Reactions of Foote and Tubbs to
Stripe Rust in 2004
Xianming Chen, USDA-ARS, Pullman, WA
C. J. Peterson, Oregon State University
8/23/04
Wheat stripe rust was
widespread in Oregon in 2004, fueled by ideal weather conditions and appearance
of new races. In the Willamette valley,
production fields of the OSU SWW variety Foote were sprayed up to three times
with fungicide to control the disease. In
eastern Oregon and Washington, stripe rust reached sufficient levels in some
fields of the new OSU variety Tubbs to warrant fungicide application. Here is our current understanding of 'what
happened' to Foote and Tubbs this past year.
Prior to 2004, the soft
white winter wheat variety Foote was highly resistant to stripe rust. A susceptible reaction type, but with low
rust severities (less than 10%), was first observed in 2002 screening nurseries
near Pullman. In 2004, Foote became
highly susceptible in western Oregon production fields, although it remained
resistant in Eastern Oregon and Washington trials. The rust race that attacked Foote in the Willamette valley has
been identified as PST-100, plus virulence on the hard red variety Summit. Race PST-100 was common among California
isolates in 2004 and predominated in 2003.
Some of the 2004 California isolates of PST-100 had new virulence on the
variety Summit. This indicates a
virulence and race change for 2004.
This race was not common in eastern Oregon or Washington this past
growing season, but could be a problem next year.
The experimental data and
field observations indicate that Foote has seedling, or major gene, resistance
that is not effective against this new race of stripe rust. Foote is not alone, as major gene
resistances of several important PNW spring wheat varieties have been overcome
in the last 2-3 years. Spring wheat
varieties now susceptible to stripe rust include Penawawa, Zak, Wawawai,
Treasure, Whitebird, Eden, Macon, and Scarlet.
In contrast to Foote, the
variety Tubbs carries genes for high temperature adult plant (HTAP) resistance
from its soft white winter wheat parents Malcolm and Madsen. HTAP resistance is not race specific. It is not unusual to find low levels of stripe
rust on the variety Tubbs into early grain fill. In western Oregon trials, under severe stripe rust conditions,
Tubbs may have up to a 10-15% infection rating with an intermediate to low reaction
type. There is minimal damage or yield
loss, however, as with warming temperatures and further plant development, the
HTAP resistance halts infection progress.
In 2004, Tubbs production
fields in the Prescott, Washington, area had unusually severe stripe rust, to
the point that fungicides were applied to control the disease. Fungicides also were applied to a limited
number of irrigated fields of Tubbs in eastern Oregon and Washington. However, in most areas, both in production
fields and test plots, Tubbs had either low levels or no stripe rust. In the Willamette valley, for example, while
infection of Foote was rated at 80% or higher by May 1, Tubbs had a rating of
approximately 5% with a low reaction type.
The rust situation on Tubbs appears to result from the interaction of unique
weather conditions with crop development stage and genetics of resistance.
HTAP resistance is generally
expressed when night temperatures are above 10 degrees Centigrade (50 F) and
daytime temperatures are above 20 degrees C (68 F). Laboratory tests for HTAP resistance are conducted using daily
temperature cycle from 10 degrees C at 2:00 am, gradually increasing to 35 C at
2:00 pm, which simulates late growing season temperatures in PNW. The temperature effect on expression of HTAP
resistance is not clear-cut, but varies with number of genes, plant development
stage, and day vs night temperatures. Heavy
inoculum and cool conditions can cause high rust severity, even with low
infection type, on plants with low to moderate levels of HTAP resistance.
Based on field tests over
the past four years, Tubbs is heterogeneous, or genetically mixed, for HTAP
resistance to stripe rust. The variety
appears to include least three types: 1) approximately 50% of the plants are
highly resistant to stripe rust and have effective HTAP resistance; 2) from 30
to 50% of the plants are moderately resistant to moderately susceptible; and 3)
less than 5% of the plants are highly susceptible do not carry effective HTAP
or seedling resistance. The second group may have fewer number of HTAP
resistance genes such that the resistance is less effective under conditions
with high inoculum load and unusually cool, wet weather or heavy
irrigation. The resistance of this
group should be adequate and effective under normal weather conditions in the
eastern PNW, but may be marginal for the Willamette valley and western areas of
the PNW.
The prolonged cool, wet conditions
in the spring of 2004, while beneficial for plant development and increased
grain yields, were excellent for stripe rust development and spread. The cool temperatures apparently were
insufficient for adult plant resistance to be fully expressed in Tubbs until later
in grain fill, particularly in plants with moderate to low levels of HTAP
resistance.
What does this mean for
2005? In the Willamette valley, alternatives
to Foote should be found for production.
If Foote is grown, is should be monitored closely in the spring and fungicides
applied as stripe rust develops. In
eastern Oregon and Washington, we believe the HTAP stripe rust resistance in
Tubbs is sufficient under normal weather conditions and without heavy
irrigation. However, it is always wise
to monitor production fields closely for the disease, particularly when there
are unusual weather events or heavy rust inoculum loads developing in the
south.
