Plectosporium Blight and
New Spray Recommendations
for 2005
Jude Boucher, University of Connecticut

Plectosporium blight, caused by the fungus Plectosporium tabacinum (formerly known as Microdochium tabacinum), is a new destructive disease of cucurbits in New England. This disease was found in Tennessee in 1988 and has since spread rapidly throughout the eastern United States. It occurred on a single farm in Massachusetts in 2000 and on at least a dozen farms in Connecticut and Massachusetts in 2003. In the fall of 2004, after two seasons of rainy weather, it was present in every field I visited from Long Island Sound to Burlington, Vermont.

Plectosporium blight is known to cause damage to a wide variety of cucurbit crops in Europe and Asia, but the strain present in the U.S. seems to primarily damage pumpkins, summer squash, zucchini and a few varieties of gourds. In wet years, which favor disease development and spread, crop losses in no-spray and low-spray fields can range from 50 to100%. Fortunately, this disease is easily recognized and can be effectively managed.

Description and Management
Plectosporium blight is favored by cool, rainy weather.  The fungus can overwinter on crop residue and can persist in the soil for several years.  Plectosporium has not been reported to be seed-borne. Tiny, one or two-celled, sickle-shaped spores are formed in lesions on vines, stems, fruit, leaves and leaf petioles.  Spores are dispersed by wind over long distances.  Lesions are small (<1/4") and white.  On vines, petioles and leaf veins, the lesions tend to be diamond to lens-shaped; on fruit and leaves lesions are usually round (Fig. 1, 2 & 3).  The lesions increase in number and coalesce until most of the vines and leaf petioles turn white and the foliage dies.  Severely infected pumpkin vines become brittle and will shatter if stepped on (Fig. 4).  Early in the infection cycle, foliage tends to collapse in a circular pattern before damage becomes more universal throughout the field.  These circular patterns can be easily detected when viewing an infected field from a distance.  Numerous fruit lesions produce a white russeting on the surface and stems that render the fruit unmarketable (Fig. 5).  Fruit lesions also allow for entry of soft rot pathogens that hasten the destruction of the crop (Fig. 6).


Disease Management Recommendations:

• A three-year crop rotation
  
• Planting in sites with good air circulation to