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Reining in Rust

Monday, December 1, 2008
filed under: Disease

Sunflower rust (Puccinia helianthi) garnered quite a bit of publicity in 2008 — partly because of its widespread incidence and partly due to the unusually early date at which the disease first appeared in certain locales.

The annual National Sunflower Association crop survey, conducted in September, found evidence of rust in 62% of the fields surveyed this year across six states and Manitoba. While that’s obviously a substantial percentage, the 2008 level actually represents a slight decline in an “inoculum growth trend” that’s been evolving the past several years. The 2007 crop survey found incidence of rust in 77% of the examined fields; the 2006 survey in 68% of the fields. The 2005 survey found rust in 60% of fields.

In North Dakota, sunflower rust typically is not observed until late July or early August. This year, however, a north central North Dakota crop consultant, Mike Hutter, found it in a commercial confection field in latter June. By early July, rust had been observed in several of the state’s sunflower-producing counties.

Sam Markell, extension plant pathologist with North Dakota State University, says Hutter’s abnormally early discovery was of the yellow-orange “aecial” pustule stage of the disease. Aeciospores spread via wind to other sunflower plants, where they initiate formation of the familiar cinnamon-brown “uredial” pustules. These uredial pustules are the form that growers and other scouts commonly find on infected plants.

Since rust is a disease for which there can be multiple cycles of spore production and plant infection, the earlier in the season aeciospores start showing up, the more potential there is for economic damage. That’s where timely scouting and fungicide applications become particularly important. In the case of the north central North Dakota field scouted by Mike Hutter, the grower sprayed it twice — once with Folicur® and once with Headline® — with good results. Had he not sprayed, yield loss likely would have been quite significant.

Many Northern Plains growers sprayed for rust this year — some once, some twice, occasionally even three times. Still others did not spray; and where rust severity was beyond the economic threshold, yields did suffer accordingly.

To gain additional insight into fungicide efficacy on rust, as well as optimum application timing, NDSU’s Markell conducted chemical and timing trials at three North Dakota locations this past season: Casselton, Carrington and Langdon.

As of 2008, three fungicides were labeled for rust control in sunflower. Two of them — Headline and Quadris® — are strobilurins. The third fungicide, Folicur, is a tebuconazole that had a Section 18 label for North Dakota as of late July and then received a full Section 3 label in August. Markell included Headline and Quadris in his trials, as well as a generic version (Tebuzol®) of Folicur. He also included yet-unregistered-on-sunflower Proline® and Prosaro® as well as an experimental fungicide.

In the chemical efficacy portion of the study, Markell’s objective was to compare the products to see how well they controlled rust. Having inoculated all the plots, he waited until rust appeared before spraying at standard recommended rates. He then evaluated rust severity at three two-week intervals (corresponding approximately to the R-5.5 (about 60% flowering), R-7 (back of plant head starting to turn a pale yellow) and R-9 (physiological maturity) stages of sunflower development. There was no second fungicide treatment.

As of the first evaluation date, rust severity was so minor at all three locations that Markell did not assign ratings. Severity started to pick up by the time of the second evaluation, but it was still below recommended levels for spraying (3% severity level). Notable differences were apparent by the time of the third evaluation, however. “At the Casselton site, the untreated control just got hammered,” with a mean disease severity of 20%, Markell reports “Headline and Quadris both had some measure of control; but the amount of disease was less with Tebuzol.”

Though the overall levels of rust were lower, test results at Carrington were fairly similar to Casselton. Both Headline and Quadris provided a significant reduction in rust severity, but Tebuzol outperformed the strobilurins. The disease simply did not develop this year at the Langdon test site, so severity ratings were very low and fungicide impact was generally similar. (It should be noted that Proline and Prosaro did as well as tebuconazole at both Casselton and Carrington. Again, however, neither of these two fungicides is presently labeled on sunflower.)

When is the optimum time to apply fungicides for the control of rust?

The easy answer is “early on.” But it’s not quite that simple, Markell points out. At both Casselton and Carrington this year, “Tebuzol gave the best control when applied right after we started seeing pustules,” he says. “But nine ounces of Headline, if put on before the pustules actually formed, gave — as a preventative — as good or better control.

“So if you’re pretty sure rust is coming, if you get Headline on at the right time you probably will fare better. But if you already have rust in your field, Tebuzol/Folicur is going to give you a little better control.”

Markell says the different chemistries of strobilurins and tebuconazoles largely explain the situations in which one group of fungicides tends to perform better than the other. The strobilurins are very effective as preventative treatments, i.e., if the rust has not established itself in leaves, they will prevent the disease from doing so. The tebuconazoles, by comparison, tend to be more effective at halting rust damage after infection has occurred.

One benefit stemming from a rust fungicide application during the flowering/ early bloom plant stage is that this also is the time when growers often treat for the seed weevil and/or banded moth. The rust fungicides can be tank mixed with the appropriate insecticide, thus getting more mileage for the application dollar.

Looking ahead to 2009, Markell says sunflower growers in his state should be alert for rust due to the large amount of inoculum that will likely carry over from 2008. “We are going to have a lot of inoculum out there, and it’s going to be widespread,” he advises. “You need to be out there scouting, because rust can creep up on you quickly.”

And don’t become complacent if you’re in a dry spell. “One thing that makes rust different from other diseases,” the NDSU plant pathologist points out, “is that you can have rust epidemics in drought years. Rust only needs dew; it doesn’t need rain. Its spores are blown by wind, not splashed.” Warm temperatures often speed up the disease’s development.

“Early infections are what cause problems,” Markell continues. “When rust comes in midway through the season, a single application of fungicide can manage it. But when it shows up earlier, you may need an additional treatment. Rust infection appearing a few weeks after flowering likely will not cause economic damage”

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. “If you see rust at 2 or 3% severity, a single application will knock it down,” Markell says.

For many growers, hybrid selection is just as important as timely scouting and spraying. There are significant differences in levels of tolerance or resistance to rust among current commercial hybrids. In a 2007 hybrid screening trial conducted by USDA-ARS research plant pathologist Tom Gulya, 18 of the 86 entries were classified as “resistant” to rust. The remaining hybrids displayed varying degrees of tolerance or susceptibility. Growers who may be planting sunflower next year in a field near where there was a 2008 ’flower field that was impacted by rust should definitely take a hard look at those hybrids that offer rust resistance, Markell says.

— Don Lilleboe

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