By: Barbara Ziesman, A.Ag, PhD Provincial Specialist, Plant Disease
The decision of whether or not to apply a fungicide can be difficult, but should always be based on disease risk and economics. The level of disease risk is a result of how much of the pathogen is present, the level of susceptibility of the host crop and the environmental conditions during the growing season. The environment plays the biggest role in driving disease cycles, with most diseases of field crops being favoured by humid conditions.
Scouting is one of the best ways to assess disease risk. The method of scouting differs slightly for monocyclic (diseases with one primary infection cycle per season) and polycyclic diseases (diseases with more than one infection cycle per season). For monocyclic diseases, such as sclerotinia stem rot of canola and fusarium head blight, the focus will be on whether or not the environmental conditions will favour disease development. Fungicides to manage monocyclic diseases need to be applied prior to symptom development. Risk assessment tools such as the sclerotinia stem rot checklist, the fusarium head blight risk maps and risk assessment chart are valuable references when making fungicide application decisions for these monocyclic diseases. These tools take into account the level of favourability of the environment in addition to other factors such as crop rotation and field history, which are known to influence disease risk.
For polycyclic diseases, such as cereal leaf diseases, anthracnose in lentil and ascochyta blight in pulses, scouting should be focused on looking for initial symptoms of disease and monitoring disease progression within the crop canopy. For these diseases, fungicides can be applied to prevent the progression of disease development and protect the plant tissues that contribute most to yield. When scouting for polycyclic diseases, visit at least five sites if the field is less than 100 acres and at least 10 sites if the field is more than 100 acres. When walking through the field, look for discoloured plants and/or small, discoloured spots on leaves and stems. Stop at each site and look down within the crop canopy, remove some plants, and closely inspect the leaves and roots. A magnifying glass or hand lens can be used to distinguish small spots and to look for chew marks or shredding, which may indicate that the damage is from insects rather than disease.
Scouting should begin at emergence and continue weekly until maturity, with the frequency of scouting increasing when the crop is most susceptible to infection, such as after rain events and when the environment is favourable for disease development. Crops will be most vulnerable to certain diseases during specific growth stages.
Some examples are as follows:
- Canola is the most susceptible to sclerotinia stem rot during flowering;
- Cereal crops are the most susceptible to loss of yield due to leaf diseases during flag leaf growth and should be scouted more regularly during this period;
- Risk of fusarium head blight in cereals is greatest during flowering (anthesis). Fungicide application decisions based on disease risk should be made at heading;
- Chickpea is highly susceptible to ascochyta blight. Scouting should begin at the seedling stage and continue until pods have well-formed seeds;
- Lentils should be scouted from the vegetative (eight to 10 node) stage, until after flowering, for both ascochyta and anthracnose; and
- Forage crops should be scouted for leaf spots prior to head emergence (grasses) or the vegetative to early bloom stages (legumes). For alfalfa, risk of blossom blight should be determined at early bloom until after flowering is complete.
Like for monocyclic diseases, risk assessment tools are available to aid in fungicide application decisions for some polycyclic diseases. For example, the Fungicide Decision Support Checklist for Control of Ascochyta Blight and Anthracnose in Lentil provides an assessment of risk based on plant stand, environmental conditions and symptom development recorded during scouting. These tools are helpful when making informed application decisions.
When the decision to apply a fungicide has been made, it is important to always follow label rates, apply the fungicide at the right time, and use the correct water volume to ensure optimum effectiveness.
- Label rates: Always follow label rates. Using lower rates may reduce the effectiveness of the fungicide and may also increase the selection pressure for fungicide insensitive pathogen populations.
- Fungicide timing: Apply fungicide preventatively. This means prior to damage of critical plant tissue for polycyclic diseases and prior to symptom development for monocyclic diseases.
- Water volume: Using the recommended water volume will help ensure that the fungicide is evenly distributed over the leaf surface. This is critical for contact fungicides. High water volumes are also very important to ensure canopy penetration when managing diseases that develop within a closed canopy.
- Fungicide placement: No fungicides on the market are truly systemic. As a result, it is critical to ensure that the fungicide is being applied with good coverage to the tissue that needs protection. This can be accomplished by using optimum water volumes and through nozzle selection.