Stripe Rust Update, May 21, 2015

Xianming Chen

Stripe rust in the PNW

Yesterday, we were checking wheat and barley fields in eastern Washington.  Winter wheat ranged from late jointing (Feekes 8) to flowering (Feekes 10.5) and mostly in boot to heading (Feekes 10 – 10.1) stages.  Spring wheat and barley ranged from tillering (Feekes 2) to middle jointing (Feekes 7) stage.  Most fields had good moisture, thanks to the rains and showers since the last week, which ended the long period of dry conditions after early April.

Stripe rust of wheat was widespread.  In commercial winter wheat fields, we found stripe rust in around 20% of about 80 fields checked in areas from Colfax to St. Johns and Lamont in Whitman County and from Horse Heaven Hills in Benton County to Walla Walla and Waitsburg in Walla Walla County.  In general, stripe rust incidence and severity were low.  It took time to find one to five leaves usually with one stripe in a commercial field in most of the areas.  In fields near Walla Walla, rust was relatively easy to found, and several leaves with multiple stripes could be found in small hot spots.  Most rust was on top leaves (infection occurred in the spring), and few spots had rust from bottom leaves (initial infection occurred last fall).  Compared to commercial fields, rust was much easier to found in experimental fields.  For example, stripe rust developed to 100% incidence (all plant had rust), 30 to 60% severity (percentage of leaf surface covered with rust pustules) on flag leaves, and 60-100% severity on lower leaves.  For spring crops, we were able to found stripe rust on susceptible varieties of both wheat and barley in our nursery near Walla Walla and a commercial field of spring wheat near Waitsburg.  Stripe rust was found on goat grasses near and in wheat fields in the Lamont area, more easily found than on wheat plants.  In addition to stripe rust, significant barley yellow dwarf was found in a winter wheat field near St. John in Whitman County and severe physiological leaf spot was found in several fields in several winter wheat fields in the Walla Walla area.

The relatively low levels of stripe rust in commercial fields compared to experimental nurseries were mainly due to resistance in commercial varieties and widely use of fungicides at the time of herbicide application.  The long period of dry weather conditions in April and the first two weeks of May slowed rust development.  Without the dry period, stripe rust could have been much severe.

As most winter wheat fields are approaching the flowering stage, we are in the second critical period for stripe rust management of this season.  Temperature has been favorable and will likely be favorable for stripe rust development in the next two to three weeks.  Moisture has been favorable since the last week and would be favorable for some areas depending upon precipitation in the next two to three weeks.  More rust incidences and higher severity will likely occur in about a week.  Our recommendation is the same as the one we made early this year that fungicide application is needed for fields grown with susceptible or moderately susceptible varieties of winter wheat.  For such fields, even for fields applied with fungicides three weeks ago, growers should start checking for rust, and apply fungicide when rust is easily found.   For spring wheat, early application of fungicide at the time of herbicide application is recommended for fields planted with susceptible or moderately susceptible varieties, and then check fields three to four weeks later to see if any new rust development.  For both winter and spring wheat, fields grown with resistant or moderately resistant varieties generally do not need fungicide application, unless susceptible reactions are easily seen.  For spring barley, fungicide application is generally not recommended as the stripe rust pressure is low.  However, barley fields should be checked on a weekly basis, and apply fungicide when rust is easily seen in the field.

Since the last update, stripe rust has been reported in southern Idaho and northeastern Oregon (such as Hermiston).

Stripe rust in the country

Since the last report, stripe rust has developed to severer levels in many regions of previously reported states, and the distribution of stripe rust has expanded further north and east.  So far, stripe rust of wheat has been reported in 20 states, including Oregon, Louisiana, Texas, Arkansas, Mississippi, Oklahoma, Washington, California, Montana, Arizona, Colorado, Kansas, Nebraska, Tennessee, Georgia, Utah, Idaho, Virginia, Illinois, and Indiana.  Barley stripe rust has been reported in California and Washington.