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Watch for sunflower diseases

Farmers interested in planting sunflowers in 2014 should be ready to protect them from sclerotinia head rot and sunflower rust

Sunflowers are generating a great deal of interest for 2014 planting. “Interest in sunflowers is very high this winter,” says Anastasia Kubinec, oilseeds business development specialist with Manitoba Agriculture, Food and Rural Development. “It’s price driven and contracts are being scooped up very quickly.”

About 90 per cent of Canada’s sunflower acres are in Manitoba, but more and more growers are experimenting with them in southern Alberta, southeastern Saskatchewan and southern Ontario. Sunflowers are grown for two markets. Confection types are grown for roasted snack foods in the shell or dehulled for the baking industry. These make up about 65 per cent of sunflowers grown. The rest are grown for oil.

There are two diseases of particular concern for sunflower growers: sclerotinia head rot and sunflower rust. There are others, but of lesser significance when it comes to yield and quality losses and control options.

Sclerotinia head rot

This is considered the most important disease of sunflowers and causes both yield and quality losses. Not unlike sclerotinia diseases in other crops like canola, dry beans or peas, sclerotinia head rot is dependent on environmental conditions during the flowering to harvesting period. Airborne spores cause the disease.

sunflower head rot

This photo shows sunflower head rot at an advanced stage, in a case of complete loss. Head rot causes both yield and quality loss. Photo: MAFRD
photo: MAFRD

If conditions are conducive, sclerotia in the soil germinate to produce apothecia or fruiting bodies, exactly as with other crops. The apothecia release spores which will infect the head. Infection is favoured by rainfall before (to encourage the sclerotia to germinate) and after (to provide a suitable environment for the spores) flowering. The spores land on the sunflower head and grow amid dead florets and pollen. After several weeks, disease symptoms become visible on the head.

The first symptoms appear as water-soaked spots or bleached spots on the back of the head, which is fleshy and moist, providing a perfect medium for the disease to flourish. If the disease progresses, it can destroy the entire head causing the seed layer to fall away altogether. If the seeds are not lost entirely, seeds from infected heads will be lighter, there will be fewer seeds and oil content is reduced. Shattering is also greater during harvest, causing further yield losses.

There are fungicides registered for use in sunflowers to prevent sclerotinia head rot, but they won’t offer complete control. The key to controlling this disease is to follow a crop rotation that reduces the incidence of sclerotia in the field. There should be at least five years between susceptible crops. Additionally, sunflowers should not be planted near fields where sclerotinia was a problem in the prior year — infection occurs from spores that are easily carried on the wind. Finally, planting the field to maximize air circulation is helpful in not providing an environment conducive for the disease to establish.

Sunflower rust

Sunflower rust is a devastating disease capable of up to 50 per cent yield losses. Severe infections can lead to smaller heads and reduced seed size, oil content and yield. Sunflower rust comes in cycles. The last cycle in Manitoba ran from 2007 to 2009. It has not been an issue since then.

sunflower rust on a leaf

This photo shows sunflower rust symptoms on the back of a leaf. If the symptoms are severe enough, the leaf will die. Photo: MAFRD
photo: MAFRD

Sunflower rust is caused by Puccinia helianthi, which is constantly changing races over time. Rust inoculum builds up on infested stubble from previous years. Given the right susceptible volunteer plants in and around the previous crop, infection can continue, with spore production and infection repeating continuously as long as conditions are suitable. If conditions continue to be ideal with short, light rainfall events and warm temperatures, infections can become severe. Some years, rust will also blow in from the south.

Early symptoms of sunflower rust appear as small, orange-to-brown spots on the upper surfaces of leaves, followed by spots on the lower surfaces of leaves. If the infection is severe enough, the entire leaf dies. Later in the season rust spots can be found on stems, petioles, bracts and the back of the heads. When the spots present as dark-brown dusty pustules, this is the most damaging stage of the infections. By the end of the season black pustules containing over-wintering spores will be found on all plant parts.

The long-term goal is to breed in resistance into sunflowers to protect against rust. Some commercial varieties currently have good-to-excellent resistance to some races, but not all. There are no fungicides registered to control rust in sunflower in Canada. Other control measures include keeping sunflowers removed from previously infected fields and keeping volunteers under control in fields and ditches.

Registered products

Three products are registered in Western Canada for the control of head rot on sunflowers. All three will provide suppression only, and won’t completely eliminate the disease.

  •  Contans (UAP): This product is a biological control that contains living fungi. It’s applied to the soil pre-seeding and post harvest. The fungi infect the sclerotia of Sclerotinia sclerotiorium and S. minor in the soil. It’s recommended that farmers use other management practices along with Contans for better control. UAP says using Contans as part of a long-term strategy will help improve disease control.
  •  Lance (BASF): Lance is a Group 7 foliar fungicide with medium resistance risk. Its active ingredient is boscalid, a carboxamide. This product can be applied to sunflowers once per season.
  •  Vertisan (DuPont): Vertisan is a Group 7 foliar fungicide with medium resistance risk. Its active ingredient is penthiopyrad, a carboxamide. Vertisan should not be applied more than twice in a row before switching to a different mode of action.

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