Fruit & Vegetable Magazine

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Downy mildew confirmed across southern Ontario

July 12, 2010  By Michael Celetti OMAFRA


Downy mildew confirmed across southern Ontario
Downy Mildew has been confirmed in several fields of cucumbers in Norfolk,
Elgin and Chatham/Kent Counties over the past week.


July 12, 2010, Guelph, Ont.
– Downy Mildew has been confirmed in several fields of cucumbers in Norfolk,
Elgin and Chatham/Kent Counties over the past week.

Some fields are reported
to have a few plants with a few lesions while other fields are reported to have
significantly more disease. The disease was also found in around the Great
Lakes Region of New York, Ohio, Michigan as well as Wisconsin this past week.

The sporadic wet weather
together with warm conditions that occurred in various parts of the province
during the last week of June was ideal for disease development and spread.
Although the hot dry conditions that occurred during the first week of July may
have stalled the disease temporarily, the wet weather during July 9 will
certainly be ideal for sporulation, spread and infection. The Cucumber Downy
Mildew Forecast by North Carolina State University for the Great Lakes Region
is rated as High Risk for local epidemic spread, which means that it is very
likely the disease will spread during the next few days.

Since the disease is
starting to spread around the region, growers are advised to apply Tattoo C
alternating with Ranman on a five to seven day schedule at this time to protect
new growth from becoming infected. Tattoo C contains two fungicides, cymoxanil
and chlorothalonil and should be included in the downy mildew control strategy.
Cymoxanil is a very powerful downy mildew fungicide that moves into leaf tissue
and can kill young but not established infections. Chlorothalonil is a
protectant fungicide that does not move into leaf tissue well. It is also the
main active ingredient in Bravo. Ranman, like Bravo, is a protectant fungicide
but with a different mode of action and good anti-sporulating properties.
Unfortunately it does not move within the plant very well. However, one of the
unique features of Ranman, is that is retained in the wax layer of the cucumber
leaf and therefore not easily washed off of the leaf during a rain event. It
also has good residual properties. Although Bravo and Ranman work well
protecting tissue from becoming infected with downy mildew spores, good
protection will only occur when applied with good coverage and canopy
penetration. Once the canopy of the cucumber crop becomes closed, good
penetration into the canopy with any fungicide is required to keep the disease
under control.

Since downy mildew is
spreading around the province, growers should keep up to date on the progress
of this disease. Successful management of this disease is accomplished when
fungicides are applied prior to the disease infecting the crop so it is
imperative that growers apply a fungicide to prevent the establishment of downy
mildew in their crops on five to seven day schedule. If possible, growers are
advised to apply a fungicide before a rainfall event, dew or prior to overhead

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