By Xianming Chen
Stripe rust has been spreading and developing quickly since the last update on April 13. The disease is now everywhere in the Pacific Northwest. When we took the early season note at Mount Vernon in Skagit County in northwestern Washington on April 18, stripe rust reached 60 percent severity on susceptible winter wheat varieties, as usual for this region. On April 27, stripe rust was very easy to find on low leaves of susceptible winter wheat varieties in our experimental fields on Conservation Farm, north of Pullman in Whitman County in Eastern Washington, where stripe rust was not found on April 12.
On May 4, we checked fields along the way to Walla Walla and Hermiston in Oregon and found stripe rust in several fields in Whitman, Garfield, Columbia and Walla Walla counties in Washington and Umatilla County in Oregon. In commercial fields, winter wheat ranged from Feekes 8 to 10.5, and stripe rust was generally low in incidence and severity, thanks to growing resistant varieties and early application of fungicides. In our experimental field north of Walla Walla, we took the second-time note of stripe rust in the winter wheat nurseries. Stripe rust reached 95 percent severity on susceptible varieties much earlier than normal. Stripe rust also reached the peak in our experimental plots in Hermiston with susceptible varieties having 100 percent severity. Some varieties were even dried out by rust.
Based on the Walla Walla data, most varieties are similar to the data of last year, but we notice significant changes on some varieties. ORCF-102 is rated as “MS” and 7 (vs. “MR” and 3 last year), becoming more susceptible. Similarly, Xerpha is “S” and 8 (vs. MR-MS and 5), Keldin “MS” and 7 (vs. “MR” and 4), CuriosityCL+ “MS” and 6 (vs. “MR” and 4) and WB-Junction “MS” and 6. The increased susceptibility is mainly due to the early start of the disease, and under the weather conditions so far, high-temperature adult-plant (HTAP) resistance did not reach to its highest level in these varieties. These changes are unlikely due to race changes, as the samples collected in March at this location are identified as PSTv-52 and PSTv-37, similar to last year.
As many fields were sprayed with fungicides more than a month ago, stripe rust has started to redevelop. Please check your fields to see if you can find new growth of active rust pustules (rust spores can stick on your figures if you rub the stripes). Based on the forecast, weather conditions will continue to be favorable for stripe rust (but not as bad as in 2011). It is better to spray fungicide again if you see rust incidence (number of leaves or plants with rust) at 5 percent or higher and the variety is in the MS and S categories or rated 5 or higher. For varieties in the MR category or rated 3 and 4, if they have more than 20 percent severity, it is worth spraying fungicide as these varieties will likely to have yield losses in the range of 5 to 15 percent. Varieties of the R category or rated 1 and 2 do not need to be sprayed unless they appear much different from what is expected in this category (such as more than 5 percent of leaves have active rust pustules). Timing of the second application is important as it should protect the crop through the rest of the growth season. Ideally plants are at the boot to flowering stage, but that can be influenced by many factors, such as the susceptibility of the variety, yield potential and schedule of air application.
HTAP resistance has been working, but the weather conditions and plant growth stages have not allowed this type of resistance to reach its highest capability of fighting against stripe rust. This type of resistance will not completely get rid of stripe rust this year, as necrotic stripes can reduce grain yield. Based on the previous forecast and current situation of stripe rust plus the weather forecast for the next two to three weeks, highly susceptible varieties (not commercially grown) would have about 60 percent yield loss, and commercially grown MR, MS, and S varieties would have 5 to 40 percent yield loss (such as Xerpha, Tubbs and ORCF-102 for the high end).
Spring wheat and barley ranged from being planted to Keekes 4. Stripe rust was found in our nurseries in Walla Walla. Fungicide application is recommended for fields grown with MS and S varieties (or rated 4 to 9 on the Seed Buyer’s Guide).
Physiological leaf spot (PLS) was found in some winter wheat fields. Do not confuse it with necrotic stripes caused by stripe rust as fungicides do not control PLS.
Stripe rust throughout the country
Stripe rust has occurred throughout the entire inland U.S. Many states have stripe rust as a number one problem, and fungicides have been used widely to control the disease. Right now, the following 23 states have reported stripe rust: Texas, Oregon, Louisiana, Arkansas, Oklahoma, Washington, Colorado, Kansas, Mississippi, California, Virginia, Montana, Indiana, Idaho, North Carolina, Tennessee, South Dakota, Kentucky, Nebraska, Minnesota, Delaware, Florida and Michigan. The early samples from Texas and Louisiana this year were identified mostly as race PSTv-52 and some as PSTv-37, similar to the last year.
Note: We would like to thank many of you who have collected and sent us stripe rust samples. Please note that stripe rust samples are better to collect when leaves are dry. If not dry, leave picked leaves open for few minutes to get rid of water before put into a glassine or paper envelope. Please do not use plastic bags to contain leaf samples as leaves will rot and rust will die in plastic bags. One to five leaves are enough for one sample (a variety or breeding line in a field), and multiple samples can be collected from fields but from different varieties or lines. Please do not dig out roots or have stems in samples. For occasional cases when leaves are free of rust but heads get infected, collect one to three heads. Keep samples as cool and dry as possible before shipping. As stripe rust fungus easily loses viability, overnight mail (FedEx or UPS) is preferred. However, sending through air mail is fine if samples are kept cool and dry and arrive within a week. My shipping address is Xianming Chen, 361 Johnson Hall, Washington State University, Pullman, WA 99164-6430 (phone: 509-335-8086). Thank you for your cooperation.