A new root rot disease of watermelon has been observed sporadically in
commercial fields in central Arizona since 2002. Affected plants were often wilted
and slower growing than healthy plants. Numerous light brown lesions were
observed on all roots. A Rhizoctonia species, initially identified by morphological
characteristics of hyphae in culture, was isolated consistently from the lesions.
Isolates were further identified by DNA sequencing as a species of binucleate
Rhizoctonia sp. Koch’s postulates were fulfilled and confirmed that the isolates
from watermelon were pathogenic, producing lesions similar to those observed
on plants in the field. DNA sequencing analysis of isolates from inoculated plants
were genetically identical to the original isolate. Although this disease has been
documented only occasionally in Arizona, it should be recognized as a potential
problem in early season watermelon.