Environment, genetics play leading roles ENVIRONMENTAL FACTORS and production practices common throughout a 2,400-mile long swath from Texas into the Canadian provinces create an ideal pathway for plant diseases to move through the U.S. wheat belt.

"Much of the wheat grown throughout the Great Plains is genetically similar," says Texas Experiment Station plant breeder David Marshall, Dallas, "and that provides an ideal situation for diseases to travel."

Marshall discussed the potential for a wheat rust epidemic recently at the Ag Technology Conference, sponsored by Texas A&M-Commerce.

Marshall says variety selection and production practices contribute to the spread of rust diseases from the southern tip of the Great Plains all the way into Canada.

"We plant from five to eight million acres of wheat per year in Texas," Marshall says. "Ninety percent is hard red winter wheat, and 10 percent is soft wheat. Also, 40 percent of the state's wheat crop is grazed to some extent. These are contributing factors to rust infection."

Marshall says forage requires early planting. "The climate most years is conducive to rust infection early in the fall," he says. "Forage wheat may serve as a rust incubator for the rest of the Great Plains.

"Also, large areas of genetically similar varieties increase the potential for a rust epidemic throughout the wheat belt."

The nature of the diseases also contributes. "Rusts have high reproductive rates," Marshall says. "Spores spread by wind, and favorable environmental factors allow for infection and re-infection."

Three rust diseases occur worldwide: leaf, stem and stripe.

"Stem rust is potentially the most destructive," he says. "Its pathogens have a low level of variability. Stem rust also is more active at higher temperatures. Control tactics depend on host resistance and early maturity."

Leaf rust, he says, is the most widespread of the three. "Leaf rust pathogens have a high level of variability. We may see 40 to 45 races in Texas compared to only three or four for stem rust.

"Leaf rust is more active at moderate temperatures and can be managed with host plant resistance and fungicides."

Stripe rust is not a common problem for Texas growers but did show up in some North Texas fields last year. "This is mostly a Northwest disease. It likes cooler temperatures, and pathogens have moderate to low levels of variability. Management includes host resistance and fungicides."

Marshall says plant breeders hope to detect varieties resistant to all three rust diseases. In the meantime, he recommends cultural practices to minimize damage.

"Early planting of susceptible varieties typically results in fall infestations," he says. "Secondary tillers may be reduced. Rust pathogens also can live a long time."

He sys stripe and leaf rust will overwinter more readily than stem rust. "The main factor for spring infection is whether rust overwintered," he says. "We're also getting more rust infection in Texas than we once thought from Mexico's Highland Plateau."

Keys to management include: - Diversify varieties.

- Plant tolerant or resistant varieties.

- Plant as late as possible.

- Eliminate volunteer wheat.

"We're trying to develop both hard and soft wheat varieties with resistance to rust. We hope to develop varieties with multiple and durable resistance," Marshall says.