Plant Disease Control

Integrated Plant Disease Management

Cereal Leaf Diseases

Cereal leaf diseases affect both the yield and quality of cereals. The following management practices are recommended for effective control of leaf diseases in all cereal crops.

• Scouting: Scout fields prior to, during, and following flag leaf emergence to check for disease levels. Flag and upper leaves are responsible for 50 per cent or more of grain-fill.
• Crop Rotation: Rotate crop types [e.g. cereal (wheat)/ oilseed/cereal(barley)/pulse] to reduce the build-up of disease inoculum in crop residue. If at all possible, do not seed the same crop in back to back years. When a short rotation is absolutely necessary, seed in the second year  a variety that is more resistant to an anticipated disease problem.
• Resistant Varieties: Provincial crop/seed guides provide a comprehensive listing of the performance of adapted varieties, including their resistance status to specific diseases.
• Foliar Fungicides: Foliar fungicides, applied at the proper time in accordance with manufacturers’ Pesticide Product labels, can control cereal leaf diseases and help to attain target yields. The greatest benefit occurs when disease pressure is high or with varieties that have poor resistance.
• Spraying Practices: Foliar fungicides should be applied preventively, before disease is well-established in a crop and already causing crop loss. Good spray coverage with minimal drift is essential. Ideally, the best time to spray is when the wind is light, humidity is above 60 per cent and air temperature is between 10 and 25oC.

Ergot of Cereals

Ergot is a fungal disease that affects most cereals and grasses in Canada. Ergot bodies contain toxic alkaloids; they should never be consumed by humans or fed to animals. Ergot is a particularly damaging disease of rye, and has also been observed sporadically over the years on cereals and grasses in the prairie provinces. After an ergot outbreak, crop residue and soil become contaminated with a higher load of ergot bodies, placing nearby grasses and cereal crops at greater risk of infection in the following seasons. This risk increases further when cool, moist weather conditions promote ergot spore production and/or when cereals experience an extended period of flowering or an induction of floret sterility due to any of a variety of agronomic or environmental factors. Once ergot is present, little can be done to control the disease in the field, so prevention is important. Planting seed contaminated with ergot bodies can potentially spread disease to previously clean fields and there are no seed treatments registered; therefore only clean, healthy seed should be used. During the field season, nearby grasses may be mowed to remove additional hosts. Prior to harvest, fields should be scouted to determine where ergot has developed, such as headlands, and those areas should be harvested separately. Viability of ergot bodies decreases after one to two years.

Fusarium Head Blight of Cereals

Fusarium head blight (FHB) causes a reduction in yield as a result of floret sterility and the loss of light weight cereal kernels during combining. More important is the effect on grain quality and food safety due to production of mycotoxins, including deoxynivalenol (DON) and vomitoxins. In Manitoba, FHB occurs throughout all crop regions and will damage wheat crops whenever environmental conditions favour the disease. In Saskatchewan, FHB has been established in eastern regions for several years, but occurs across the province, particularly in wet years.

• Field Management of FHB: Weather is by far the greatest factor in development of FHB. The disease is most likely to develop when the plants are flowering, temperatures range from 15 to 30oC and high moisture is continuous for 48 to 60 hours. If conditions remain warm and moist, the  pathogen can continue to sporulate and spread to other kernels or heads. Under these optimum conditions, crop management has little impact on FHB outbreaks. Production practices, which lead to reduced tillering and shortened flowering duration, could reduce the risk period of FHB infection.
• Crop Rotation and Crop Selection: A break of at least one year – preferably two years – is advised between cereal, grass and corn production. In fields of wheat on wheat stubble,   the incidence of FHB was about one and a half times higher than in fields of wheat planted into pulse crop residue. Regardless of the rotation, producers should consider planting cereals that are less susceptible to FHB. Results from previous years show that durum and soft white wheat varieties are more susceptible than hard red spring wheat varieties. Barley is more resistant than wheat, and oat is more resistant than either wheat or barley. Refer to provincial seed guides for FHB disease ratings for each variety. Planting two or more varieties of wheat with differing flowering times or varying planting dates will help reduce the risk of infection. Susceptible crops should not be planted on infected corn stubble. Corn trash is slower to decompose than cereal trash, and acts as a source of inoculum for a much longer time period.

Late Blight of Potatoes

One of the major threats to Manitoba and Saskatchewan’s potato industry is the fungal disease late blight. At present, there is no fungicide registered for use on potatoes that is capable of eradicating the fungus from infected plants. As a result, producers are forced to adopt preventive management to control this disease. One of the main components of this strategy is the application of fungicides at specified spray intervals. This interval varies with the type of fungicide used. Shortening or lengthening of this interval should be based on current weather conditions and the status of the disease in the crop.

In Manitoba, potato producers can make use of a weather-based late blight risk forecasting program. The purpose of this program is to predict when environmental conditions are most conducive to disease development and issue warnings based on those parameters.

Accurate weather monitoring and scouting techniques are very important for achieving the most effective use of fungicides. Combining precise weather forecasting with spray interval scheduling may lower input costs for the farmer and lead to a more productive, higher quality crop. These weather monitoring systems monitor key environmental variables, such as relative humidity, temperature, leaf wetness and precipitation.

The following practices are recommended for effective disease management:

• Scouting: To effectively schedule preventative fungicide applications and eliminate unnecessary fungicide use, local weather forecasts should be used to identify conditions conducive to disease development. Scout fields regularly to identify diseases and pests that may be developing. Low areas in rolling or hilly fields and in wind-protected areas near trees lines should be specially checked.
• Crop Varieties: (There are no known commercial resistant varieties currently available in Canada). Where practical, the use of short season varieties may help reduce the period of use for fungicides.
• Healthy Seed: Obtain seed from sources with effective disease management practices. The use of certified seed is legislated in Manitoba and Saskatchewan. Grade seed carefully while cutting and discard suspicious looking tubers and seed pieces.
• Cull Clean-up: Avoid leaving tubers, including debris or slivers from seed cutting, in cull piles for any length of time. Follow a program of sanitation for storage facilities and equipment to eliminate sources of the disease. Dispose of cull piles in an approved manner so they do not serve as a source of disease inoculum for future infections. Dispose   by burying, using a cover, spreading out on the field over winter, or feeding to livestock.
• Sanitation: Follow a program of sanitation for storage facilities and equipment to eliminate sources of disease. Sanitation consists of cleaning and disinfecting all equipment, storage, and tools that contact potatoes from seeding through harvest and storage. Since most disinfectants are inactivated by soil and plant debris, it is essential that equipment and storage is thoroughly cleaned with a pressure washer or steam cleaner with detergent before disinfectant is applied. Treated surfaces should remain wet for at least 20 minutes for the disinfectant to destroy disease organisms.
• Cultural Practices and Rotation: Use proper cultural practices including a one in four-year potato crop rotation; proper hilling to reduce disease and greening in tubers; manage irrigation to avoid an excess or deficit of soil moisture; schedule irrigation throughout the day so it is not extending the natural dew period and prolonging leaf wetness; if late blight is discovered destroy hot spots of infected fields; control weed hosts (especially nightshades) and remove and destroy volunteer potatoes. Use appropriate weed control practices in rotational crops to control those weeds that may be hosts of diseases in potatoes.
• Foliar Fungicides: Preventative fungicide applications are most effective in controlling late blight. Follow product label guidelines for most efficient and safe use of products. Labels of newly registered products also provide information on resistance management. In this context – medium to high risk of resistance fungicides (e.g. Group 7 – boscalid and Group 11 – strobilurins) should be rotated or mixed with low risk fungicides (e.g. mancozeb (M3) and chlorothalonil (M5)).
• Farm Visits: The following recommendations are provided to prevent the spread of potato diseases from field to field or between farms. All people serving the potato industry should use these sanitary practices.
  1. Contact the grower for permission to enter fields and other facilities on the farm.
  2. Keep your vehicle clean and whenever possible, avoid driving your vehicle into fields or potato handling areas.
  3. Carry a boot brush and a supply of disinfectant in your vehicle at all times. Quaternary ammonia (General Storage Disinfectant) is recommended as it is also registered for bacterial ring rot disinfection.
  4. Wear coveralls or other protective outerwear that can be discarded or disinfected regularly.
  5. Clean, washable, footwear is recommended and rubber boots are preferred.
  6. Clean, wash, and disinfect your boots thoroughly on arrival at each field/farm/storage shed and before leaving.
  7. Remove dirty outerwear, including boots before entering your vehicle.
  8. Any tools to be used during the farm call (potato forks, shovels, soil probes, knives, etc.) should be cleaned and disinfected before and after use.
  9. Maintain a detailed logbook of field/farm/storage shed visits.

Canola Diseases

Sclerotinia stem rot has been one of the most prominent diseases affecting canola in Manitoba and Saskatchewan for the past 25 years. An important factor for disease development is environmental conditions. The disease is much more widespread and severe during wet years. Fungicide applications are an important element in controlling the development and spread of sclerotinia stem rot. Fungicide spray decisions are based on soil moisture, weather conditions, crop stage and density, and disease history. The Sclerotinia resting bodies (sclerotia) require moist soil conditions for up to 10 days for germination to occur and the spore-bearing structures (apothecia) to form. Usually these conditions do not occur until the crop canopy closes. The spores released from the apothecia utilize the canola petals as a food source and fall into the canola canopy where they infect plants. Lesions form up and down the stem, wilting leaves and eventually killing the plant. Fungicide should be applied between the 20 to 50 per cent flower stages to protect the petals from being colonized by the spores.

Blackleg caused by Leptosphaeria maculans affects canola and most crucifer field and vegetable crops. After many years of low incidences, due to resistant canola varieties grown in the prairie provinces, the disease is gaining importance again. High frequency of canola in crop rotations, accompanied by changes in the pathogen populations, has led to higher incidences and severities   in some fields. For an effective control, a four-year crop rotation is highly recommended.

Clubroot is a soil-borne disease caused by a microbe, Plasmodiophora brassicae. Clubroot affects the roots of cruciferous field crops such as canola, mustard, and camelina, as well as cruciferous vegetables and weeds. Clubroot has become a significant problem for canola growers in some areas of Alberta and the pathogen has been detected in Saskatchewan and Manitoba. Clubroot is a regulated pest in Saskatchewan under The Pest Control Act. Currently there is no provincial legislation that regulates clubroot in Manitoba.

Invasion of the interior of the host roots alters hormone balance and leads to increased cell division and growth, resulting in clubroot galls. These deformed roots have a reduced ability to absorb water and nutrients leading to stunting, wilting, yellowing, premature ripening and shrivelling of seeds. The cause of these above-ground symptoms can be confirmed by digging up suspect plants to check roots for gall formation. Clubroot affects canola yield and quality to a similar degree as other diseases affecting water and nutrient uptake, and its impact depends on soil conditions and the growth stage of the crop when infection occurs. Spore germination, infection and disease development are favoured by warm soils, high soil moisture and low soil pH; however, the disease can still occur under conditions outside of the optimum parameters. Infected roots will eventually disintegrate, releasing resting spores into the soil, which may then be transported by wind, water erosion, animals/manure, shoes/ clothing, vehicles/tires or earth tag on agricultural  or  industrial field equipment. Resting spore numbers will decline over time  when non-host crops are grown, but a  small  proportion  can survive in soil for up to 20 years. Clubroot is primarily a soil-borne disease; it does not infect seed but it may be found in soil attached to seed or other plant parts. There are currently no seed treatments or foliar fungicides registered for control of clubroot on canola.

The following best practices are recommended for prevention and management of clubroot:

1. Plant susceptible crops, including resistant varieties, no more than once every four years. Although crop rotation will not prevent the introduction of clubroot to fields  that are free of the pathogen, it will restrict clubroot development by limiting the increase of clubroot resting spores and preventing the increase of clubroot inoculum, as well as help alleviate the impact of other plant pathogens.
2. Scout crops regularly and carefully.
   • Identify suspicious above-ground symptoms including wilting, stunting, yellowing and premature ripening of canola or other susceptible crops.
   • Field entrances and approaches are likely to be contaminated with clubroot spores first. Therefore, symptoms will often appear there first.
   • Confirm cause of above ground symptoms by checking the roots for galls.
   • Send sample of symptomatic plants into a commercial lab for confirmation of diagnosis.
3. Practice good sanitation by restricting movement of potentially contaminated soil to non-contaminated regions.
   • For Saskatchewan and Manitoba producers, this means restricting entry into their fields of vehicles, field machinery or oil rig equipment with earth tag from infested regions unless it has been properly sanitized. Ask questions about where the equipment is from and what sanitation measures have been used before the equipment left the infested area, dealer or auction site.
   • Cleaning steps may include: removal of crop debris and soil, washing of equipment with a power washer using hot water or steam and misting with disinfectant (1 to 2 per cent bleach solution), followed by an additional rinse with water.
   • Other agricultural products, which could carry soil,  should be carefully checked for excess soil and if possible be from clubroot free areas.

For more information on clubroot, visit www.clubroot.ca, www.saskatchewan.ca, or www.gov.mb.ca/agriculture.

Pulse Crop Diseases

There are a variety of pulse crops produced in Manitoba and Saskatchewan including field pea, field bean, lentil, chickpea and soybean. Pulse crops are adapted to different regions and will require unique agronomic and disease management practices. Some diseases will attack all pulse crops, e.g. sclerotinia (white mould) and seedling/root rots caused by Aphanomyces euteiches, Pythium, Rhizoctonia, Fusarium and Botrytis species. Some diseases may occur on more than one type of pulse crop, but the pathogen species infecting each is often specific to that crop. This is the case for the ascochyta blights, powdery mildews and anthracnose. It is important to source information on pulse disease control from grower organizations such as the Saskatchewan Pulse Growers (www.saskpulse.com), Manitoba Pulse Growers (www.manitobapulse.ca), provincial specialists, and field diagnostic guides. Most foliar diseases are favoured by warm, moist conditions and lush crop canopies, but root rots and powdery mildew can be present in dry years as well. In general, pulse disease management will need to include the following practices:

• Use of clean seed and seed treatments: Plant certified seed or seed that has been tested at an accredited lab and known to have high germination and zero or acceptable levels of seed-borne disease. Seed treatments will help protect seed and seedlings from low levels of seed-borne and soil-borne pathogens. However, there are no seed treatments registered for control of Aphanomyces euteiches.
• Crop Rotation: It is important to keep at least three years between the same type of pulse crop to allow for the breakdown of crop residue on which disease pathogens survive. Longer rotations may be required for Aphanomyces euteiches, due to long-lived resting spores in the soil. Since there are diseases that affect more than one type of pulse crop, it is still important to maintain at least two years between different pulse crops.
• Crop Varieties with Disease Resistance: Refer to provincial seed guides for varieties adapted to your region. When available, choose varieties with disease resistance.
• Scouting and Foliar Fungicide Application: Begin crop scouting at the vegetative stages for aggressive diseases such as ascochyta blight in chickpea. Scout for other foliar diseases at early bloom, e.g. ascochyta blight and anthracnose in lentil. It is too late to apply fungicide to control sclerotinia (white mould) once symptoms are observed, and/or the canopy has closed, so forecasting to determine risk is necessary.
  • Use foliar fungicides only when disease risk and potential loss are significant. Rotate fungicides or use tank mixes from different fungicide groups to prevent development of resistant pathogen populations.

Effects of Weather

How do fungicides work?

Do not apply foliar fungicides during periods of dead calm or when winds are gusty. Avoid application immediately after a rainfall and delay spraying if rainfall is imminent. Contact fungicides are always more sensitive to wash-off by rainfall than systemic fungicides, because their mode of action relies on drying on the leaf surface. Failure of a contact fungicide to dry on the leaf surface may result  in a loss in efficacy. Systemic fungicides are less sensitive than contact fungicides, but still need sufficient drying time and be fully absorbed by plants prior to rainfall. Consult the label or product manufacturers for rainfast period for individual products.

Pathogen Resistance (Insensitivity) Management

Any fungal pathogen population may contain strains naturally insensitive to a fungicide and other fungicides within the same Group. A gradual or total loss of disease control may occur over time if these fungicides are used repeatedly in the same fields. Other resistance mechanisms that are not linked to site of action but specific for individual chemicals, such as enhanced metabolism, may also exist.

To delay fungicide resistance/insensitivity:

• Where possible, rotate the use of a fungicide, (and others within the same Group) with different Groups that control the same pathogens.
• Where possible, tank mix fungicides with a high risk of developing insensitivity with other fungicides from a different Group.
• DO NOT apply more than the maximum number of applications listed on the label. Avoid consecutive sprays of a fungicide, or other fungicides in the same Group, in a season.
• Fungicide use should be based on an integrated pest management (IPM) program that includes scouting and accurate recording related to pesticide use and crop rotation. An IPM program also considers cultural, biological and other chemical control practices.
• Monitor treated fungal populations for signs of fungicide insensitivity. If disease continues to progress after treatment with a product, DO NOT increase the use rate. Discontinue use of the product and switch to another fungicide with a different target site of action.
• Contact your local regional crops specialist or certified crop advisor for any additional pesticide management and/or IPM recommendations for specific crops and disease problems in your area.

Fungicide Modes of Action

Why are fungicides needed?

• Control of disease during crop establishment.
• Increase productivity of crop (photosynthesis) and/or reduce blemishes.
  • Maintain yield and/or market value.
• Improve storage life and quality of harvested plants / grain / produce.
  • Prevent spoilage and/or production of mycotoxins.

How do fungicides work?

There are several ways to define ‘mode of action’:

• Timing:
  • Preventative: fungicide must be present on plant surface before the pathogen and repeated applications are required to protect new growth.
  • Curative: pathogen may already be present (post- infection, pre-symptom kick-back activity).
  • Eradicant: (post-symptomatic activity).
  • Inhibitive: prevents spore germination or sporulation.
• Placement:
  • Contact (AKA protectant): immobile – must come in direct contact with the pathogen.
  • Systemic (AKA penetrant): mobile – can move within plant.
• Movement:
  • Intra-plant Movement: within crop via vapour phase or redistribution by rain.
  • Passive Absorption – by diffusion.
  • Apoplastic Movement: xylem-mobile; move within free space and cell walls, upward through the transpiration stream (with water).
  • Symplastic Movement: phloem-mobile (common characteristic of herbicides and insecticides but very few fungicides).
• Spectrum:
  • General, Non-specific, or Broad Spectrum: fungicide affects pathogen in multiple ways.
  • Specific or Narrow Spectrum: fungicide targets a specific metabolic site in pathogen or against critical enzyme or protein. Genetic changes or naturally insensitive fungi have a greater chance to overcome the fungicidal effect (resistance/insensitivity).
• Composition:
  • Inorganic Fungicides: sulfur or metal ions such as copper.
  • Organic Fungicides: contain carbon atoms.
  • Biopesticides: suppressing pest populations using naturally occurring organisms or natural products derived from plants.
• Biochemistry:
  • Primary basis to classify fungicides, developed by Fungicide Resistance Action Committee (FRAC) using their general Mode of Action on fungi, and their chemistry.
  • All fungicides within a group share a common mode of action and resistance mechanism.
  • Fungicides within a group may have different chemical structures.
  • Resistance management strategies required wherever resistance is known or there is a risk of resistance development