Sclerotinia Control

Disease ‘hate’ triangle

Diseases in general follow a common pattern and require the interaction of three separate factors (the three corners of the triangle): a favourable or conducive environment, a susceptible host, and the pathogen (disease causing organism). Independently they are no cause for alarm, but when they occur simultaneously, watch out. Take sclerotinia for example.

Pathogen: Sclerotia are irregular-shaped, hard black fungal resting bodies that can overwinter in the soil and canola residue. In some instances, sclerotia which cause infections have remained dormant in excess of five years before germinating during favourable conditions.

When sclerotia germinate, they produce apothecia (small fruiting structures that resemble tiny golf tee-like mushrooms). These apothecia grow in the soil and eventually release ascospores, (wind-borne spores that can travel up to 1 km) which are responsible for plant infection.

Ascospores do not infect canola plants directly. They must first land on flowers, fallen petals, or existing pollen attached to the stems and leaves. Ascospores metabolize the pollen, thus providing them with the necessary food source to germinate, grow, and infect the plant.

Results: Symptoms develop 3-5 weeks after flowering begins. Pale-grey to white lesions can be seen on stems, branches and pods, at or above the soil line. Sclerotia infections often develop where the leaf and the stem join because the infected petals lodge there. Infected stems appear bleached and tend to shred. Hard black fungal bodies, of varying sizes, develop within the infected stems, branches, or pods. Plants with girdled stems wilt and ripen prematurely. Severely infected crops frequently lodge, shatter at swathing, and make swathing more time consuming. Yield losses can range anywhere from 5 – 100% pending severity.

Best practice makes perfect

Managing sclerotinia in your fields means knowing your disease risk, monitoring the weather, and being on top of agronomic practices that can help keep disease from taking hold in your crop. These practices start with the soil and don’t stop until the disease has been successfully kept at bay.

Rotation is an important part of disease management, but it’s often not as effective with sclerotinia as it is with other diseases. The sclerotia that cause the disease can live for up to five years in the soil, and can affect or be hosted on the major broadleaf crops grown in western Canada. Growers should still carefully consider their rotation because spacing out affected crops will reduce the build-up of the disease inoculum in the soil.

“We’ve seen sclerotinia in almost every broadleaf crop on the Prairies,” says Roger Rotariu, Marketing Manager for Oilseed Fungicides, Insecticides and Seed Treatments with Bayer CropScience Canada. “In the past we had a three to four year rotation away from broadleafs, but that’s not the case anymore. With tighter canola rotations, there’s more sclerotia in the soil and we know it can generally survive until the next broadleaf crop rotation.”

There have been few options in terms of selecting seed for sclerotinia resistance, but that is changing. Breeders are developing seeds which have altered the plant structure making it difficult for sclerotinia to infect the plant. Bayer CropScience has taken a different approach in their seed development and is a year away from the introduction of a new InVigor hybrid with a built-in defence mechanism that protects the canola plant from within.

“We’ve completed one year of trials and on average we saw a significant reduction against sclerotinia when compared to the check,” says Rotariu. “We anticipate when you combine this seed with a fungicide like Proline you’ll be able to reduce your infection rate by over 90%.

Once the seed is in the ground your only option to control sclerotinia is a fungicide application. There are a number of risk assessment tools, weather guides and petal tests available to aid growers in making a spraying decision. Proline alone will control about 80% of sclerotinia stem rot, compared to 65-70% from some of the other sclerotinia products on the market.

Many growers take a proactive approach knowing that if they get the disease before they can see it, they will have better control. Others wait until they see evidence of the disease, and watch the weather for signs of the warm, humid conditions that help the disease flourish. The use of a fungicide when the field is between 20-50 percent bloom and before signs of infection are present is ideal for high-risk crops.

“Today if you have a healthy, thick crop, you have to assume that you are at risk for sclerotinia,” says Rotariu. “If you wait until you see it you’re too late for optimal control. Growers should look at everything – the weather last year, disease in your area, seed choices and how your crop looks this year. Once you have gathered all of that information, which is usually a couple of weeks before flowering, you need to take a leap of faith and treat the crop if you want the best outcome for sclerotinia control.”

Other agronomic decisions include growing a variety with a smaller canopy, or seeding at lower seeding rates. While a less lush canopy may help move the air through the plant better and slow the growth of the disease, a thick canopy is often preferable to growers for other reasons. No-till systems have also been proven to reduce the number of apothecia that are able to produce spores, and also keep the disease out of the band of soil that is closest to the surface.

Following harvest, especially if it has been a year with high disease pressure, growers can choose to apply a soil treatment that will kill overwintering spores in the soil. It takes several months to work, but it is an option to reduce sclerotia prior to the spring season.

The best practices for managing sclerotinia take place before there is any sign of disease. When growers take into account all the factors that can lead to an outbreak, they can make management decisions that help keep sclerotinia from devastating their yield.