High-Temperature Adult-Plant Resistance: How Warm is Warm Enough?

High-temperature adult-plant resistance, frequently referred to as HTAP resistance, is used widely in PNW winter and spring wheat varieties to help reduce the impact of stripe rust. This time of year, when we’re transitioning into warmer spring temperatures, is when I often get the question “how warm does it need to be for HTAP resistance to kick-in”?  It’s an important question that can impact management decisions, such as whether to make another fungicide application.

Unfortunately, I haven’t had a good answer to that question, and the scientific literature is not a lot of help either. So I recently sat down and discussed this topic with our resident stripe rust expert, USDA-ARS scientist Dr. Xianming Chen, to help clarify the situation. It turns out not to be a simple answer because there is no single temperature at which HTAP becomes active.  HTAP resistance is not like a light that is on or off – rather, more like one with a dimmer switch where it increases gradually.

In the discussion below, I go into the nitty-gritty of HTAP resistance. However, the short answer to the question above is that temperatures close to or above our area averages allow HTAP resistance to be effective against stripe rust and below-average conditions reduce the effectiveness of HTAP resistance. In general, when wheat plants are near or beginning to joint, it takes nighttime temperatures in the 50’s and daytime in the 70’s for HTAP resistance to become active. Nighttime temperature is just as important as the daytime, and HTAP resistance may be delayed or reduced if days are warm enough but nights are too cool. HTAP resistance also can decrease if temperatures cool-off after it becomes active. These temperatures are also good for stripe rust, which is favored by temperatures from 46° to 54°F for infection and 50° to 75°F for disease development.

However, this scenario varies considerably depending on the genes a particular variety contains. There are more than 30 genes for HTAP resistance in PNW wheat varieties; some varieties have one or two genes, whereas others have five or more.  Some HTAP resistance genes are more sensitive to growth stage, some to temperature, and still others to both growth stage and temperature. In addition, some genes provide more effective resistance than others. Varieties like Madsen with very good resistance have a combination of genes that allows resistance to become effective early and be less sensitive to changes in temperature.

To understand HTAP resistance, we also need to understand all-stage resistance, also called “seedling” resistance.  All-stage resistance genes are expressed throughout the life of the plant but are most apparent in seedlings when HTAP resistance is not expressed.  All-stage resistance is not usually influenced by growth stage or temperature, but it is influenced by races of the stripe rust fungus. Varieties with only all-stage resistance are very resistant or very susceptible throughout the growing season, depending on the pathogen races present. In contrast, wheat varieties with only HTAP resistance are susceptible as seedlings but become resistant in later stages and are not influenced by races.

So what are races and why do they matter? A race is one type of pathogenic specialization that occurs in plant pathogens. In the case of the stripe rust fungus, each race is capable of infecting some but not all wheat varieties, depending upon which resistance genes the variety contains.  Another way to think about this is that some resistance genes are effective only against some races (specific resistance), whereas others are effective against all races (non-specific resistance).

You might think that all-stage resistance is ideal because it is highly effective throughout the life of the plant. Unfortunately, pathogens have the ability to change and new races can and do emerge that overcome all-stage resistance genes, rendering the plant susceptible. One way to deal with this problem is to develop varieties that contain a combination of genes that are specific and non-specific; most PNW wheat cultivars contain both all-stage and HTAP resistance genes.  Although HTAP resistance is stable and effective against all races, it is not as effective as all-stage resistance; that is, some disease may develop on resistant plants.
Some new varieties have combined effective all-stage and HTAP resistance.  For example, spring wheat Seahawk has all-stage resistance genes Yr5 and Yr15, which are effective against all stripe rust races identified in the United States so far, and the HTAP resistance gene from Alpowa.  Spring wheat variety Expresso has combined HTAP resistance genes from Express with the all-stage resistance gene Yr15.  These combinations provide effective stripe rust resistance throughout the life of the plant.

So back to the question of “how warm does it need to be for HTAP to kick-in” – it depends on the variety and the genes it contains. And this is the main reason that breeding for stripe rust resistance, as well as resistance to other diseases, is ongoing – trying to find those magical combinations of genes that result in varieties with very good resistance and stay one step ahead of changes in the pathogen.

More information on stripe rust, including photos showing rust percentages and a table listing fungicides with their active ingredients, use rates, and growth stage restrictions, is available under the Foliar Fungal Diseases in the Disease Resources section of the WSU Wheat and Small Grains website. Additional information is also available on Dr. Chen’s website.

Washington State University