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 irrigation.