Wooster, Ohio
June 27, 2007
Downy mildew has been found on
cucumber farms in northern Ohio, and
Ohio State University vegetable
experts are asking growers to take measures now to prevent
damage from this potentially devastating disease.
Sally Miller, a plant pathologist with the Ohio Agricultural
Research and Development Center (OARDC) and a vegetable-crops
specialist with OSU Extension, said the fungal disease was
confirmed in single fields in Medina County (June 21) and Erie
County (June 25). Neither field had been treated with
fungicides.
The Ohio outbreaks follow a report of the disease in
southwestern Ontario, Canada, on June 8.
“We don’t know the source of the downy mildew spores in the Ohio
outbreaks, but given the reports so far and the prevailing
winds, growers with cucumber fields south and east of
southwestern Ontario, as well as those near confirmed outbreaks
in Ohio, should be particularly vigilant,” Miller said.
Miller is recommending that all cucumber growers in the northern
part of Ohio step up scouting of their fields and protect their
crop with fungicides. She said fungicides should be applied on a
7- to 10-day schedule under hot and dry weather conditions, and
on a shorter schedule under cool, moist conditions.
Recommended products include Gavel (five-day pre-harvest
interval, or PHI), Previcur Flex (two-day PHI), Tanos (three-day
PHI) and Ranman (no PHI). These protectant fungicides must be
tank-mixed with either Bravo (no PHI) or Dithane (five-day PHI).
It’s also important to alternate these products to avoid
development of fungicide resistance.
Organic growers have limited fungicide options available, but
those in high-risk areas should apply OMRI-approved,
copper-based fungicides to their cucumber crop on a weekly basis
when weather conditions are favorable for the disease.
Last year, downy mildew cost cucumber farmers in Ohio and
Michigan millions of dollars in fungicide application costs and
crop losses.
In North America, downy mildew spores overwinter only in Mexico
and the southernmost tier of the United States. The fungus can
be carried northward by winds or storms, but Miller said a
possible source of the current Ohio outbreaks may actually be
greenhouse production in Canada.
“The weather in Ohio has been largely warm and dry this growing
season,” Miller pointed out. “But Wayne and Medina counties have
received approximately once-weekly thunderstorms, and the field
in Huron County received about one inch of rain around June 4
and 1.4 inches on June 20. This may have created favorable
conditions for movement of downy mildew spores and crop
infection.”
Cucumber is the most susceptible of all cucurbit crops to downy
mildew. But in the past, the disease has been spotted on squash
and pumpkins several weeks after its appearance on cucumbers.
Growers, Miller added, should also step up scouting of pumpkin
and squash fields at this time and apply protectant fungicides
if weather conditions are favorable, particularly if the disease
has been reported in nearby cucumbers.
Caused by the fungal organism Pseudoperonospora cubensis, downy
mildew first appears as pale green areas on the upper leaf
surfaces. These change to yellow angular spots. A fine
white-to-grayish downy growth soon appears on the lower leaf
surface. Infected leaves generally die but may remain erect
while the edges of the leaf blades curl inward.
For up-to-date information about downy mildew, log on to
http://www.ag.ohio-state.edu/~vegnet; contact Miller at
(330) 263-3678,
miller.769@osu.edu or contact your county OSU Extension
office.
OARDC and OSU Extension are the research and outreach arms,
respectively, of Ohio State’s College of Food, Agricultural, and
Environmental Sciences. |
|
