Sclerotinia management: Common questions

July 6, 2016 - Issue 16

Here are a few questions we often get on sclerotinia stem rot management. Answers and links may help with your management decisions.

What conditions favour sclerotinia stem rot? Moisture is the key factor in sclerotinia stem rot risk. Good soil moisture and a few rains in the period starting two weeks before flowering and carrying through to infection after flowering will greatly increase the sclerotinia stem rot risk. Low rainfall and low humidity through these stages will greatly reduce the disease risk, and fungicide applications rarely provide a benefit in these conditions. Temperature can influence severity. Night and morning lows of 15°C are ideal for sclerotinia, especially with heavy dew. Infection may not advance during the heat of the day, but it will grow at night. Infection can be high in hot weather as long as the moisture is there. Anything that increases humidity in the canopy microclimate can also increase the risk. These factors include dense canopies and lodging. Click here for more.

Do I have enough yield to justify fungicide? If conditions favour infection and canola has yield potential of 30 bu./ac. or greater, then a fungicide application at 20-30% flower is warranted. Fungicide may also pay for yield potential below that level.

How do I decide when to spray a variable field? Split applications — two lower rate applications 7 to 14 days apart — can be worthwhile if conditions are good for fungal growth and the crop flowers for a long period. Split applications might also be best for crops with plants at multiple stages. The first application can be made when first plants reach recommended staging. If necessary, a second application can be made when the remaining plants are ready to be sprayed. Not all products are registered for a split application. See the table at the bottom for products that are available for split application or multiple applications.

Can I spray patches only? Of course, as long as you can identify the areas to spray. However, custom applicators may not be interested in working around patches, so this is something you may have to do on your own.

When is it too late to spray? If conditions are dry at early flower and then it rains at 40% to 50% flower, spraying at the end of the window may be effective — especially if branching or strong plant recovery from heat or drought stress extends the flowering period. Applications after 50% flower are not on fungicide labels, and may be inside the pre-harvest interval for some fungicides.

In general, late applications are not as effective as applications at 20% flower since the main stem flowers first, and these flowers are more likely to drop on main leaves and against the main stem. These main stem petals lead to infection that girdles the main stem. Also, earlier infections have more time to develop and cause more damage to the plant. At or after 50% flower, most of the flowering is on side branches. These petals tend to drop onto upper leaves and side branches, causing minimal damage to the main stem. However, if the crop lodges, infection on side branches can spread to main stems. Using spray volume and pressure that drives fungicides down into the canopy may still provide some protection at these stages.

Remember that fungicide does not provide a curative benefit, so any infection present before application will not be stopped. A late application may stop subsequent infection, but infection that is already present can spread throughout the plant and from plant to plant in cases of extreme lodging, reducing the effectiveness of this late application.

Should I spray hailed crop? Canola beat down by hail at early flowering can recover, and start to bolt and re-flower again. In that case, growers may see a benefit from spraying for sclerotinia if yield potential is above 25-30 bushels per acre, and conditions are moist (rain, humidity and/or dew). Keep in mind that some fungicides are registered for only one application per year. If you already sprayed that product before the hail, switch to a different one.

Updated June 23, 2016. Email whetterj@canolacouncil.org if you have further updates.

Updated June 23, 2016. Email whetterj@canolacouncil.org if you have further updates.

A comprehensive article on sclerotinia stem rot.

Canola Watch