Stripe rust disease viruses of wheat currently cause an estimated 2 percent to 25 percent yield loss every year in California wheat production. An average of 2 percent in yield loss statewide was documented in 1998, 2001, 2007 and 2008, according to Lee Jackson, CE specialist, small grains, UC Davis. That figure jumped to 15 percent in 2006 and skyrocketed to 25 percent in 2003. The really disturbing trend is that until 1998, only trace amounts of stripe rust were even seen in California.
Three rust diseases, stripe rust, leaf rust and stem rust, occur on wheat. Rust infections reduce plant vigor, root growth and grain filling, resulting in fewer and smaller kernels. Stripe rust — Puccinia striiformis — is the most significant in California. It infects plants earlier in the season than other rusts because it can survive at lower temperatures. A year-round source of host plants such as volunteer wheat, barley and wild grasses compounds the problem and ensures continuity from one season to the next.
A grower's best defense is to plant a resistant variety. Under the most severe disease pressure, varieties or cultivars that are “highly susceptible” to rust stripe can sustain 75 percent or more in yield loss, according to Jackson. Those that are merely “susceptible” can still suffer 50 percent to 70 percent or more yield loss. While resistant varieties are key, plant breeders are having difficulty staying ahead of the curve. Rapidly mutating races of stripe rust pathogens are throwing the entire industry for a loop.
Bottom line: don't rely totally on a resistant cultivar to protect you from stripe rust. The reason is simple, yet complex. It starts with the rapid development of the disease.
“From the time a spore lands on a leaf, to the process of germination, penetration, infection, production of new spore living pustules, and dissemination of new spores — we're talking 7-10 days,” Jackson says. “If the weather is conducive, there are always new spores being produced. Making matters worse, the organism has a potential for a 10,000-fold increase per spore generation. Those are amazing numbers.”
Ultimately, that just means the pathogen can quickly mutate into races that overcome any resistant variety that breeders can throw at the pathogen.
“These new races that overcome the resistance of the new cultivars, are really the crux of the problem,” Jackson says. “We've seen 6-17 races every year in California since 2000, and 137 races have been detected in the U.S. as of the end of the 2007 season. There will be more as we get 2008 data. The pathogen is extremely diverse.”
Up until 1998, there were not many problems with rust stripe in California. “Varieties such as Yecora Rojo and Anza lasted for 20 years after they were first released,” Jackson says. “Something happened fundamentally in the pathogen population that led to the serious epidemics that we've seen not only in California, but also across the country beginning in about 2000.”
One of the fundamental problems in dealing with rust stripe disease in California is geography. “The Central Valley is one large mixing basin for spores,” Jackson says. “Spores produced in one end of the Valley get caught up in prevailing winds and weather systems and transported the length and breadth of the Valley. When they're deposited, they can infect plants as long as conducive weather conditions exist.”
Since the Central Valley operates as a unified air basin, any stripe rust infection of any variety by any particular race of the disease puts other regions at risk.
“That's why it's not a good idea to think you're protected down in the southern part of the state just because you haven't experienced what's infecting the Sacramento Valley yet,” Jackson says. “It can easily be transported down south.”
Breeders are working hard to identify new sources of cultivar resistance. They're looking for genes that are more durable, longer lasting and less likely to be overcome by the pathogen than the genes that are currently in play. “We're also trying to put together combinations of genes and develop new cultivars that will delay the pathogens' ability to overcome resistance,” Jackson says.
Currently in the Sacramento Valley, the best bets for varieties resistant to stripe rust are Patwin, Expresso, Blanca Royale and Blanca Fuerte, according to Doug Munier, University of California Cooperative Extension farm advisor in Butte, Glen and Tehama counties.
However, even resistant varieties are not much of a guarantee. Vigilance is still the best defense both short-term and long-term. Chemical control is an option, but application timing is critical. The goal is to protect the flag leaf from infection and to protect the plant during the grain-fill period.
“Since disease can increase quickly and become quite severe it's very important for growers to detect infections as early as possible in the growing season so they can do something about it,” Jackson says. “We have a lot of fungicides that are very effective, but if disease becomes severe before fungicides are applied, the effectiveness is compromised.”