Sunflower growers should watch out for downy mildew, especially if spring conditions are favourable for the disease
A 2011 survey of Manitoba’s sunflower fields found downy mildew present in nine out of 11 fields surveyed. “Last year was a bit of a heavy downy mildew year because of the fact that the spring was very wet. The downy mildew in general for sunflower is favoured by wet soil and temperatures around 15 C to 20 C around the seedling stage,” says Dr. Khalid Rashid, an Agriculture and Agri-Food Canada (AAFC) research scientist at Morden, Manitoba.
Downy mildew spores survive in the soil for years — in fact, hardy spores can last for 10 years. Water allows the spores to move freely through the soil, infecting sunflower roots. Such infections are severe and systemic, and plants infected through the roots will die without producing seeds. Infected plants can also spread the disease later in the year, although secondary local infections on the leaves don’t usually affect yield.
The number of infected plants ranged from trace to 10 per cent in last year’s survey. Rashid surveys sunflower fields each year, and he has seen more severe infections in the past.
“Not many fields would have 40 per cent of infected plants, but there are cases where we see Manitoba fields that have 30 per cent infected plants. And in some cases I’ve seen over the years those fields were plowed under because we observed that at the seedling stage. I remember one day, one field had more than 60 per cent, so the grower just put it under and planted canola or something else,” says Rashid.
A 10 per cent infection rate doesn’t necessarily add up to a 10 per cent yield loss. Though severely infected plants die, nearby sunflowers often compensate, making the actual yield loss difficult to calculate. Rashid estimates that compensating sunflowers can sometimes cut the yield loss in half, though he hasn’t measured the yield loss.
Rashid recommends scouting for downy mildew when the crop is three to four inches high, especially if farmers have low spots in their fields. Infected seedlings will start to blanch, or turn yellow, from the top leaves to the bottom. If there is enough humidity, within a week or so grey fungus will be visible on the underside of leaves. Plants infected at the seedling stage may survive to grow to a half-metre, but they will be severely stunted and will eventually die.
Secondary infections, spread by infected plants, can create small lesions on otherwise healthy plants. Rashid says these secondary infections are minor, though when combined with rust, sclerotinia, or other diseases, the situation is more complex.
Eradicating downy mildew is difficult, so farmers need to think about managing the disease. A three- or four-year crop rotation helps cut the number of soil-borne spores in the soil, reducing disease severity. The fungus that causes downy mildew only targets sunflowers, meaning other crops aren’t vulnerable to the spores. Volunteer sunflowers host the disease, so controlling weeds during the rotation is also important. Sanitizing equipment can help prevent soil-borne spores from spreading to clean fields, but the wind is likely to spread spores as well.
Commercial hybrid sunflower seed is treated with a mixture of fungicides, reducing the likelihood of introducing the disease to a clean field through seed. Farmers getting their seed from another source should make sure the seed has been treated.
“They can choose resistant hybrids. But then the downy mildew hybrid may not have enough resistance to rust. So they have to be very careful to choose a hybrid that has a good package of different disease resistance, not just one disease,” says Rashid.
No Canadian labs are certified to study infected seedlings and identify races, but Rashid says his lab can handle a small number of infected samples. Interested farmers can put individual infected plants in separate paper bags and send them by courier to Rashid. Rashid adds that the samples need to be collected when the sporulating fungus starts growing on the lower sides of leaves. He suggests that farmers call him before collecting samples (his number is 204-822-7220). Rashid’s lab may be able to help individual farmers identify the specific downy mildew race infecting their fields. His lab also provides information to companies developing resistant varieties.
Rashid cautions farmers to watch for downy mildew even if this spring isn’t as wet as 2011. “It’s all relative terms when we say wet season. It doesn’t have to be total soil saturation… If the roots come in contact with the inoculum right away, they don’t need moisture. The moisture helps the spores to move, that’s all.” †