PPDL Picture of the Week for

May 25, 2015

From Small Lesions

Dan Egel, Vegetable Pathologist, SWPAC, Botany & Plant Pathology, Purdue University

From small lesions, mighty diseases may grow.  The lesion on the watermelon leaf in Figure 1 is small and would be easy to miss. However, a commercial grower brought me this seedling and asked me what the problem might be.  My diagnosis:  bacterial fruit blotch (BFB).  BFB causes leaf lesions on young seedlings as seen here or on leaves of mature plants. But the lesions that watermelon growers worry about are the ones on mature fruit (Figure 2).  

Under warm, moist conditions, the bacteria that cause BFB may spread from leaf to leaf and from leaf to fruit. More details about the biology of bacterial fruit blotch can be found here.  An update to management recommendations for commercial growers may be found here
A lesson to be learned from this story is to have small problems, or potential problems, diagnosed early. The grower who brought me the seedling mentioned above has now located healthy seedlings, which he will plant instead of diseased ones. Contact the Purdue Plant and Pest Diagnostic Laboratory to get an official diagnosis of your plant problem.  

​Click image to enlarge

1 tn 
Figure 1:  This single lesion of bacterial fruit blotch on a watermelon seedling seems small, yet if diagnosed early can save the grower problems later.
2 tn 
Figure 2:  If undetected and untreated, bacterial fruit blotch can cause large unsightly lesions on watermelon fruit.​