Some forecasters are predicting a hot, dry summer across much of western Canada, which means farmers could see some signs of drought stress — such as bleached heads — in their cereal crops. But how can they be sure these symptoms aren’t the result of something else, like fusarium?
“In a lab, we can tell if it’s fusarium by looking for spores on the glumes. If they aren’t present yet, we can get the spores to form by putting glumes or seeds onto a plate to allow the fusarium to grow for a few days. A pinkish colour is the surefire way to tell it is fusarium,” says Faye Dokken-Bouchard, provincial plant disease specialist with the Saskatchewan Ministry of Agriculture. “Sometimes you can even see spores on the glumes in the field, and there will be a pinkish or orange colour to the spores. Damage from anything else will not result in pink or orange colour.”
Another telltale sign of fusarium might be individual glumes affected as opposed to the whole head. “If the infection took place when flowering started, then the symptoms will often be localized to the middle of the glume because flowering starts in the middle florets and continues up and down the head,” says Dokken-Bouchard. “If infection is severe enough, however, it can affect more than individual glumes.” Infected glumes may not produce seed, or have withered, chalky kernels.
Drought stress: tricky to call
Famers who know their fields well, and know where the wet spots usually are, may see some subtle differences to help them figure out what’s causing bleached heads. During moisture stress, symptoms are likely to be present throughout the entire field. However, even during drought, the whitened heads might appear in scattered clusters in the field, says Pratisara Bajracharya, a plant pathologist with Manitoba Agriculture, Food and Rural Development.
“Even in drought conditions, there may be areas of the field which would have a little better supply of water than the other areas, so in those areas if the damage is due to drought stress only, you should see less white heads, so they would be present in scattered clusters,” she says. “Sometimes if there is double seeding in an area, that area would be particularly bad during a drought too.”
When plants don’t have sufficient water they make physiological sacrifices in an attempt to set some seed. Depending on the stage, this can mean premature ageing of leaves, heads or the entire plant. High salinity can also play a role, particularly during drought stress. Low temperatures can also stress plants and damage immature heads. Excess moisture early in the season can cause plants to develop shallow roots and make them more sensitive to heat stress later in the season as they develop.
Could herbicide damage also be responsible for white heads in cereals? It’s unlikely, says Bajracharya, especially if there are no other telltale signs of herbicide damage, and as long as the tank has been properly cleaned out and the herbicide sprayed at the appropriate time and at the correct labelled rate.
The problem with fusarium
Fusarium can infect crops at different stages, so early on it can cause seedling blight and emergence problems if it’s on the seed or already present in previous crop stubble. The first line of defence is for farmers to use clean seed. Seed treatments can also take care of most of these problems, says Bajracharya. “But if farmers see plants wilting and not doing well early on in the season they might want to bring them in for testing because it’s hard to know if there is a fusarium infection or not at this stage,” she says.
How effective seed treatments are will depend on the species of fusarium present on the seed and the percentage of infection. “If there is two to three per cent of fusarium graminearum present, which is the species that causes mycotoxins in the finished grain, it might be worthwhile to invest in seed treatment, but at five per cent the seed treatment might not work very efficiently. If the predominant species of fusarium are other species then a seed treatment could be effective up to 10 per cent.”
Seed treatments won’t prevent fusarium head blight during later growth stages but will help with early season issues like poor emergence and seedling blight caused by fusarium.
Fusarium spores can also blow in with the wind and cause infection at the flowering stage, even if there isn’t a pre-established population of fusarium in the field. Fusarium infection depends largely on weather patterns, which should be the next clue. “For fusarium infection, a prolonged period of hot and humid conditions is required,” says Bajracharya. “The infection starts when it’s hot, but there is moisture present because of rainfall, and normally symptoms would start developing three days after the infection has set in. So there is a rainfall event and farmers are seeing some of these symptoms it’s more likely to be fusarium because rainfall was involved.”
Unfortunately, once the infection is established and farmers see symptoms, they can’t do anything to protect the rest of the crop. “Fusarium head blight is a monocylic disease, which means there will only be one disease cycle per season, so unlike polycylic diseases (where you can look for early symptoms and apply fungicide to prevent disease from spreading), symptoms mean the damage is done. The rest of the crop cannot be protected, but assuming the crop is at the same stage, it’s also not going to be susceptible anymore because fusarium head blight can only infect flowering crops,” says Dokken-Bouchard.
The seed may still be infected, and even if there is no visible kernel damage, and therefore no grade impact, farmers should get the seed tested if they plan to plant it, as infected seed may still negatively affect seed germination and stand establishment.
Spray if infection is likely
Farmers should consult provincial fusarium head blight risk forecast maps, which are published online daily during the prime infection periods. These maps help predict the level of risk for infection in different areas based on weather station data from the previous few days. If local weather forecasts predict conditions conducive to infection, and crops are come into flowering, each farmer must decide whether or not to spray fungicides based on whether infection is likely to occur.
“Timing of a fungicide application is crucial, so we are emphasizing that farmers spray only during flowering when the crop is most susceptible to fusarium infection,” says Bajracharya. “All of the fungicides registered for fusarium head blight are only for the purpose of suppression and they don’t provide complete control. Because of that we recommend that they spray only when infection is more likely. Using the forecast maps would tell them the fusarium risk for that day and if the crop is flowering, and the risk is high, an infection could be likely so they may want to spray at that time.”
“Even after harvest, Fusarium graminearum will continue to produce mycotoxins as long as the fungus is living, so it’s important not to harvest when the moisture content is more than 14 per cent if it is for grain,” says Bajracharya . “For green feed it is recommended that at the time of harvest, it is at least 20 per cent dry. It’s something to keep in mind because even if you couldn’t avoid the infection, you could still lower the amount of mycotoxin accumulation because that’s one of the factors in determining the quality of grain.”
Although no one can plan ahead for drought, planning can definitely help reduce the risk of fusarium infection. Rotating non-host crops such as pulses or alfalfa and ensuring there is at least one, and preferably two, years between host crops such as wheat, barley and corn can help reduce the level of pathogens that survive in crop residues.