“Sunflower rust does not require an alternate host like cereal rust does.”
– KHALID RASHID
Sunflower rust cut yields and bushel weight significantly in some Manitoba fields this year.
Seventy-four per cent of the sunflower fields surveyed in Manitoba this year had rust ranging from a trace to 40 per cent of the leaf area being infected, said Khalid Rashid, research scientist, oilseed crops pathologist with Agriculture and Agri-Food Canada based in Morden.
“Those fields that have 40 per cent leaf area affected I bet yield must have dropped by 50 per cent right away,” he said.
“Rust drains the energy from a plant and the leaves dry very quickly. The field looks like it has been desiccated early.”
No fungicide control
Since there are no registered fungicides in Canada to fight the fungal disease, farmers must take other measures to protect their crops.
One is rotation. Most sunflower growers already do a good job of that, says Anastasia Kubinec, Manitoba Agriculture, Food and Rural Initiatives’ (MAFRI) oilseed specialist. But farmers need to go a step further and avoid planting sunflowers adjacent to fields that were infected the previous year, she said.
Although sunflower rust – Puccinia helianthi – can blow in from the United States, the disease can also overwinter here on sunflower residue.
Spore production and infection continuously occur, states MAFRI’s website. “This ‘repeating stage’ is the most damaging with spores being spread by wind to other fields.”
“Sunflower rust does not require an alternate host like cereal rust does,” said Rashid. “The black spores that overwinter germinate and complete their life cycle on the sunflower itself.”
It can happen on volunteer sunflowers or on the seedlings of the newly emerged crop.
Early symptoms appear as small, orange-brown spots on the upper surfaces of leaves, followed by spots appearing on the lower surfaces of leaves, the website says. A heavy infection can cause the entire leaf to die. Rust spots may also be found on the stems, petioles, bracts and back of the head.
In late July or early August, rust appears as dark-brown, dusty pustules on the surface of the leaves, leaf petioles and the back of heads. This is the most damaging stage of rust infection during the season. The black pustules of overwintering spores will be found on all plant parts towards the end of the season.
No sunflower hybrid is completely resistant to sunflower rust, Rashid said, but some offer better tolerance than others. Part of the problem is there are several races of rust.
Early infections, right up to the R6 stage (ray flowers starting to wilt), have the potential to do the most damage, Kubinec said.
It’s bad enough to lose yield, but reduced bushel weight can affect the crop’s marketability. Confectionery sunflower buyers want a relatively large “nut” once the hull has been removed. But even oilseed processors need a decent-size seed from which to extract the oil.
“I’ve heard of guys with a rust infection taking off 1,400 pounds (an acre), but their test weight was under 18 (pounds per bushel), which is really bad,” Kubinec said. “Buyers like it around 25. This year everything is low. It doesn’t matter whether you had rust or not.”
Kubinec suspects cool growing conditions may be a contributing factor.
Many of the hardest-hit sunflowers fields this year were in the southwest, but some fields in south-central Manitoba were hurt too. Rashid said he knows one farmer who estimates his yield loss at 50 per cent.
The fungicide Folicur is registered in the U. S. for sunflower rust. And Rashid is doing research into effectiveness of Headline, Folicur, Proline, Stratego and Dithane. (He expects to present some of the findings at this year’s agronomist conference.)
If sunflower rust threatens Manitoba crops next year there an emergency use registration from the pest Management Regulatory Agency (PMRA) is possible, said Jeanette Gaultier, MAFRI’s pesticides, minor use and regulatory specialist.
“The last (major) outbreak (of sunflower rust) was 2003 so it’s hard to predict right now if it will be an issue next year,” she said. “If it is a big enough issue we would go the emergency registration route. But for now we will work for a label expansion on some product to be determined.”
Gaultier will meet with farm commodity groups in March to pick pesticides they want registered through the minor use process, including perhaps, one to control sunflower rust. It usually takes several years, even for a minor use registration. [email protected]