NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665

March 4, 1999

Wheat Producers Alerted to Implications of Management Decisions

Fusarium head blight (scab) has been the No. 1 enemy of this region's wheat producers for much of the 1990s, but researchers at North Dakota State University and with USDA's Agricultural Research Service say wheat growers should not totally disregard another nemesis: leaf rust.

The good news is that currently available fungicides can aid producers in this two-front war. But as producers prepare to do battle against diseases in 1999, they should consider all the implications of their management decisions.

"There's no doubt that we've had more rust in the past few years than we've had in many years, perhaps in decades in some areas," says Jack Rasmussen, an NDSU plant pathologist.

Wheat leaf rust is caused by a fungus whose spores overwinter in the South and move northward on wind currents. Rasmussen says rust levels in states like Texas, Kansas and Nebraska are good indicators of the rust potential for North Dakota. The rust fungal spores typically begin infecting spring wheat leaves in southern North Dakota in late May or early June. The disease recycles every 10 days in the field and a heavy build-up can occur on susceptible varieties. Durum varieties are generally quite resistant to leaf rust.

Part of Rasmussen's research involves monitoring commercial wheat varieties in test plots and in fields to determine rust severity during the production season. He also evaluates experimental lines for leaf rust resistance or susceptibility.

In cooperation with Jim Miller, a USDA-ARS plant pathologist in Fargo, the North Dakota Agricultural Experiment Station operates rust nurseries for spring wheat near Fargo, Carrington and Langdon. Rasmussen says the Carrington nursery had high rust levels in 1998, but there was less rust in Langdon where temperatures were cooler.

Six to eight hours of dew on leaves and temperatures ranging from 60 F to 80 F are the ideal conditions for leaf rust development, Rasmussen says. Under favorable conditions, spores germinate and penetrate into leaves. Flag leaf severity on susceptible varieties may range from 40 percent to nearly 100 percent, with corresponding yield losses of up to 30 percent for severely infected susceptible varieties. Leaf rust also causes reductions in test weight.

Rasmussen says there are wheat genes that protect adult plants from leaf rust, but because these genes are expressed predominantly in adult wheat plants, it has been difficult for geneticists to tag and map these genes. As a result, researchers can't say with certainty which wheat cultivars possess these protective genes, but current NDSU varieties probably carry two such genes, Lr13 and Lr34.

It seems unlikely that 2375 and AC Barrie carry those two genes, Rasmussen says. Unfortunately, 2375 and AC Barrie are popular with North Dakota wheat producers because both cultivars possess more scab tolerance than other commercial varieties.

"There have been no real major changes in the race populations this past year. It's just that some of the cultivars producers are planting are more susceptible to leaf rust," says Dave Long, a plant pathologist at the USDA-ARS Cereal Disease Laboratory in St. Paul.

Long analyzes the rust samples coming to the St. Paul lab for race determination, and he also prepares the rust loss report for the United States. Long believes adult plant resistance in spring wheat varieties is one of the reasons why leaf rust is in check and yield losses have been low. For 1998, leaf rust destroyed about 2.5 percent of the Kansas wheat crop. In North Dakota, the loss was about 1 percent, although losses were more severe in some areas.

Wheat leaf rust was a problem for many producers in northeastern North Dakota last year. Particularly heavy infestations occurred in Walsh and Ramsey counties and in the northern half of Nelson County, says Terry Gregoire, area extension specialist for cropping systems in Devils Lake. Fortunately, fungicides treatments often are cost effective, he says.

"We lost as much yield to leaf diseases as we did to scab last year, probably more, and 2375 was the worst for rust. But we saw some leaf rust on AC Barrie also," says Brad Brummond, NDSU extension agent for cropping systems in Walsh County.

In 1998, the U.S. Environmental Protection Agency approved the North Dakota Department of Agriculture's request for a specific exemption to use Folicur 3.6F in a single application for controlling scab in wheat and barley. The EPA has yet to decide whether to clear Folicur for scab on wheat and barley this year. However, the state has submitted a request to the EPA for a Section 18 crisis exemption to use Folicur in 1999 for scab, says Marcia McMullen, NDSU extension plant pathologist. This fungicide, as well as other registered products, helps control leaf rust.

"The control of leaf rust is a side benefit of treating for scab," Rasmussen says. Looking to the near-term future, he says two NDSU experimental spring wheat lines with enhanced scab tolerance also appear to have more leaf-rust resistance than either 2375 or AC Barrie.

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Source: Jack Rasmussen (701) 231-1027

Editor: Dean Hulse (701) 231-6136