Pumpkin Disease

Pumpkin Disease Update - (Rick Latin) - Many growers have called 
expressing concern over an apparent lack of fruit set in their 
pumpkins.  As you know, poor fruit set is not likely due to an 
infectious disease problem.  High daytime and nighttime 
temperatures during most of July may be responsible for the 
production of fewer female blossoms.  Also, high temperatures may 
keep bees from their normal activities and may interfere with the 
pollination process itself.  Finally, if there is any kind of 
fruit set and a stressful period begins, it is possible that young 
fruit will be aborted to save the earlier set.
Be that as it may, it is possible for virus infection to 
contribute to poor fruit set.  If the infection occurs early in 
the development of the fruit, or before fruit are formed, then it 
is likely that the plant will produce only a fraction of the fruit 
it normally would set, and those probably will be of very poor 
quality.  The viruses are very common in the Midwest in late 
summer.  We are all familiar with the mosaic or 2-4-D type of 
symptoms that appear on foliage.  Severe damage is associated with 
early infection and appearance of these symptoms.  If the symptoms 
are just occurring now, then it is not likely that the virus will 
affect your crop.  
The virus most likely survives in perennial plants and weeds in 
wooded areas, along fence rows, and in ditch banks.  I suspect 
that transmission to other weeds occurs in the spring.  Sometime 
during the summer, aphids may enter a field after feeding among 
the weeds, and transmit the virus to pumpkins.  The viruses also 
can be mechanically transmitted.  By that I mean that it can be 
carried and spread with plant sap if a tractor drives through the 
field or a crew of workers damage plants during cultivation.  It 
is my opinion that the aphids may be responsible for introducing 
the virus into the field, but the spread and increase is probably 
due to mechanical transmission.
Powdery mildew apparently got off to a late start this year and 
is not likely to cause much damage across the region.  If you can 
keep it in check for another week or so, then its affect on the 
crop will remain minimal.  I might be concerned about downy mildew 
outbreaks between now and October.  A few cases of downy mildew 
have been discovered.  A wet September might increase our concerns 
and downy mildew management efforts.  Phytophthora blight remains 
a concern, despite the hot dry weather.  I suggest that you 
continue protective sprays, especially if the field has a history 
of the disease or if you have heard of an outbreak nearby.  It is 
probably too late to do anything about it, but the bacterial leaf 
spot pathogen appears to be fairly widespread.  This pathogen also 
attacks young fruit.  Symptoms of fruit infection include raised 
white scabs or blisters on the face of the pumpkin.  This disease 
was identified within the past few years.  We are still in the 
process of learning about how the pathogen survives and spreads, 
and what kinds of conditions prompt serious losses.
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