant, whereas ‘Klondike Blue Ribbon’ and ‘Louisiana Queen’ were most susceptible. Other varieties were intermediate in susceptibility and not always in the same order. The incidence of bacterial rind necrosis varied from year to year. For example, ‘Sweet Princess’, the most tolerant variety, had 10.0, 14.8, and 21.8 percent affected fruit in 1972, 1973, and 1974, respectively. ‘Klondike Blue Ribbon’, the most susceptible variety, had 39.1, 64.8, and 73.7 percent affected fruit in those years.
Some of these same varieties were evaluated in the Imperial Valley of California. ‘Klondike Blue Ribbon’ was the most tolerant variety and ‘Sweet Princess’ was intermediate in susceptibility. These results are at odds with those obtained in Florida.
What is known about bacterial rind necrosis can be summarized as follows: The disease has been reported from several watermelon-growing areas; Erwinia sp. most often has been associated with the diseased rind areas; other bacterial organisms isolated from either diseased or healthy fruit cause typical symptoms at inoculation sites; the incidence of the disease varies among varieties and growing seasons; and varieties may not always respond to the disease in the same way.
Watermelon bacterial rind necrosis appears to be genetically controlled, but its frequency is affected by an interaction with the environment. 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.