Stripe Rust Update, June 28, 2012

Xianming Chen

Stripe rust on winter wheat

Winter wheat has generally passed flowering stage with some fields close to maturity and the stripe rust management season has been basically over in eastern Washington and northern Idaho. By June 27, stripe rust had developed to 100% severity and dried up all leaves on susceptible entries in our experimental plots near Walla Walla and in our experimental fields near Pullman, highly susceptible varieties had up to 100% severity and still actively producing spores. In commercial fields, stripe rust with various reactions (resistant to susceptible) were found in different fields generally with low levels of severity (2 to 10%) and mainly not active rust pustules. The low levels of rust severity are resulted from variety resistance and/or application of fungicides. With the widespread rain on July 26, some fields will be observed with more head infection. Head infection in general does not do as much damage as leaf infection. Because winter crops are further in dough to maturity stages, no further fungicide application is recommended.

Stripe rust on spring wheat

Now, we have approached the critical time for stripe rust control in spring wheat as the crop has reached middle jointing (Feekes 5) to flowering (Feekes 10.5) in eastern Washington and northern Idaho. In our experimental fields, stripe rust had reached 50% severity near Wall Walla and 30% near Pullman on highly susceptible varieties by June 27. In commercial fields, stripe rust was found in much lower levels (1-10% severity and also incidence). The widespread rain on July 26 has created ideal conditions for stripe rust infection. Such infection should have occurred since then and will occur at the nights during a week or so as the moist condition will produce adequate dew on plants for stripe rust to infect at night. In the next 10 days, the forecasted night temperatures (mostly in the 50s and low 60s F) will be ideal for stripe rust infection (rust spore germination and penetration into leaves) and the daytime temperature (mostly in 70s and low 80s F) will be ideal for the fungus to produce spores (sporulation), especially in the Palouse region and further north and east. Under such very favorable moist and temperature conditions, it just takes about 10 to 14 days for stripe rust fungus to develop from infection to sporulation. Fungicides should be applied before sporulation occurred on most leaf surface (using 5% plants infected or 5% leaf surface observed rust as a general fungicide application guideline). Fields grown with susceptible and moderately susceptible varieties should be sprayed from now to before the crop reaching flowering time, the earlier the better as rust infection has already occurred and the weather will become dryer and warmer in July. If a field of susceptible or moderately susceptible variety was sprayed with fungicide more than three weeks ago and the crop has not passed flowering stage, a second application may be needed. This year, high-temperature adult-plant (HTAP) resistance in many varieties has worked and will work better as the weather is relatively warmer than the same period of last year in most PNW areas. For assisting you to make decision if a fungicide application is needed or not, I include an early stripe rust note taken yesterday for spring varieties in the 2012 WSU Wheat Variety Trials grown in our experimental fields near Pullman. In general, varieties with infection type (IT) 5 or higher, severity over 10% should be sprayed. Varieties with ITs 0 to 3 or severity less than 10% may not need spray. Please check your fields for rust before spray.

Barley stripe rust

Stripe rust was found on susceptible spring barley entries in our experimental field near Walla Walla and Pullman. No rust was found in commercial barley fields. Barley fields may not need fungicide application as the rust pressure is still low. However, please check your fields for stripe rust and use the 5% rust level guideline to determine if the fields need to spray or not during the next two weeks.

Wheat stripe rust in other states

The widest distribution of wheat stripe rust throughout the country in the record history is just like the situation in 2010, from the west coast to the east coast and from Texas to North Dakota, entering into Alberta and Ontario, Canada. However, the disease has caused and is continually causing more damage in North Dakota and eastern Montana than 2010, due to the high moisture and low temperatures. In these regions, stripe rust has occasionally showed up and stopped quickly, but the disease has lasted so long and developed so severe this year. Most wheat-producing areas throughout the country have got stripe rust levels high enough to justify fungicide application.

* Scarlet was clearly susceptible (IT 8) in other two locations near Pullman, although severity was low.