WAWG
By Dr. Xianming Chen
So far, we have completed recording stripe rust data of our winter crop nurseries in various locations including Central Ferry, Mount Vernon, Wallla Walla, Lind, and Pullman (Palouse Conservation Field Station, Plant Path Farm, and Spillman Farm) in Washington and Hermiston, Ore.
Wheat stripe rust developed to 90-100% severities on susceptible varieties in all locations except the Spillman Farm site that apparently has shallow soil and stripe rust developed only up to 60% severity on the susceptible check. We have not noticed obvious differences in stripe rust reactions among popular varieties except Bobtail, which had distinct reactions in different nurseries even at the same locations. This variety caught our attention as we first noticed rust (infection type 5 and severity 50%) on it in the WSU Winter Wheat Variety Trial nursery when we took the first-time note in April and saw rust (infection type 5 and severity 20%) on the same entry at Central Ferry in May. The variety in the same nursery had infection type 8 and severity 20% at the Palouse Conservation Field Station, near Pullman. However, the Bobtail entries in our regional winter wheat variety nursery and the Oregon Wheat Breeding nursery at the same location did not have rust. A few years ago, Dr. Chris Mundt noticed stripe rust on Bobtail in Corvallis, Ore., and in that year, Bobtail was rust free in the experimental fields around Pullman. Based on this year’s observations, Bobtail may have two different sources of seeds, one is still highly resistant to stripe rust while the other is or has become moderately susceptible. From next year and forward, growers need to pay attention to stripe rust if planting this variety.
In commercial fields, stripe rust was found in some fields of both winter and spring wheat in Adams, Walla Walla, and Whitman counties, mostly below 1% severity. The stripe rust season is generally over for winter wheat, and the disease currently has low potential to cause damage to spring wheat as the weather has been dry. Fungicide application is not recommended except for fields planted with susceptible or moderately susceptible varieties under irrigation or in areas with high moisture.
Stripe rust in the country
Since the last update (May 21), stripe rust has been reported in Michigan, Nebraska, Idaho, and Virginia). So far, 12 states (Louisiana, Texas, Georgia, Washington, Oregon, California, Oklahoma, Kansas, Michigan, Nebraska, Idaho, and Virginia) have been reported to have stripe rust.
Special notes on Yr5 and Yr15 resistance
Yr5 and Yr15 are among few wheat genes that confer effective all-stage resistance against the stripe rust pathogen populations in the world. These two genes have been used quite intensively in breeding programs within the last two decades or so. The stripe rust isolates virulent to Yr5 have been reported in Australia and India many years ago and in China and Turkey in the recent years, but the virulent isolates have not spread even in these countries. Similarly, virulence to Yr15 was also reported in a couple of countries many years ago but has not caused much concern until this year. This spring, stripe rust was first found on wheat varieties carrying Yr15 in the UK and later found in several European countries including Denmark. Apparently, the rust has caused damage in commercial fields grown with wheat varieties carrying Yr15. This is an alarm to the world.
In the U.S., we have not found any stripe rust isolates with virulence to either Yr5 or Yr15. Starting in 2010, we have included Yr5 and Yr15 into our set of wheat lines used to identify races of the wheat stripe rust pathogen so that every wheat stripe rust sample has been tested on these lines. In addition to every year’s tests of stripe rust samples since 2010, we have also tested stripe rust collections from 1968 to 2009. All stripe rust isolates we have tested so far are not virulent to either of the genes.
Starting using Yr5 and Yr15 in wheat breeding programs in the U.S., the wheat research community has been calling for and setting up a stewardship of developing new varieties with these two genes in combination rather than just one of them to extend their lifetime of effectiveness. So far, about 15 to 20 wheat varieties, mostly of spring wheat, with the two genes together are commercially grown. Examples are Seahawk grown in the Pacific Northwest and Patwin 515 in California. About 80% of California wheat acreage is currently grown with various “515” varieties (carrying both Yr5 and Yr15). To my knowledge, only spring wheat variety Expresso has Yr15 but no Yr5. As Expresso was developed from Express by adding Yr15 and Yr17 (not effective against the predominant races), it also should have the high-temperature adult-plant (HTAP) resistance genes from Express and the Yr17 donor (Yr17 is linked to a HTAP resistance gene). Because of the combination of Yr5 and Yr15 in the U.S. wheat varieties, the Yr15 race from Europe or the Yr5 race from Asia will not be an immediate threat to the U.S. varieties with these genes even if the races spread to the U.S. However, virulent mutants may arise from the rust population within the country and the pathogen is able to combine different virulent genes. We all need to pay attention to rust in the fields. When you see stripe rust on previously resistant varieties or just like to know if stripe rust on any varieties is the same or different from the past, please collect and send leaf samples with stripe rust to us (Xianming Chen, USDA, WSU, 410 SE Dairy Rd, 114B – 101, Pullman, WA 99164) for virulence testing and race identification.