Sclerotinia stem rot

Last Update: Friday, July 21, 2017 9:22:08 AM

Table of contents

    Tips for Best Management 

    • The fungus Sclerotinia sclerotiorum occurs in all the canola growing areas of Canada, causing stem rot, which is one of the most destructive diseases of canola. 
    • Sclerotinia stem rot of canola is extremely variable in occurrence and severity from year-to-year, region-to-region and field-to-field. 
    • Foliar fungicides remain the main control strategy. Several tools are available to help growers assess risk and plan for fungicide application. 
    • Varieties with some tolerance to sclerotinia stem rot are available and efforts continue to improve varietal resistance, in both the public and private sectors.


    About Sclerotinia Stem Rot

    Overview

    The fungus Sclerotinia sclerotiorum, which occurs in all the canola growing areas of Canada, causes stem rot of canola. Sclerotinia stem rot, or white mould as it is sometimes called, is one of the most destructive diseases of canola. The severity of sclerotinia stem rot is extremely variable from year to year, region-to-region and even from field to field. Sclerotinia has become more serious as canola production has increased, likely due to a combination of more acres of canola in rotations and management practices that contribute to high yields, but also produce dense canopies, which are a better microclimate for disease development. Wet weather has also favored disease development in recent years. [1]

    In 2010, sclerotinia was fairly widespread in canola across much of western Canada, with up to 90% of crops surveyed in some areas showing symptoms of infection. In all provinces, the mean incidence of the worst affected fields was between 25% and 27%, with the mean sclerotinia severity rating ranging from a low of 2 to a high of 3, which indicates the disease will likely affect at least one-quarter to half of seed formation and filling on the plants. The 0 to 5 rating scale starts with 0 having no symptoms to 5, where affected plants have a main stem lesion towards the lower part of the plant and the potential to affect seed formation and filling for the entire plant.

    A sclerotinia stem rot lesion starts on the stem.

    Photo by Justine Cornelsen

    Plants are infected with sclerotinia when the canola crop is in bloom. Variation in the disease incidence (percentage of plants infected) and severity among fields not treated with fungicide is due to:

    • differences in the quantity of infectious spores
    • crop density
    • crop height and vigour
    • severity of lodging
    • rainfall/humidity/leaf wetness
    • soil moisture
    • temperature
    • crop canopy/plant architecture
    • timing of flowering in relation to favourable environmental conditions for sclerotial germination and/or plant infection
    • genetic tolerance of some varieties 


    With the right combination of spore load, crop density and weather conditions or irrigation, heavy infections can develop almost anywhere. Even after plants are infected, the severity of stem rot symptoms and the resulting effect on yield will vary according to where the infections occur, crop density, lodging potential and especially the stage of crop growth at the time of infection. Infections of lower main stem or branch tissue initiated soon after early bloom will result in more severe symptoms. 

    Yield losses reflect reduced seed filling per infected plant, the amount of preharvest shattering, and the percentage of infected plants in a crop. In general, when conditions for the disease are favourable and infections are initiated during early flowering, yield reduction per infected plant can equal 50% or more. Yield losses can be attributed to fungal growth within stems and resulting lodging, which leads to some or all of the following:

    • smaller and fewer seeds
    • premature ripening
    • shattered pods
    • loss of smaller, shrunken seeds during combining


    Sclerotinia also affects over 400 other plant species including other broadleaf crops such as mustard, sunflowers, beans, lentils, peas and others as well as most broadleaf weeds, such as chickweed, stinkweed, hemp-nettle, thistles, shepherd's purse, narrow-leafed hawk's-beard, false ragweed, wild mustard and others.  

    Disease Cycle

    Figure 19: Sclerotinia Disease Cycle

    Spore-producing apothecia

    Sclerotinia trapped inside undecomposed stubble

    The sclerotinia disease cycle is shown in Figure 19.

    The stem rot fungus overwinters as sclerotia in the soil, in stubble at the soil surface and mixed with seed. Sclerotia are compact masses of hyphae that can remain viable in the field for five years or more. The level of sclerotia in the soil will largely reflect the crop rotations used and will increase under short rotation canola and where susceptible pulse or forage crops are grown or where significant broad-leafed weed populations occur, especially in wetter regions of the prairies. Where levels of rainfall are reduced, conditions are likely not conducive for development of the disease and production of sclerotia. Each year some sclerotia will germinate when conditions are suitable but others will remain dormant. Germination produces either mycelium (microscopic filaments), which may infect plants, especially root tissues, in direct contact with sclerotia, or spore-producing apothecia (small golf-tee shaped structures).

    Nearly all infections in canola result from airborne spores produced by apothecia at the soil surface. However, for sclerotia to germinate and produce apothecia, they require prolonged moist soil conditions (at least 10 days above the wilting point) and moderate temperatures of 15 to 25°C. Normally, such conditions do not occur until the crop canopy closes and permanently shades the soil surface. Since this is typically at the late rosette stage, with the 10- day delay, apothecia appear as flowering starts. Only sclerotia in the top few centimetres of soil will produce functional apothecia, since the apothecial stalks are rarely longer than 5 cm (2"). Deeply buried sclerotia will not produce apothecia but can remain dormant. If brought near the surface by cultivation, they may germinate.

    A single sclerotium can produce up to 15 apothecia, either at one time or over a period of weeks. Apothecia only grow from sclerotia and not from any plant tissue or residue in the soil. Occasionally apothecia can emerge from inside plant tissue from sclerotia trapped inside undecomposed stubble.

    In western Canada, apothecia normally begin to appear in June but most develop during flowering. Apothecia can continue to develop until late September but the critical period for causing damaging infections is from early to full bloom. Research has shown that plant infection rarely developed before the plants were in the mid-flowering stage and that apothecia rarely appeared in the field before plants were in bud.

    Apothecia are typically 5 to 15 mm (0.2 to 0.6") in diameter. The upper end of this range is about the size of a small fingernail or slightly smaller than a dime. Apothecia produce millions of microscopic spores that are released into moving air currents. Even the lightest breeze easily carries the spores across a field or into adjacent fields, possibly as far as several kilometres (miles). Honeybees can also carry spores.

    Spores normally cannot infect the leaves and stems directly, they must first grow in dead petals or other organic material adhering to leaves and stems. The petals provide the food source necessary for the spores to germinate and produce the hyphae that grow and eventually penetrate the plant. Moist conditions from rainfall or heavy dew, which may keep leaves and stems wet for two to three days, are also necessary for infection. Radiation, relative humidity and temperature influence ascospore survival.

    Ascospores are fairly short-lived and only remain viable for up to 4 to 5 days. Research has found that the average ascospore survival was < 50% after 2 days and <1% after 3 days of field exposure. Other factors such as exposure to radiation, higher temperatures (>25C) and high relative humidity (>35%) can result in more rapid loss of ascospore viability after 1 to 2 days on canola petals.  Ascospores released and deposited on petals as the crop is coming into full bloom create the highest risk of infection, depending on weather and environmental factors. [2],[15],[16],[17],[18]

    When leaves and leaf axils or bases are infected the fungus can spread down into the stem. The fungus grows and invades healthy stem tissue when conditions are favourable. A dense canopy provides higher humidity and better conditions for disease development.

    Heavy stands tend to lodge, and stem rot will spread from plant to plant by direct contact, especially if wet weather delays swathing. Spread will also occur in wet swaths. The fungus eventually forms new sclerotia in diseased plants that are returned to the soil at harvest, completing the disease cycle.

    Influence of environment

    Rainfall and soil moisture are necessary for sclerotia germination, spore production, and spore germination and growth. Frequently water from heavy dews dripping off the plant is enough moisture for sclerotia germination. Ideal canola growing weather is also ideal for sclerotinia.

    Lesion development is favoured by humid conditions and temperatures between 20 and 25 C.  Dry, warm conditions will impede or stop infection and lesion development, but established lesions may resume growth when favourable conditions return.


    Identify Sclerotinia Stem Rot

    Symptoms

    Girdled stems

    Sclerotinia lesions on canola stem

    Sclerotinia lesions on canola root

    Sclerotinia inside canola stem

    Sclerotinia stem rot develops late in the season, with the first visual symptoms appearing by the end of flowering. Two to three weeks after infection, soft watery lesions or areas of very light brown discolouration become obvious on the leaves, main stems and branches. Lesions expand, become greyish white, and may have faint concentric markings. Plants with girdled stems wilt, ripen prematurely and become conspicuously straw-coloured in a crop that is otherwise still green.

    The stems of infected plants eventually bleach, taking on a whitish appearance and infected tissues tend to shred and shatter very easily. Infected plants may produce fewer pods per plant, fewer seeds per pod or small, shrivelled seeds that blow out the back of the combine. The extent of damage depends on whether the main stems or branches are infected and at what stage during flowering infection occurs. Severely infected crops frequently lodge, shatter at swathing and are difficult to swath.

    When the bleached stems of diseased plants are split open, a white mouldy growth and hard, black resting bodies (sclerotia) are visible. Sclerotia vary in size and shape. They may be small and round like a canola seed or up to 2 cm (0.8") long and cylindrical, ovoid or irregular in shape. Under moist conditions, sclerotia and the white mouldy growth may also occur on the surface of infected areas of the plant. At harvest the sclerotia are either threshed out with the seed or left in the field.


    Sclerotinia Forecasting and Risk Assessment

    Forecasting systems have been developed for stem rot in canola that use either petal testing, a checklist or other tools based on environmental conditions. While no forecast system is 100% accurate they do provide practical direction in making a decision whether a fungicide application is warranted.

    Factors Involved in Sclerotinia Forecasting

    Many factors influence a forecasting system and its ability to predict the actual incidence of disease. Most predictive models evaluate several environmental and crop variables such as:

    • field cropping history
    • field disease history
    • apothecia presence
    • rainfall
    • soil moisture
    • weather forecast
    • canopy density


    Other important variables affecting the incidence of the disease include:

    • changing inoculum levels during flowering
    • heat units
    • daily and weather related inoculum fluctuations
    • light penetration
    • leaf area index
    • crop height
    • leaf wetness


    Field and nearby field cropping and disease history are an indirect means of measuring the potential for presence of spores. While apothecia produced from sclerotia within the field are considered the main source of spores, spores produced in nearby fields and blown into the crop can also be important in disease development. [3],[4],[5]

    Research has shown that most ascospores are deposited within 100 meters of the source, and therefore adjacent fields can be an important source of inoculum. Fields with sclerotia at distances of greater than 500 metres to 2 km would likely represent a minimal source of inoculum, as most ascospores would have been deposited within 100 to 400 m of the apothecia from which they were produced. [19],[20],[21]

    Scouting for Apothecia

    The presence of apothecia is a good indicator of the potential for spore production. Make sure what you see are apothecia of the sclerotinia stem rot fungus. They are tan or honey-coloured, 5 to 15 mm across, and tops are cupped like a golf tee. They will be growing from sclerotia in the soil or decaying canola.

    There are other fungal structures or mushrooms produced by saprophytic fungi, meaning that they survive on decaying crop residue that can occur in similar conditions but are not apothecia. The spores produced from these fungi do not contribute to increased sclerotinia risk. Above the soil surface the stalks of apothecia tend to be short, so mushrooms with stalks that extend above the soil surface (e.g. 25 mm or more) are unlikely to be apothecia of sclerotinia. Mushrooms with rounded tops or bright colours are not likely apothecia of the sclerotinia fungus.

    Canola Disease Scouting & Risk Assessment Card

    However, scouting for apothecia may be quite difficult, as sclerotia are not found in high numbers in most infested soils. Therefore, the sampling site or the individual carrying out the sampling may influence accurate estimates of apothecia numbers. Also each apothecium produces large numbers of spores so relatively few apothecia are needed to cause a localized high level of disease. Sclerotia and thus apothecia may also be highly aggregated in an individual field and therefore may be missed during field scouting activities.

    Scout low wet (but not flooded) spots in the field that provide better conditions for sclerotia to germinate, nearby fields that had canola or field peas with some level of sclerotinia infection in the past two to three years and/or other dense canola or other crops. Scout any crops, including heavy cereal stands that are dense enough to produce conditions favourable for sclerotia germination and with a history of sclerotinia in the past. Low wet spots and a dense canopy protect the sensitive apothecia from desiccation and allow them to release infectious spores over a longer period. There is the potential in wet years that sclerotia may not remain viable in saturated waterlogged areas, however the remainder of the field can still have viable sclerotia and produce ascospores and infection.

    Weather

    Rainfall and soil moisture are necessary for sclerotia germination, spore production, and spore germination and growth. Ideal canola growing weather is also ideal for sclerotinia. Soil moisture indicates sclerotia germination and, therefore, the potential for ascospore production rather than infection and disease development. Frequently water from heavy dews dripping off the plant is enough moisture for sclerotia germination.

    Weather forecasts can improve the reliability of the disease forecast because sudden weather changes can cause infestations to occur unexpectedly or high-risk fields may show limited disease development. Hot, dry weather can greatly reduce the risk of sclerotinia infection.

    Sclerotinia Stem Rot Checklist

    Sclerotinia incidence can vary greatly among fields and years, making scheduled routine spraying of fungicides unprofitable. However, when sclerotinia risk is high, preventative fungicide applications can effectively lower disease severity and improve yield. Assessment of disease risk within each field is essential to improve the odds that fungicides are only applied when it is economical to do so. [6],[7],[8]

    Figure 25. Frequency and Distribution - Fields with 25% Infected Plants

    Determining whether or not the risk of sclerotinia warrants a fungicide application is a very difficult decision. This decision depends on many variables including:

    1. the potential yield and price of the crop,
    2. cost of the fungicide,
    3. current and predicted rainfall, and
    4. presence and level of disease inoculum, e.g. petal infestation, sclerotia/apothecia
    5. and host vulnerability/susceptibility.


    A checklist developed in Sweden can be useful in helping to assess disease risk in fields. Growers should assess the crop and fill out the checklist for each field shortly after first flower (when 75% of the canola plants have at least 3 open flowers). Usually this occurs during the last week of June or the first week of July. Read each question and add the point value assigned to the answer that best describes the conditions for that field. The total of the risk points from all questions represents the overall risk for sclerotinia (see discussion in next paragraph). It is important to assess the risk early in the flowering period so that arrangements for chemical and application can be made if required. Continual monitoring up to full bloom is advisable as risk can change dramatically from early flower to full bloom.

    The greater the risk score for a field the higher the probability of a positive economic return to a foliar fungicide application. Swedish researchers who developed this model (through evaluation of 800 fields over a 10-year period) found that with a total of less than 40 risk points, the risk of heavy infection (disease incidence exceeding 25%) was low (Figure 25). A threshold value of 40 risk points accurately identified 75% of the fields that needed spraying for sclerotinia but also 16% of the fields that did not need a fungicide. A threshold of 35 risk points increased the accuracy to almost 90% of the fields that needed spraying but also included more than 20% of the fields that did not need spraying. Therefore, they concluded that under their conditions if the risk points were 40 or higher, it was likely worth spraying. If the risk points were less than 40, it was not likely worth spraying. This provides a good guideline for use of the checklist in Canada, but growers should keep in mind that it may vary a bit under our field conditions, and also depending on fungicide cost and commodity price.

    Using this checklist effectively requires scouting for apothecia, usually in nearby cereal crops following canola or other host crops (e.g. beans, sunflowers) in the rotation. The same moist soil conditions conducive to apothecia production can also favour the development of many other types of mushrooms or fruiting bodies. Accurate identification of apothecia is critical to effectively determine the risk of stem rot (see sections 3 and 4 of the Canola Disease Scouting and Risk Assessment Card).

    Sclerotinia Stem Rot Resistance Evaluation Protocol

    To determine the amount of resistance or tolerance a cultivar has to sclerotinia stem rot, the canola industry has agreed to use the protocol developed by Dr. Lone Buchwaldt (Agriculture and Agri-Food Canada) and the Pathology Sub-committee of the Western Canada Canola/Rapeseed Recommending Committee. This protocol outlines the steps necessary in order to correctly test for sclerotinia resistance and to correctly label the amount of resistance for cultivar descriptions and marketing purposes.


    Petal Test

    The petal test method developed by the University of Saskatchewan is based on the fact that the amount of disease that develops in the field is determined by both the number of spore-carrying petals and by weather conditions. A combination of weather conditions, inoculum levels and crop canopy density affect how many plants actually become diseased. Prediction of stem rot risk is based on the numbers of spores present during flowering. [3],[4],[5]

    The purpose of a petal testing kit is to test canola petals for the presence and level of fungal ascospores.  Contact Discovery Seed Labs in Saskatoon for more details: (306) 249-4484; e-mail: info@seedtesting.com.

    The petal test involves:

    • collecting canola petals, starting at early flower
    • placing petals in plates containing a culture medium specifically designed to encourage growth of the sclerotinia fungus
    • scoring canola flower petals in the culture plates by identifying colonies of sclerotinia
    • calculating the percentage of petals that were infected and estimating the risk of disease from the results

     

    Sclerotinia infected flower petals

    Figure 24: An Estimation of Potential Yield Loss Based on Average % Canola Petals Infected with Sclerotinia Spores at Early Bloom

    The average percentage of infected flower petals can be used to estimate the probable percentage of diseased plants that could develop in the field being evaluated. Typically, if levels of petal infestation remain less than 10 to 20% from early to full bloom, the risk of economic levels of stem rot is minimal. However, should levels be above 30 to 40%, then economic levels of stem rot can occur, especially when moderate temperatures and rainfall occur and crop yield potential is at least 30 to 40 bushels per acre. 

    Use the chart in Figure 24 as a guide to estimate probable percent yield loss. The University of Alberta and AAFC are developing a new technique for petal testing - based on PCR, which hopefully should be available in the near future.

    When only the average percent infected petals at early bloom are considered it is easy to flow through the chart to estimate the probable percent yield loss.

    Research at the University of Guelph shows that measuring soil moisture levels a week previous to and the week of petal collection helps increase the accuracy of the petal test. A soil that is just above the wilting point will have enough soil moisture to germinate sclerotia.

    Crop Canopy Density

    Crop canopy density can influence the risk of sclerotinia, given favourable environmental conditions. [9] The crop canopy can be modified by seeding rate, row spacing and fertility, which is the main factor impacting crop canopy density. Generally a high population of rapidly growing plants leads to faster canopy closure and thick, dense canopies. While these dense canopies often have the greatest yield potential, they tend to maintain high soil moisture levels beneath the canopy, increasing the germination of the resting bodies (sclerotia) of the stem rot fungus. Moist soil conditions for approximately 10 days or more induces sclerotia to form apothecia and subsequent spore release. Moist conditions in the lower canopy also increase the chances of infection being initiated when petals carrying spores land on lower leaves or leaf axils. In short season growing areas, B. rapa varieties tend to have lighter canopies and generally lower infection levels.

    Grade Loss

    One additional risk to economic returns that occasionally results from sclerotinia stem rot is downgrading of canola due to sclerotia in the seed sample. Tolerances for sclerotia are very low. Table 7 of the Canada Seed Act, which applies to mustard and canola, has a separate standard for sclerotia. When performing a purity analysis on the crop, sclerotial bodies are reported per 50 grams. The standards are as follows:

    Grade -- Maximum number of sclerotial bodies per 50g
    Canada Foundation No. 1 -- 1
    Canada Foundation No. 2 -- 2
    Canada Registered No. 1 -- 1
    Canada Registered No. 2 -- 2
    Canada Certified No. 1 -- 1
    Canada Certified No. 2 -- 2
    Common grade -- 2


    Management of Sclerotinia Stem Rot

    Sclerotinia stem rot of canola is extremely variable in occurrence and severity from year-to-year, region-to-region and field-to-field. [1],[11] Foliar fungicides remain the main control strategy.  Several tools are available to help growers assess risk and plan for fungicide application. Resistant/tolerant varieties are available and efforts continue to improve varietal resistance, in both the public and private sectors. [1],[9]

    Variety selection and genetic resistance/tolerance

    The genetics of canola varieties selected may affect the risk profile of a field. Sclerotinia has been observed to be severe in heavily lodged fields or areas within a field. Therefore choosing cultivars that resist lodging should be beneficial to reduce sclerotinia severity. Genetics can also influence crop canopy density, which can in turn affect the amount and severity of sclerotinia infection.

    All canola varieties with flower petals can be infected by sclerotinia stem rot. The first form of sclerotinia resistance available in Canada was in canola varieties with the apetalous trait. Plants with this trait avoided some sclerotinia infections as they produced flowers without petals. Since flower petals are the main food source for sclerotinia spores their absence greatly reduces, but does not necessarily eliminate the risk of disease development.

    The recent introduction of varieties with some physiological tolerance to sclerotinia is another management tool for growers. These varieties have demonstrated a significant ability to reduce sclerotinia severity in the field and protect yield when the disease is present, although a fungicide application may be still be warranted depending on the conditions.

    Research and plant breeding efforts continue to improve varietal resistance, in both the public and private sectors. AAFC has had some success in finding new sources of resistance. In 2011, two varieties with improved sclerotinia tolerance were available among the glyphosate tolerant (Roundup Ready) varieties, and some varieties in the glufosinate tolerant (Liberty Link) system were very close to commercialization.

    Crop Rotation

    Crop rotation is not always effective because of the pathogen's large host range and its ability to survive for years in the soil as sclerotia. Also, air-borne ascospores can blow in from nearby fields, where they are released by apothecia germinating from sclerotinia left from previous broad-leaved crops. This further reduces the impact of a diverse crop rotation. Regardless, the opportunity for high sclerotinia stem rot disease levels is maximized if broad-leaf crops are grown consecutively due to increased potential for spore production within the field.

    Cereals and grasses are not susceptible, and can help reduce viable sclerotia in the soil through decay and germination in the absence of susceptible hosts. Avoid seeding canola adjacent to a field that had a heavily infected crop the previous year. Control of susceptible weeds and volunteer plants in cereal crops helps to avoid replenishing viable sclerotia levels.

    Seeding

    Use clean pedigreed seed, free of sclerotia. Since crop density is an important factor, avoid exceeding the recommended seeding rate. Research at the University of Manitoba found that increasing seeding rates two or three times above normal seeding rates can lead to lodging, which can increase sclerotinia infection. [10] Lighter seeding rates or wider row spacing may increase ventilation within the crop canopy and reduce the moist soil microclimate required for sclerotia to germinate, helping to reduce sclerotinia levels. However, keep in mind that seeding rates need to be adequate to produce a minimum of 5 to 6 viable plants per square foot (ideal population is 7 to 14 pl/ft2) or yields may be compromised regardless of sclerotinia levels. Also, frequent rainfall and/or persistently high humidity may create a suitable environment for infection in spite of an open canopy.

    Growers reported that fall seeded canola tended to have less sclerotinia infection than canola sown in early to mid-May.

    Harvest

    Growers can apply fungicide for sclerotinia control and achieve good to excellent results in the standing crop. Unfortunately, this disease may progress rapidly in the swath in wet years particularly in B. napus cultivars. Do not swath canola crops with significant incidence of infection if rain is forecast, particularly if the crop is immature (green) when cut. In wet compacted swaths, particularly on the turns, sclerotinia rot can progress rapidly. The disease can be detected by a rotten "egg like" smell coming from the swaths. This problem is obviously more prevalent in the wetter regions of western Canada. The heavier and more compact the swath, the greater the likelihood for sclerotinia to rot the swath before combining.

    Steps to follow to control sclerotinia in the swath:

    • do not swath immature stands (at least 30% of the seed must be ripe, optimum is 50-60% seed colour change among the healthy plants)
    • do not swath if rain is in the immediate forecast
    • avoid compacting swaths
    • use a high cut to allow for better drying
    • avoid heavy swaths on the turns
    • consider direct combining the crop, but recognize that shattering losses from prematurely ripened infected plants will be significant 


    Up to one-third of a canola crop may be lost in the swath due to sclerotinia rot. Additionally, this damages and reduces the quality of the seed and increases the number of sclerotia.

    Foliar Fungicide

    Where required, the use of fungicides not only increases yields but also reduces dockage due to sclerotia contamination of the seed and small, shrivelled seed. Since the cost of spraying a fungicide is high and sclerotinia disease incidence varies greatly among years, regions and fields, systematic spraying is not profitable. By applying a fungicide only when necessary, yield losses due to heavy infestations as well as unnecessary fungicide applications can be avoided. [1],[7],[8]

    There are several fungicides registered for sclerotinia stem rot control in canola*: Astound, Lance, Proline 480SC, Quadris, Quash, Rovral Flo, Vertisan. There are also bio-fungicides available including Contans WG and Serenade Max/ASO. [1] (See Biological Control section for more details on these bio-fungicides).  Spray when the crop is as yellow as possible to ensure that the majority of the petals will be covered with the fungicide. Some of these products are registered for split application, which is an advantage as it will provide longer protection if the bloom period is extended due to cool, wet conditions or uneven maturity resulting from staggered emergence or excessive branching with low plant populations.

    *Always consult your provincial crop protection guide for an up-to-date list of registered products.

    If you plan to use a fungicide for sclerotinia control you must decide when to spray. Among the chemical fungicides, the recommended window for application generally falls somewhere between 20 and 50 per cent bloom, with optimum timing typically around 30 per cent bloom. Sample several plants over the field and assess the number of open flowers. One way to check for bloom stage is to find the main stem, pull off the secondary branches, and count only the open flowers on the main stem. Generally, it takes a crop from two to four days to move from first flower to 10% bloom (Table 4).

    Table 4. Identification of Flowering Stages of Canola
    Flowering StageB. napus Canola (flowers - main stem)B. rapa Canola (flowers - main stem)
    10% 10 6 to 7
    20% 14 to 16 10 to 12
    30% 20 14 to 16


    At 30% bloom, a field of canola is said to be in full bloom-when the maximum number of flowers are open at one time (Figure 26 and 27). 

    At 30% bloom, a field of canola is said to be in full bloom-when the maximum number of flowers are open at one time.

    The objective of the fungicide application is to cover as many petals as possible while ensuring that some chemical also penetrates into the canopy to help protect potential infection sites (such as leaf axils and bases). The chemical is only active on those petals that are present at the time of spraying. The chemical will not protect petals that emerge after spraying, but some chemical coverage within the canopy may help to restrict infection. The fungicides can also not cure infections that have already penetrated into plant stems, hence the need to apply the fungicide prior to significant petal drop when conditions are conducive to sclerotinia infection.

    If the above objective is achieved, the maximum number of fungicide-covered petals will fall into the canola canopy (lower leaf axils, leaves and shoots). Infection of the canola plant will only take place from sclerotinia-infected petals. When the petals fall into lower leaf axils the presence of one or more petals carrying fungicide will likely prevent sclerotinia infection.

    For growers without the equipment to spray for sclerotinia, due to the very short time of four to eight days from first flower to 20 to 30% bloom, book a custom applicator as early as possible.

    Research at AAFC Saskatoon compared 5 methods of fungicide application over three years to determine the effect of nozzle type and pressure on sclerotinia stem rot infection of canola and crop yield. [11] Variation in product performance among years indicated that environmental conditions had a major effect on crop development, sclerotinia stem rot infection, the effect of fungicides and subsequent yield. Overall, conventional flat fan nozzles (TeeJet XR), low-drift venturi nozzles (CFFC TurboDrop) and hollow cone nozzles (TeeJet TXVS-8) were all effective at reducing disease symptoms. Both 275 and 550 kPa (40 and 80 psi) provided similar performance although increasing the venturi nozzle pressure to 550 kPa (80 psi) improved disease control slightly. These results indicate that venturi nozzle technology is appropriate for use with foliar fungicides for sclerotinia control in canola provided pressures are adjusted to optimize nozzle performance.

    Economics of Fungicide Application

    The decision to apply a fungicide to prevent sclerotinia stem rot depends on 4 key questions:

    1. Have conditions been moist over the past few weeks for apothecia development and survival?
    2. Is the canopy thick and is yield potential high?
    3. Does the forecast call for more rain and/or humidity in the next week?
    4. Is the pathogen present in sufficient quantities?


    If the answer is "yes" to all 4, then spraying is recommended. If the answer to some of the questions is no, then the decision is more difficult. Utilize the stem rot checklist to aid the decision, but also consider canola price and overall profit potential of the crop before investing more in it. Canola canopies with yield potential of less than 30 bushels/acre are less likely to suffer sufficient sclerotinia infection to make spraying economical, unless conditions for infection are ideal. Crops with an open canopy could be at lower risk, particularly if warm and windy weather allows them to dry out in the afternoons. However, if frequent rains and high humidity keep the plants wet there is still a threat. Rainfall at flowering and thick canopies increase the sclerotinia risk, especially if you're in an area where sclerotinia sprays are often warranted.

    Canola Council of Canada research shows that it may be economically justifiable to apply fungicide when field scouting indicates that disease levels will reach 15 percent in Brassica napus (Argentine canola) and over 30 percent in B. rapa (Polish canola) by crop maturity. This figure is based on an $8 return per bushel ($353 per tonne) and an application cost of the fungicide at around $22 per acre ($55 hectare).

    As a rule of thumb, the potential yield loss in a field can be determined by:

     % Potential Yield Loss = % Infection x 0.5

    1"For example, an Argentine canola field with a 50 percent main stem infection would have an approximate yield loss of 25% (50 % infection x 0.50 = 25). The actual yield losses depend on the variety, weather and time of infection.

    The estimated percent yield loss can be used to estimate the bushel loss due to sclerotinia infection if not treated using the following formula. If this value is lower than the cost of a fungicide application/acre, then a fungicide application is not recommended.

    Yield Loss/acre = % Potential Yield Loss   x   Estimated Yield   x   $/bu

    Biological Control

    In 2010, United Agri-Products (UAP) introduced Contans, a biological control product registered for control of sclerotinia in field crops such as canola, soybeans, dry edible beans, sunflower and safflower. The active ingredient is Coniothyrium minitans, a fungus that colonizes and slowly degrades the sclerotia when it comes in contact. The product is a pre-emergent bio-fungicide that requires several months for the fungus to destroy the viability of the sclerotia. Since this product is new, little published research and limited field data is available on how well it controls sclerotinia incidence and severity in canola.

    Researchers at AAFC, the University of Manitoba and University of Lethbridge are studying various biological control options for sclerotinia in canola. Researchers have identified bacterial strains that appear to have biocontrol activity against Sclerotinia sclerotiorum, as well as a fungicide, however these are still in development. [12],[13],[14]

    Leave A Check Strip

    Researchers and agronomists recommend that when you do spray with a fungicide and/or a biological control product, leave a check strip to see how the spray worked. Just leave a strip unsprayed and then go back during your pre-harvest scout and compare sclerotinia infection levels. If you have a yield monitor on the combine, you can also compare yield differences. Consider leaving additional strips in the field (or multiple fields) to increase confidence in the comparison of disease levels and final yields.

    Research 

    There are various research projects underway by researchers and plant breeders from government, universities and industry working on developing new resistant germplasm and cultivars. Other projects are focusing on improved weather modelling and field assessment methods for S. sclerotiorum risk and fungicide application timing.


    References/Sources

    [1] Kutcher, H.R, F. Dokken-Bouchard, T.K. Turkington, W.G.D. Fernando, S. Boyetchko, L. Buchwaldt, D. Hegedus and I. Parkin. 2011. Managing sclerotinia stem rot in canola.  Canola Research Summit, Winnipeg, MB  April 12-13, 2011

    [2] Turkington TK. 1991. Factors influencing a petal-based forecasting system for sclerotinia stem rot of canola. PhD Thesis. University of Saskatchewan, Saskatoon.

    [3] Home-Grown Cereals Authority (HGCA). 2004. Sclerotinia in oilseed rape, prediction and control. Topic Sheet No. 77. London, UK.

    [4] Home-Grown Cereals Authority (HGCA). HGCA Winter oilseed rape: Fungicide Performance project 2009 (RD-2007-3457). Sclerotinia Update, January 2010.

    [5] Turner, J., C. Young,, A. Riding, and P. Gladders. Sclerotinia Control in Oilseed Rape: Progress With Quantitative Diagnosis and Development of a Web-Based Forecasting Scheme. Project Report No. OS56. 2002. Home-Grown Cereals Authority (HGCA). London, UK.

    [6] Canola Council of Canada. Sclerotinia Stem Rot Checklist. https://canola-council.merchantsecure.com/canola_resources/product11.aspx

    [7] HGCA Sclerotinia Decision Guide Tool. Control. http://www.hgca.com/minisite_manager.output/2997/2997/Sclerotinia%20Decision%20Guide%20Tool/Sclerotinia%20Decision%20Guide%20Tool/Control.mspx?minisiteId=24

    [8] HGCA Sclerotinia Decision Guide Tool. Fungicides. http://www.hgca.com/minisite_manager.output/2998/2998/Sclerotinia%20Guide/Sclerotinia%20Guide/Fungicides.mspx?minisiteId=24

    [9] Kutcher, Randy. Kelly Turkington, K. Rashid, Stephen Strelkov, Ralph Lange, and Stewart Brandt. 2006. Management of Oilseed Diseases. SSCA Conference.

    [10] Jurke, C.J. and W.G.D Fernando. 2006. Effects of seeding rate and plant density on sclerotinia stem rot incidence in canola. Phytopathology and Plant Protection.

    [11] Kutcher, H.R. and T.M. Wolf. 2006. Low-drift fungicide application technology for sclerotinia stem rot control in canola. Crop Protection. Volume 25, Issue 7. Pages 640-646.

    [12] McLaren, D., R. Conner and D. McAndrew. 2009. Impact of Timing, Rate and Application Technology on Biological Control of Sclerotinia Stem Rot of Canola caused by Sclerotinia sclerotiorum. Canola Agronomic Research Program Final Report #2006-03

    [13] Fernando, W.G.D.,  S. Nakkeeran, Y. Zhang and S. Savchuk. 2007. Biological control of Sclerotinia sclerotiorum (Lib.) de Bary by Pseudomonas and Bacillus species on canola petals. Crop Protection. Volume 26, Issue 2. Pages 100-107.

    [14] Hung-Chang Huang and R. Scott Erickson. 2007. Biological Control of Sclerotinia Stem Rot of Canola Using Ulocladium atrum. Plant Pathology Bulletin 16:55-59

    [15] Turkington, T.K., R.A.A. Morrall and S.V. Rude. 1991. Use of Petal Infestation to Forecast Sclerotinia Stem Rot of Canola: The impact of diurnal and weather-related inoculum fluctuations. Canadian Journal of Plant Pathology, 13:4, 347-355

    [16] Turkington, T.K. and R.A.A. Morrall. 1993. Use of Petal Infestation to Forecast Sclerotinia Stem Rot of Canola: The Influence of Inoculum Variation over the Flowering Period and Canopy Density. Phytopathology 83:682-689.

    [17] Clarkson, John P., John Staveley, Kath Phelps, Caroline S. Young and John M. Whipps. 2003.  Ascospore release and survival in Sclerotinia sclerotiorum. Mycol. Res. 107 (2): 213-222.

    [18] Qandaha, Issa S.  and  Mendozab L. E. del Río. 2011. Temporal dispersal patterns of Sclerotinia sclerotiorum ascospores during canola flowering. Canadian Journal of Plant Pathology Volume 33, (2): 159-167.

    [19] Qandaha, Issa S.  and  L. E. del Río. 2006. Dispersal of Sclerotinia sclerotiorum ascospores in canola fields from area source of inoculum. Department of Plant Pathology North Dakota State University, Fargo 58105. http://NDwww.ars.usda.gov/SP2UserFiles/ad_hoc/.../2006Posters/Qandah.pdf

    [20] Ben-Yephet, Y. and S. Bitton. 1985. Use of a  Selective Medium To Study The Dispersal of Ascospores of Sclerotinia Sclerotiorum. Phytoparasitica 13:1.

    [21] Meindert D de Jong, Graeme W. Bourdôt, Geoff A. Hurrell, David J. Saville, Hans J. Erbrink & Jan C. Zadoks. 2002. Risk analysis for biological weed control - simulating dispersal of Sclerotinia sclerotiorum (Lib.) de Bary ascospores from a pasture after biological control of Cirsium arvense (L.) Scop. Aerobiologia 18: 211-222.