Sclerotinia is one of the most widespread diseases in canola. Farmers need to use a number of strategies to mitigate its impact. Management starts with cultural controls such as rotation and includes the use of fungicides, biological controls and the use of partially resistant varieties. Not all options are terribly effective and need to be combined for maximum effectiveness.
Management begins with understanding why sclerotinia is so widespread in the first place. It has a wide host range, with many broadleaf plants and crops being susceptible to it including canola, mustard, pulse crops, soybeans, carrots, sunflowers and even weeds like wild mustard, shepherd’s purse, stinkweed and thistles.
Secondly, when we look at the life cycle of sclerotinia, illustrated in Figure 1, we see that the spread of sclerotinia in canola is from many, many spores that originate on the soil surface and land on the petals of the canola. These infected petals then fall off the flower and land in the leaf axils of the plant (where the leaf joins to the stem). The spores use the petals as their initial food source and that’s when they infect the plant.
This is why when we look at one of the potential cultural controls such as rotation, its impact on the incidence of sclerotinia in canola is quite low. Even though we may have a good break between consecutive canola crops in any given field, the widespread use of other susceptible broadleaf crops in rotation and weeds both within our own fields and also adjacent fields mean that there is usually a generous supply of inoculum available to infect our canola fields when the conditions are right. Therefore, even though rotation is often mentioned as a control measure for sclerotinia in canola, its role is usually relatively minor in the traditional canola areas of Western Canada.
A second potential control measure is with the use of a biological control such as Contans. Contans, distributed by UAP Canada, is the commercial name of a biological control product that contains a fungus called Coniothyrium minitans.This fungus destroys the resting sclerotia bodies present in the soil. This product has been used for many years in Europe and has been shown to reduce the level of sclerotia bodies in the soil and therefore reduce the level of sclerotinia infection that occurs within the field that it is applied in.
There are however several concerns with its application methods and potential effectiveness. First, according to the label, the product should be applied approximately three months before the infection is going to occur. That means, practically speaking, that for most of Western Canada, that the product will need to be applied in the previous fall. Second, the product requires some incorporation within the top five to 10 cm of soil. That means, in most cases, using tillage, which will not be a popular option for those practising zero tillage. And although the product may end up significantly reducing the sclerotinia infection within the field that it is applied in, it will not control the spores that are produced from neighbouring fields, especially those that are situated upwind. So it’s a tool that can reduce the level of infection within a field, but will not reduce the risk from neighbouring fields.
Farmers do have accese to newer varieties from Pioneer Hi-Bred, such as 45S51 and 45S52, that offer some moderate resistance to sclerotinia. On a scale of one to nine, where one is highly susceptible and nine is highly resistant, these varieties are rated by their developer as between five to 5.5. That means, that under low to moderate sclerotinia pressure, these varieties will offer a level of resistance to the development of sclerotinia, such that the need for a fungicide application may be eliminated. However, under high sclerotinia pressure, these varieties can still develop significant amounts of the disease such that the application of a fungicide would still be economically warranted.
Resistant varieties could be a good option for growers in areas where sclerotinia is not regularly a significant problem or where disease pressures are usually only moderate. The use of one of these varieties may offer farmers enough protection that they wouldn’t need to look at a fungicide application unless disease pressures were high. They may also be useful under high-pressure situations where farmers might potentially be looking at split fungicide applications and the use of a partially resistant variety could save them one application.
And finally, farmers have access to chemical fungicides. These products were the first to be developed to help in sclerotinia control and remain one of the better control options. However, they remain relatively expensive and may need to be applied more than once under heavy and prolonged disease pressure.
The main risk with a fungicide application is to properly assess whether sclerotinia pressure is significant enough where fungicide application would be economic. That’s the biggest problem with this disease. Whatever control measures you look at, you need to apply them in advance of actually seeing any disease on the plants. So the biggest challenge remains to properly assess the relative risk of infection and the potential value of the application. The more favourable the weather conditions are for development of the disease and the more favourable the crop conditions are (heavy, lush canopy that remains wet for many hours), the better the chances for a greater economic response to the fungicide application.
Farmers in many areas are looking at fungicide application for sclerotinia as insurance. That is, if they have a heavy crop with good-excellent yield potential and conditions that are suitable for disease development, then they more or less routinely apply fungicide to protect that yield potential.
To sum up, sclerotinia is one of the big diseases that concern us in canola production. There are a number of methods that can be used to reduce its yield damaging effects on our crops. There is no one best solution for all growers. However, after understanding the strengths and limitation of each of the available control strategies, growers can use one or more of the strategies to help them in controlling this disease on their farms and help them in producing a healthy bountiful and profitable canola crop.
JohnMaykoisasenioragri-coachwith Agri-TrendAgrologyandoffersagri-coaching servicesintheMundare,Alta.,areawherehe alsofarmswithhisfamily
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The Sclerotinia Disease Cycle
The sclerotinia fungus,Sclerotinia sclerotiorum,produces sclerotia in the stems of infected plants. These sclerotia sometimes look similar to ergot bodies or mouse feces. Sclerotia fall to the soil at harvest and can survive on or in the soil for several years.
If the soil is moist (at or close to field capacity) for 10 to 14 days, the sclerotia may germinate to produce tiny mushroom-like fruiting bodies that resemble golf tees called apothecia. These apothecia can produce millions of airborne spores and are produced not only in canola fields, but also in fields of other crops that grew susceptible crops or weeds in the past. The spores escape from the canopy and may be blown by winds to nearby fields.
The spores do not infect healthy green plant tissue, but need a food source. As canola petals die and fall onto lower portions of the plant, any spores on the petals may germinate and begin to grow, especially when the canopy stays wet for long periods of time. Once growth is established on the dropped petals, infection continues into the surrounding tissues. Infections may continue to spread as long as the canopy remains wet for many hours. Sclerotinia development may stop in dry weather, but it can resume once wet weather returns.
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SCLEROTINIA STEM ROT CHECKLIST
(for each factor, circle the risk points that apply to your field)
NUMBER OF YEARS SINCE LAST CANOLA CROP
DISEASE INCIDENCE IN LAST HOST CROP
RAIN IN THE LAST TWO WEEKS
REGIONAL RISK FOR
— Source: Canola Council of Canada
More than 6 years
3 –6 years
1 –2 years
Low (1-10%) Moderate (11-30%)
Less than 10 mm
10 –30 mm
More than 30 mm
To help farmers assess the risk of their canola crops to sclerotinia infection, the Canola Council of Canada has published a Canola Disease Scouting &Risk Assessment Card that is available on the web at: https://canola-council.merchantsecure.com/canola_ resources/product11.aspx. Using experience from Sweden, where this model was developed, it’s recommended that when the risk score totaled 40 or more, there was a good chance of an economic response to fungicide application.
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