Time to assess sclerotinia risk in canola

Now is the right time for canola growers to assess their crops for susceptibility to sclerotinia.

Fields considered at high risk should ideally be sprayed with a fungicide at 30 per cent bloom. Where germination was uneven due to the cool, dry spring, flowering will be uneven too. Under those conditions farmers should consider a split fungicide application, said Derwyn Hammond, the Canola Council of Canada’s agronomy specialist based at Brandon, Man.

“With a split application a farmer would spray at 20 per cent bloom and then re-assess to see if a second application is required,” he said in an interview July 2. If conditions turn dry and hot a second application might not be necessary.

Sclerotinia, a fungus disease, needs moisture at several stages before it can successfully infect canola. The process begins with sclerotinia — little black fungal bodies that can survive in the soil for up to four years. They germinate in the summer, producing apothecia — tiny mushrooms that can release millions of spores that can travel at least a mile by wind to infect a canola plant. Germination requires moist soil (at least 10 days at or near field capacity, according to Manitoba Agriculture, Food and Rural Initiatives) and temperatures of 15° to 25°C before and during canola flowering. Each sclerotium can produce up to 15 apothecia at one time or over a period of weeks, according to the Canola Council’s web site. Spores can survive 21 days.

Apothecia are typically five to 15 mm in diameter and look like tiny golf tees or gramophone horns.

If apothecia are found in a canola field or nearby fields of any crop, there’s potential for infection under the right environmental conditions. Dense crops are more at risk than thin ones.

Spores cannot infect the plant directly. They land on canola flower petals, which provide the food needed to germinate, grow and eventually penetrate the plant. The infected leaves fall and some land in the leaf axils or at points of stem branching where the airborne or petal-borne spores land and droplets of water can be frequently found.

Under dry, hot conditions infections might not occur. But to be effective, fungicides must be applied before the infection spreads from the fall petals to the main plant occurs. Farmer must make the decision to spray or not before knowing whether the right conditions will exist.

Given relatively high canola prices, Hammond said many farmers will opt to spray and protect their crop if they find lots of apothecia nearby.

Winter wheat and early barley fields that have been planted to canola, sunflowers or pulse crops during the previous couple of years are good places to look for apothecia. Those crops provide a canopy and the moist conditions the sclerotia need to germinate.

Fungicides work best when applied to most of the canola petals before they fall. Hammond said 30 per cent bloom occurs when 20 flowers are open on the main stem of a canola plant or when there are one or two flowers on a secondary stem.

— Allan Dawson is a reporter for the Manitoba Co-operator.

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