Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
I recently received an email from a grower that was having problems with watermelon rind necrosis (also known as internal rind necrosis or bacterial rind necrosis). In my career, I have seen this disorder or disease only two times where significant numbers of melons were affected.
Watermelon rind necrosis is characterized by the presence of a corky, red to brown layer of dead tissue that occurs on the inside of the rind of affected fruit but that does not extend into the fruit flesh. Early stages of rind necrosis can be noticed as small discolored water-soaked areas in the rind. Rind necrosis can be found in immature fruit as well as mature fruit.
Presence of a few melons with watermelon rind necrosis (WRN) can cause rejections of the whole load as not saleable. In the US the disorder shows up sporadically but can affect significant acreage (Florida and Georgia have had problems in the past).
Over the years plant pathologists have been able to isolate a number of different bacteria from necrotic areas in the rind and in some literature the disorder is often called bacterial rind necrosis. However, no one bacteria has been identified as the specific causal organism.
Bacteria can be isolated from healthy watermelon rinds and reside there without causing disease. The current theory of how WRN develops is that stressful environmental conditions trigger a hypersensitive response in the fruit rind to resident bacteria and cells near bacterial populations die; however, this has not been verified experimentally. There is some evidence that water stress may be involved and some association with abnormally shaped melons (prominent lobes).
In past “outbreaks” in Florida and Georgia, severity differed by variety. For example, in one area the seedless varieties Gypsy, Melody, and Bold Ruler as well as the seeded pollenizer Sweet Harmony had much higher severity than Crunchy Red (seedless) and Mardi Gras (seeded). Unfortunately, because the disorder cannot be established in controlled trials, the susceptibility of many newer varieties is still unknown. However, when a variety is identified with the problem in breeding trials it is not advanced.
Watermelon Rind Necrosis (variety Fascination)
Closeup of necrotic area in watermelon rind