Features Agronomy Diseases
Research continues to look for sunflower rust control

Rust on sunflower was widespread through Manitoba in 2008, with 74 percent of fields surveyed showing symptoms. South of Winnipeg, in the Red River Valley, severe infestations were noted. “2008 was another high rust year for sunflowers, and was similar to the 1990, 2003 and 2004 epidemics,” says Khalid Rashid, a research scientist with Agriculture and Agri-Food Canada (AAFC) at Morden, Manitoba. “The rust can cause localized losses of up to 50 percent in sunflowers.”

Fungicide application may help to control sunflower rust outbreaks. (Photo courtesy of Dr. Khalid Rashid, AAFC Morden)

Sunflower rust is caused by the fungus Puccinia helianthi, and the occurrence and severity of epidemics vary from year to year and region to region, depending on environmental conditions, races of rust, and the resistant genes in commercial hybrids. Rashid explains that there are several virulent races of the fungus, which makes the fungus much more resilient. The fungus also completes its sexual life cycle with five spore stages on the sunflower plant, which also makes managing the disease more difficult.

According to Rashid, early rust infections and subsequent epidemics are influenced by several factors. Infested stubble with the black spore stage of the fungus, which is present in the spring from the previous year, can provide a source of inoculum to young sunflower plants. Susceptible volunteer plants in or around last year’s crop can also provide ample supply of primary inoculum. High humidity overnight and short periods of light rain at each cycle of the infection in the brown spore stage can help to raise the infestation levels. The rust also thrives in temperatures between 10 and 30 degrees C. “In other words, the rust does well in the environmental conditions typical of southern Manitoba where sunflowers are grown,” says Rashid.

Shifting races make plant breeding difficult
With several races present in rust infections, Rashid says that plant breeders have to work to develop sunflower hybrids with multiple race resistance. He believes that the prevalent races have changed in Manitoba during the last five to eight years. “Unfortunately, the picture isn’t as simple as a few races and developing genetic resistance to them. There are prevalent races within the major race lines, so even year to year, there can be differences in the races within Race 300, for example,” says Rashid.

Effects of Fungicides on Sunflower Yield, 2008. (Courtesy of Dr. Khalid Rashid, AAFC Morden.)

Effects of Rust and Fungicides on Kernel Density (g/L), 2008 (Courtesy of Dr. Khalid Rashid, AAFC Morden)

In 2003, Race 100 accounted for 12 percent of the prevalent types, Race 300 for six percent and Race 700 for 82 percent. In 2008, no Race 100 was found in the survey (down from the mid-80s through 2005-07), while Race 300 (336, 326, 320, 324, 337) accounted for 62 percent of infections and 38 percent of Race 700 (736, 724, 726, 734).  Race 500 also showed up at low levels in 2006 and 2007, but not in 2008. 

Commercially, some hybrids have resistance to some races, but not all of the races. Unfortunately, because the races change from year to year, the best that the growers can do is to select a hybrid with a combination of resistance and good agronomic features. 

Rashid cites the case of nine different sunflower differential lines used for variety development. A differential line like HA-R3 has proven to be resistant to only two of four Race 700 races.  None of the differential lines were resistant to race 777, and only a few to race 737, 736, and 774.  “Genetic resistance will only provide part of the answer,” says Rashid.

Fungicide research investigates benefits
With a lack of genetic resistance to some races, foliar application is the next option that growers may consider down the road. While fungicides are registered in the US for control of rust on sunflower, only Headline, which received emergency registration in July 2009 is registered for that use in Canada. This emergency use registration was made possible through the co-operation of the National Sunflower Association of Canada, Manitoba Agriculture, Food and Rural Initiatives and BASF Canada.

Rashid is part of a group, including the National Sunflower Association of Canada, and several private companies that are working on a Minor Use registration for fungicide application on sunflower rust. He recently completed a four-year study using a confection sunflower hybrid that compared 11 fungicides with three application treat ments each. The application timings were one early application at late flowering; one late application two weeks later; and two applications, early and late. 

Observations included leaf area infected at 10-day intervals, stem area infected at the end of the season, green leaves at the end of the season, kernel density, kernel weight and yield. 

Generally, all fungicides reduced rust infection and severity, but not all provided a yield or quality advantage.  Proline, Folicur, Headline, Dithane and Stratego all reduced the disease level index by 40 to 50 percent while providing a 10 to 20 percent yield improvement. Bravo, Tilt and Lance reduced the disease index up by 30 percent, but had no significant improvement in yield. 

For disease reduction, Rashid found that two applications were better than one application, and two applications were slight better for improving yield. The effectiveness of early and/or late applications varied between years depending on the earliness of the rust infection and disease development. Generally, early application at late flowering was the most effective when there was an onset of rust on the middle leaves. 

For kernel density, which is important in the confectionary industry, similar results were observed in the disease and yield responses. There was not much difference between treatments on kernel weight.

While Canadian sunflower growers wait for more fungi-cide registrations, Rashid says that they must rely on agronomic management to help avoid the most damaging effects of rust. This includes using hybrids with the best rust resistance, rotating away from last year’s rust-infested fields, and controlling volunteers in and adjacent to last year’s infested fields in ditches and on fields.