Agriculture Reference
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losses due to rust in wheat were then attributed
to stem rust (caused by P. graminis Pers.:Pers. f.
sp. tritici Eriks. & E. Henn.). A common attitude
was that P. triticina caused little or no damage to
wheat (Chester 1946). This mistaken opinion was
most likely because the disease only affected the
leaves, did not cause grain shriveling, and was not
visible when the grain was harvested. Replicated
yield loss studies in fi eld plots with sulfur or fun-
gicide treatments determined that leaf rust was a
major cause of yield loss in wheat.
Mains (1930) examined the importance of leaf
rust on yield in the 1920s and determined that the
commonly grown soft red winter wheat cultivars
that were susceptible to leaf rust suffered losses
that ranged from 25% to over 90%. Even a resis-
tant cultivar had losses over 10% due to the pre-
mature death of heavily infected fl ag leaves.
Caldwell et al. (1934) determined that seven
winter wheat cultivars which varied from resis-
tant to susceptible suffered losses from 15%
to 28% due to leaf rust infections. In the early
hard red winter wheats Johnston (1931) showed
that resistant and susceptible cultivars suffered
maximum losses of 22% and 55%, respectively,
due to leaf rust. In 1938 leaf rust caused a yield
loss of 25%-30% statewide in Oklahoma (Chester
1939). In hard red spring wheat in Canada, losses
due to leaf rust were over 50% in susceptible
cultivars and from 12% to 28% in resistant cul-
tivars (Peturson et al., 1945; Samborski and
Peturson 1960). Chester (1946) developed a pre-
dictive curve for estimating yield loss due to leaf
rust based on the growth stage in which wheat
was defoliated, using data from 68 yield studies.
Losses ranged from an average of 10% if defolia-
tion occurred in the dough stage of grain develop-
ment to an average of 95% if defoliation occurred
in the jointing stage. In a test with isogenic spring
wheat lines that differed by only a single leaf rust
resistance gene, Dyck and Lukow (1988) showed
a 22% difference in yield between the resistant
and susceptible lines. Martin et al. (2003) deter-
mined that isolines of winter wheat that had resis-
tance genes Lr41 and Lr42 had a 63% and 26%
yield increase compared with lines that lacked
these genes. Herrera-Foessel et al. (2006) esti-
mated yield losses over 50% due to leaf rust
in durum wheat ( T. turigidum ssp. durum ) in
Mexico.
Leaf rust continues to cause regular losses in
present-day wheat cultivars. In 2007 yield loss
due to leaf rust in the hard red winter wheat crop
in Kansas was estimated to be 14% (Kansas
Department of Agriculture, Topeka, Kansas).
Khan et al. (1997) developed a yield loss model
for southern US soft red winter wheat that pre-
dicted a 1% yield loss for every 1% increase in
rust severity at the milky-ripe stage of grain
development. Leaf rust resistance is a high prior-
ity in wheat germplasm developed at the Interna-
tional Maize and Wheat Improvement Center
(CIMMYT). Wheat production in Africa, South
America, and Asia where CIMMYT wheat germ-
plasm is grown has suffered 1%-20% losses due
to leaf rust (Marasas et al., 2004). CIMMYT has
estimated a 27 : 1 benefi t-to-cost ratio for develop-
ment of leaf rust resistant wheat cultivars. This
same study also determined that breeding for leaf
rust resistance in wheat was economically justifi ed
even if yield losses in areas with yields of 4 t ha −1
were only 0.2%-0.8%.
Effects on grain and fl our quality
In the study by Caldwell et al. (1934) 75% of the
yield loss to leaf rust was due to reduced number
of kernels per head. Susceptible winter wheat cul-
tivars with leaf rust infection had 14%-17%
fewer kernels than the same cultivars treated
to control rust. In their study the weight of
individual kernels was also 6%-7% lower in the
leaf-rust-infected treatments, and kernel weight
was reduced from 1.5% to 12%, depending on
the resistance level of the cultivar. Martin et al.
(2003) showed that kernel weight decreased by
9%-14% in susceptible winter wheat lines com-
pared with their resistant isolines. Other studies
with susceptible and resistant spring wheat
types showed a larger effect, with a 6%-39%
reduction in kernel weight (Waldron 1936;
Peturson et al., 1945). Leaf rust infections that
occur before fl owering will result in fewer kernels
per head, while infections that occur during
grain fi lling will result in lighter kernels (Chester
1946). Thus different yield components may
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