Panhandle Perspectives - Sept. 6, 2016

Stripe Rust: Summary and planned research

Robert M. Harveson, Extension Plant Pathologist, Panhandle R&E Center, Scottsbluff

Author’s note:  This article will summarize the stripe rust story that has been the subject of the Panhandle Perspectives column for the last several weeks. Other portions in this series have described the disease’s history and geographic distribution, biology and life cycle, reasons for recent severe epidemics, and lastly disease management options.

Panhandle Perspectives
Bob Harveson

Stripe rust is a serious disease of wheat worldwide, and caused by the fungal pathogen (Puccinia striiformis f. sp. tritici). Over the last decade, the incidence and severity due to stripe rust has increased dramatically. In fact, damage to wheat production has been so widespread and severe in Nebraska that it appears that this disease has now displaced the virus disease wheat steak mosaic as the most important and economically damaging disease of wheat in the state.

The disease has occurred historically wherever wheat is grown under cool, moist environmental conditions during the season. Thus it has been regarded as a low temperature disease and problematic only during cool weather.

Since 2000, the disease has emerged to consistently induce severe economic losses in Nebraska under very warm conditions that were previously thought to be impossible. Prior to this date, it was primarily restricted to the cool and damp Pacific Northwest and California. After 2000, it began to appear nationwide on all forms of wheat, causing significant losses in new locations where it rarely ever was present – including more than 20 states from coast to coast and throughout the entire Great Plains from Texas to North Dakota.

The formation of new pathogen races is being proposed now to explain the high levels of yield loss and widespread nature of its recent appearance. These races have been demonstrated to cause disease more rapidly and be more aggressive at higher temperatures than previously observed.

Furthermore, they have seemingly displaced the old isolates across the expanded geographic ranges, suggesting that the new isolates were more fit than older ones. In doing so, they have additionally overcome the most effective genetic resistance used in the United States wheat crop, converting previously resistant cultivars to susceptible ones.

These remarkable changes indicate that the old pathogen races occurring in cooler climates (before 2000) are distinct from the invasive new isolates occurring in warmer climates after 2000. This suggests that the “new” races were likely recently introduced into the United States from a still-unknown location rather than arising as a genetic modification (mutation) from older existing, native isolates.

Managing this disease will require several new concepts. First we need to realize now that this pathogen is now capable of causing disease under conditions not previously considered. We also need to accept the fact that it will take an integrated approach to successfully control stripe rust, consisting of multiple control measures, including both fungicide applications and use of resistant cultivars (see Panhandle Perspectives – Sept. 4, 2016).

New Research Planned

Due to the severe stripe rust epidemics in Nebraska wheat crops over the last decade, we intend to begin studies in 2016-17 that attempt to help producers better manage this disease. As mentioned in last week’s article, the first step is to accept the fact that this disease appears to be an endemic problem that we will have to consistently face each year now.

Thus we need to establish a system that will predict when the disease may be more problematic and base the management decisions on this model. This model will be referred to tentatively as the Stripe Rust Disease Management Decision Model.

Although not a true disease forecasting model, it will allow growers to estimate disease potential in their own fields based on a series of factors which need to be present for disease to cause damage. These factors, among several others, include level of genetic resistance in cultivars, crop stage at time of infection, 10- to 14-day weather forecast, time of the season infection occurs, incidence of disease in fields, and yield expectations/inputs in crop.

These studies will be conducted at several sites throughout the Panhandle and with additional sites in North Dakota. The idea is to estimate the risk of disease development based on the factors mentioned above occurring at each site, and make fungicide applications based on the model. We will begin this fall with a resistant and susceptible variety planted at each site and will compare several fungicide treatments (first sign of disease, first appearance of flag leaf) with an application based on the predictive model’s characteristics. These studies will be done in collaboration with Dipak Santra (UNL alternative crops breeding specialist), and Andrew Friskop (North Dakota State University extension plant pathologist).