With all the effort that goes into breeding disease-resistant crops you might think it should be time we didn’t have to worry about them. The reality is that no matter what variety you use, its genetic resistance is just one part of stopping disease from robbing you of yield and grades.
That was the message Dr. Kelly Turkington brought to the audience at the Southern Alberta Conservation Association meeting. “Don’t depend only on a resistant variety,” said the Agriculture Canada crop disease researcher. “Resistance may not last forever, and it often isn’t 100 per cent effective. Don’t make it your only management strategy.”
Turkington advises adding other strategies to fight disease and says using several strategies at the same time is the most effective disease control.”
He says disease shows up when three things are present – the pathogen, a susceptible host and a favourable environment. If you depend only on genetics to limit the host’s susceptibility, the pathogen can change quite quickly to overcome the host’s resistance,
“Our cropping systems are inherently vulnerable to plant diseases,” Turkington said. “We have developed very uniform varieties that maximize yield and quality, while disease resistance and grain quality can be antagonistic – high levels of one may come at the cost of the other.
“Then we aggregate these genetically uniform plants in space and time, with large fields, short rotations or even mono-cultures and only a handful of varieties and a few crops in an area. So, any disease organism that hits a vulnerable crop at the right time can take out many fields.”
Turkington says a dense, uniform crop with a closed canopy provides the ideal microclimate for crop diseases. For some crops such as beans, ultra-wide row spacing to open the canopy and managing irrigation to avoid a wet canopy at the highest-risk time can make the crop less hospitable to disease organisms such as sclerotinia.
The healthier the crop, the better it can fend off disease attack, says Turkington. You can also make the crop more resistant to disease by ensuring it is as healthy as possible by providing ideal growing conditions – seeding to the ideal depth, at the right time, with healthy seed, good fertility and weed control.
PREVENTION, NOT CURE
Fungicides protect the crop against imminent attack by disease organisms. But it can’t rescue a crop that’s already suffering from the disease. Fungicides work by preventing the disease organism from getting into the plant tissues. Those that are already inside the plant causing symptoms you can see have already done their damage.
“Most fungicides generally don’t move to new leaves as the crop grows,” says Turkington. “You want to protect the top three leaves of the plant, which contribute most to yield and grain filling. Applying fungicides early in the season along with herbicides does not directly protect the plant. If you spray a fungicide early in the season and rainy, humid or dewy conditions develop late in the season, you may see some disease loss.”
Turkington’s research in barley shows that spraying early – at the 2-3 leaf stage, whether with a full or half rate of fungicide, doesn’t reduce leaf disease levels to the same extent as a flag leaf stage application, which provides direct protection of upper-canopy leaves. The best strategy to protect the crop from scald or net blotch is to target the upper leaves of the crop canopy with a single timely application around flag-leaf emergence.
Overall, it’s a dilemma figuring whether the benefit of fungicides justifies the cost, especially if you have scheduling difficulties to work around. The risk of damaging levels of leaf diseases is the main thing, but other factors such as whether you’re more risk-averse or frugal can also play a role in deciding to spray or not to spray.
“If you’re risk-averse, and opt for fungicide, flag-leaf emergence is the most effective time to spray,” says Turkington.
Rotation is still the best way to control disease, separating the pathogen and the host with time. The more diverse the rotation, the lower the level of disease, but if you can’t switch crops, at least switch varieties, Turkington says. His work switching barley varieties has shown some success in managing barley leaf diseases.
To protect a crop from any disease, Turkington says is to grow it in rotation, use healthy seed of a variety with low susceptibility, promote crop health with good agronomics and use management to minimize infection, possibly applying a fungicide should the risk warrant it.
“There’s no magic bullet for crop disease,” he says. “Combine multiple strategies and use integrated pest and crop management. Using an integrated approach with multiple strategies, will prolong the usefulness of sources of genetic resistance and lower the risk of developing fungicide resistance.”