The virulent 777 race is only one rust species. Three things sunflower growers can do to protect their crops
If it seems like rust is becoming a bigger problem for Manitoba sunflower crops, that’s because it is. Researchers have identified a new virulent race of sunflower rust that made up nearly two-thirds of the rust cases in Manitoba during the 2016 growing season. Left unchecked, the disease deprives the plant of nutrients, and can reduce seed size and oil quality, resulting in yield losses of up to 30 per cent.
Dr. Khalid Rashid, research scientist at Agriculture and Agri-Food Canada’s Morden Research and Development Centre, says “rust” is a common name for a disease that attacks all crops, but the species that impacts sunflowers is sunflower-specific. Other species of rust affecting cereal crops such as wheat and barley require alternate hosts to complete their life cycle. Sunflower rust does not require an alternate host to complete its sexual life cycle and produce new races.
“The smaller the micro-organism, the shorter the life cycle and the better the adaptability,” says Rashid. “Because sunflower rust does not require a host plant, it mutates quickly and has a short life cycle. This is what happened with the sunflower over the years — new races are always developing.”
There’s an international three-digit coding system that scientists use for classifying races of sunflower rust. Rashid says it’s a common classification used for testing a race on all nine sunflower genotypes, three groups at a time. Rashid’s team brings rust isolates into the lab from sunflower fields across Manitoba, and they have identified the rise of a particularly virulent strain of sunflower rust in recent years: 777.
First appearing in 2009, and comprising 23 per cent of the isolates collected at the time, “777” has seen some highs and lows. But in 2016, the race was back in abundance, comprising 64 per cent of the sunflower rust in Manitoba.
“It’s a very virulent race, and seed companies do not offer hybrids with genetic resistance to that race yet,” Rashid says.
Here are three things farmers can do.
1. Select resistant seed
Rashid says growers should pay particular attention to rust resistance when selecting hybrids for the 2017 growing season.
“Growers should look for hybrids that offer the most available resistance to sunflower rust,” Rashid says.
2. Choose the right field
Rotation, he says, also plays an important role. Rashid advises growers to avoid planting sunflowers close to last year’s sunflower fields. That’s because rust overwinters on sunflower stubble, and in spring it produces the first spores that infect seedlings. In the spore stage, rust appears as orange-coloured lesions on the plant’s cotyledons.
“Scouting is key here,” says Rashid. “If growers start seeing lesions in early spring, rust is forming. By staying away from last year’s field they’ll avoid overwintering inoculum.”
It’s important to note that even planting sunflowers a distance from last year’s crop won’t keep sunflower rust entirely at bay. Chances are good that airborne rust inoculum will still find its way to the crop, traveling by wind from the south. But that inoculum will take two to three weeks to find sunflower crops. Those extra weeks will allow the crop to have a better start than it would if it were prone to local inoculum from nearby stubble at the seedling stage.
3. Scout and (sometimes) spray
Rashid says growers should scout fields every few days. Rust spores infect the plants and produce lesions with fresh inoculum within 10 days, and the new spores produce a new flux of infection every 10 days. Growers should assess the situation and plan to apply fungicides.
However, he notes economics always factor in to any decision to spray. If the expected yield is high, and the price is high, fungicide application may save economic loss. But if rust arrives later in the season, and if the crop is otherwise healthy, fungicide may not be the best solution.
“Sunflowers have a lot of foliage — a lot of leaf area,” Rashid says. “It can have one to two per cent rust infection and still produce reasonable yield. Sclerotinia and other diseases will complicate that story, and can make the crop less likely the thrive.”
For information about fungicides that are approved for use on sunflower rust, Rashid says growers should consult the Manitoba Guide to Field Crop Protection https://www.gov.mb.ca/agriculture/crops/.