Downy Mildew Confirmed in Two Ontario Counties

Michael Celetti, Plant Pathologist-Horticulture Crop Program Lead, OMAFRA

Downy mildew was confirmed in at least one field of cucumbers in Elgin County and Kent County over the past week. With the unsettled weather conditions in Ontario over the last couple of weeks which was favourable for infection and disease development, more cases of downy mildew will most likely be reported in Ontario over the next week. However, with the return to warmer drier conditions forecasted for many parts of Ontario over the next few days, disease spread and infections should slow down a bit. Regardless, Michigan State has been trapping downy mildew spores in their cucumber growing regions since the end of June.

The conditions for infection, disease development and spore production were ideal over the past few weeks in many regions of the province.

Spore sacks (sporangia) of downy mildew are observed on the underside of the leaf lesion and appear as tiny black specks.
Spore sacks (sporangia) of downy mildew are observed on the underside of the leaf lesion and appear as tiny black specks.

The disease spreads by very tiny wind-blown spore sacks (sporangia) that look like dark oval or lemon shaped specks on the underside of the leaf lesion. The spore sacks can best be observed on the underside of the leaf lesion with a good hand lens (12-20x) either in the morning or after a period of rain when the humidity is very high. When these spore sacks land on a leaf surface, they germinate at temperatures between 17-21oC, which were the night time temperatures experienced in Ontario over the past week or so. They also require the presence of free moisture as either dew or rain on the leaf surface which has occurred in many fields over the past week or so. If free water is not present and the conditions are not right for germination, the spore sacks remain viable for up to 16 days with the relative humidity between 38-71%. When the spore sacks germinate, they release 8-12 swimming spores which tend to swim towards the breathing pores or stomates on leaves where they form a cyst. The cysts then germinate and infect through the stomates and the pathogen colonizes leaf cells within the vicinity of the stomates for a short period of time before producing more spore sacks. More spore sacks will be produced from the lesion that develops when the relative humidity at the leaf surface is 100% for approximately 6 hours. The bottom line is that it only takes 4 days from the time of infection to the production of more spore sacks when night time temperatures are 15oC and daytime temperatures are 25oC. These were close to the conditions experienced in many fields over the past week.

The disease can spread very quickly and become epidemic within a very short period of time. If left uncontrolled, entire fields can be completely wiped out within a week or two under optimum environmental conditions. The disease reduces leaf area responsible for cucumber fruit initiation and filling and therefore can affect yield significantly. In addition, diseased leaves senesce prematurely and expose any fruit that does develop to sunburn, resulting in further losses to the quality of yield.

All cucurbit growers should keep up to date on the progress of this disease. Growers are advised to apply effective targeted fungicide such as Ranman 400SC/Torrent 400SC, Zampro, Presidio or Tattoo C on a 5-7 day schedule to keep downy mildew at bay. Never apply the same fungicide from the same family back to back in the field. Always follow the label and if possible, apply a fungicide before a rainfall event, dew or prior to overhead irrigation.

More information on downy mildew management in Ontario can be found here: http://www.omafra.gov.on.ca/english/crops/hort/2012-downy-cucumbers.htm.

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