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Managing Rust: Research Provides More Answers

Thursday, April 1, 2010
filed under: Disease

After two years of research on managing sunflower rust, North Dakota State University extension plant pathologist Sam Markell is breathing easier when recommending when and what to spray to control sunflower rust.

Markell has had a number of research plots in various locations over the last two years, with rust spores introduced in each of the plots to ensure infection. In summary, when rust shows up in the reproductive growth stages (the “R” plant development stages), it can likely be managed with one well-timed fungicide application.

Markell suggests that when rust severity approaches 3% on the upper four leaves, it’s time to pull the trigger. And if you can wait to spray until the bloom stages (R5), a second application will be unnecessary. In fact, his research shows that one application during the R5.2 to R5.5 period was as effective as three applications during other stages of development. An application at R-6 (petal drop) or later is too late and had little consequence on yield.

Sunflower growers can choose from three different fungicides with two different modes of action: Tebuconazole (Folicur® and generics), Pyraclostrobin (Headline®) and Azoxystrobin (Quadris®). Markell emphasizes that all the products will work to manage rust, but there are a few subtle differences. For example, he found that Headline is most effective in the early stages of rust onset, while Folicur is most effective in the later stages. “Headline and Quadris protect the crop from infection; but the key is to apply the fungicide soon after disease is present on the upper leaves — maybe a trace to 1 or 2%. Folicur and the generics have a bit more kickback, so they would have the edge on the other products when disease severities get to 2% or greater,” Markell reports.

The current economic threshold — that point at which a fungicide application is offset by the increased yield that comes from controlling rust — is 3% rust coverage on the plants’ upper leaves. “During the early bloom stages (R5.2-R5.5), we have seen yield increases from applications made around the 1-3% severity range on upper leaves.

After about 3% or higher, things can get out of control rather quickly if the environment is favorable for rust” Markell says. “However, if you don’t see 3% until R6 or R7, don’t worry. Fungicides have shown no yield benefit beyond the bloom stages, and it is unlikely that rust will rob any yield at that point anyhow.

“At the end of the season, rust is a great defoliant,” Markell adds.

One of growers’ more common questions is whether a fungicide and insecticide can be tank mixed and sprayed at the same time during early bloom? The answer is yes — and many producers are doing just that. Markell suggests it’s always best to check with the product manufacturers (and always follow label directions); but he is unaware of compatibility issues with any of the available insecticides.

One area where Markell and other pathologists are not comfortable in making recommendations is in early onset of rust. Generally, rust does not show up in the northern region until later in the season. However, there have been instances where rust was found in the vegetative stages in northern North Dakota. In one commercial field, the producer sprayed early and left a check strip. He saw a 1,200-lb increase in yield where a fungicide was used.

Rust tends to show up earlier in the High Plains. Colorado producer Leon Zimbelman says that there is always rust on the bottom leaves of his irrigated confections. But in 2009, the rust spread to the upper leaves — and he and his contract growers pulled the plug and sprayed fungicide, which took care of the problem. “We usually don’t see the pustules moving to the upper leaves; but 2009 was cooler than average and obviously was conducive to rust,” Zimbelman observes.

Markell and others will be looking at early infection in 2010. He has received a grant from the North Dakota Harmoniza-tion Board to conduct rust nurseries in four locations, and he’ll collaborate with CHS to create a fifth. He will inoculate rust spores in trials at Carrington and Langdon, N.D., to ensure early disease onset. Markell also will collaborate with Vision Research Park to locate a commercial field where rust is occurring early in the season in northern North Dakota, and set up a cooperative trial with Dr. Robert Harveson of the University of Nebraska-Scottsbluff. Harveson also hopes to find a commercial field with early natural infection. This two-year project will round out the issues related to fungicides and rust. “After this project is complete, we will have a good base to recommend application timing and fungicide choice in early and late onset of rust,” says Markell.

Impact of Rust on Yields

Rust is an explosive disease when conditions are right. Moisture, as in the form of dew, is necessary for the spores to multiply. A splashing rain is not needed. Temperatures between 55-85 degrees are needed as well. A typical High Plains summer day is usually above 85 degrees, which will stop the disease. Cool nights in the northern region will stop it as well.

But under the right conditions, the disease can take its toll on yields. A good example is a confection sunflower yield trial at the NDSU Carrington R & E Center in 2009. The plot was infected naturally, and no fungicides were used. Several hybrids showed good resistance — and that showed up in the yields. Given the results below, it’s apparent infections in the reproductive stages can reduce yields by half without the aid of a fungicide. — Larry Kleingartner

% Rust (Upper Plant) # Hybrids Avg. Yield
0 - 6 3 1,633 lbs
11 - 20 5 1,075 lbs
25 - 35 6 870 lbs
40 - 50 2 801 lbs
Source: ND and SD Hybrid Sunflower Performance Testing, 2009
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