20 May 2005
Stripe rust follow-up
By John W. Burns, WSU Extension Agronomist
The following is a collective summary of comments/considerations from X. Chen, USDA/ARS, Plant Pathologist, and K. Kidwell, WSU Spring wheat breeder and K. Campbell, Wheat Geneticist, USDA/ARS during a recent meeting on May 18, 2005 to discuss the current stripe rust situation in eastern Washington. We realize every field situation is different and it is very important to look at each field on a case by case basis, particularly in making fungicide treatment applications.
OVERVIEW: This is a unique year. At this point pathologists think we are dealing with existing races of stripe rust and not new races. Historically, the source of stripe rust for eastern Washington originates in northern California and the Willamette Valley of western Oregon. Both of these regions have had heavy infections in recent years, creating a vast innoculum load that is dispersed via the wind into eastern Washington. In addition, due to the early fall seeding dates in central Washington and the later maturity dates of spring wheat in eastern Washington and northern Idaho, host plants are available for the pathogen nearly 12 months of the year. Another feature to consider has been the weather. We have had very mild winters and relative wet springs, a perfect combination for stripe rust development. Separately, these factors could be considered a problem, but collectively, as in this year, a crisis may be emerging.
WINTER WHEAT: In commercial fields, winter wheat crop is generally in good shape and does not need to be sprayed, except for a few fields where stripe rust has developed to 5 to 10% severity, which need to be sprayed. X. Chen, 18 May 2005.
Generally the majority of soft white winter wheat cultivars have enough High Temperature, Adult- Plant (HTAP) resistance to stripe rust. This resistance is provided by a number of different genes and expresses resistance like the name implies (high temperatures (high 70’s day/50’s night and adult (when plants begin stem elongation – booting). In contrast, a majority of HRW wheat varieties are very susceptible to stripe rust (such as Hatton). The WSCIA Seed Buying Guides for winter and spring wheat provide a good snapshot of variety resistance to stripe rust. With such heavy rust pressure this year, varieties such as Eltan that are listed as MS (moderately susceptible) are showing some stripe rust appearing on leaves. In general, the level and severity of rust will not warrant a fungicide application and the HTAP resistance will ‘kick-in’ to provide protection.
SPRING WHEAT: Stripe rust is very common in spring wheat fields, mostly less than 2% severity and on the first leaves from the ground surface. The earlier the field was planted, the more leaves that are showing rust. X. Chen, 18 May 2005.
There is a general consensus of opinion that spring wheat crops in the seedling stage are all vulnerable to stripe rust this year. Most of the major spring wheat and barley cultivars have some level of high temperature, adult-plant resistance (HTAP); however, even these varieties may not have enough seedling resistance to overcome the high number of different races of stripe rust that are present. Alpowa spring wheat is probably the best example of a variety with good HTAP but no seedling resistance against predominant races.
If a field has 5 to 10% stripe rust severity or incidence, it should be sprayed, unless you know 90-95% of plants in the single variety are resistant. X. Chen, 18 May 2005. With spring wheat we are dealing with small plants that have a much more difficult time in overcoming stripe rust damage than larger winter wheat plants.
Currently, winter wheat is passed the seedling stage and we only need to worry about the adult resistance or HTAP, whereas with the spring wheat, the concern is the ability to identify stripe rust at the seedling stage, estimate the infection/percentage, and determine if a fungicide treatment is required. Two photos below show what plant pathologists use to record stripe rust infection type for winter and spring wheat. The scale for both winter and spring is the same: 1 (GOOD - no rust or symptom) to 9 (BAD - abundant rust). Generally stripe rust found on leaves that are in the 1-3 type would not require a fungicide treatment (resistant). 4-6 types might require a single treatment (intermediate) and 7-9 types are extremely severe (susceptible).
HOWEVER, it is important to note that the symptoms in the photos look different on adult wheat plant leaves Fig. 1 and wheat seedlings (Fig 2). Wheat seedlings do not show as vivid stripe rust streaks. Severe infection on seedlings may simply appear as chlorotic blotches with a few rust pustules. We cannot underestimate how important it is to recognize that diagnosis for adult and seedling stripe rust severity based on blotches, stripes and sporulation is different.
FIGURE 1: Basic scale and expanded scale for recording infection types of Stripe rust on ADULT WHEAT PLANTS
FIGURE 2: Basic scale and expanded scale for recording infection types of Stripe rust on WHEAT SEEDLINGS
Attached (click here) is a spreadsheet provided by X. Chen, USDA/ARS Plant Pathologist that provides stripe rust ratings for all varieties entered in the WSU Extension Variety Testing Program trials in 2004. There are different locations listed and within each location are two columns (infection type [T] and percent [%]). These are ratings of the type (above photos) of stripe rust for each variety found in 2004 and the percent of the leaf tissue that was infected with stripe rust. These tables can be used to compare varieties susceptibility to stripe rust under natural infections. For 2005, the ratings will be much more severe.
We hope this helps. Please let me know if you have any questions. This information will also be posted on the Agronomy Updates section of the WSU Extension Variety Testing Web site (http://variety.wsu.edu)
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