LEAF RUST OF WHEAT
Donald E. Hershman
Leaf rust of wheat, caused by the fungus
Puccini recondita f. sp. tritici, can cause heavy yield losses
in wheat. Growers frequently underestimate the losses caused by leaf rust
because the disease never destroys an entire crop in Kentucky and seldom
causes severe shriveling of the grain. Yet the disease reduces the number
of Kernels per head, as well as grain test weight. Grain from severely
rusted plants is also lower in protein content.
Some leaf rust occurs in the state
every year, but variations in the weather and the amount of rust overwintering
in the southern states produce large year-to-year differences in leaf rust
development. Yield loss is most severe when the disease occurs early in
the spring on susceptible varieties and continue until the crop is mature.
Small, round-to-oval, raised, orange-red,
dusty pustules are scattered or clustered mostly on the upper surface of
the leaves and leaf sheaths of infected plants. Each pustule contains many
thousands of microscopic, orange-red rust spores.
Leaf rust frequently starts on the
lower leaves and gradually progresses up the plant to the flag leaf. However,
when massive levels of spores are windblown into the field after flag leaf
development, pustules may be more noticeable on the flag and the leaf just
below the flag than on lower leaves. Some pustules may also develop on
the stems (culms), and occasionally on the awns and glumes of the head.
As the season progresses, the pustules become more and more numerous until
30 to 50 percent or more of the total leaf area may be destroyed. Such
severely infected leaves usually shrivel and die prematurely.
As the wheat matures, other dark gray-to-black,
flattened pustules (telia) of about the same size may develop in large
numbers, mostly on the undersides of the leaves, leaf sheaths and culms.
These pustules contain the overwintering spores (teliospores). The teliospore
stage does no damage to the wheat crop and may not even occur if plants
become infected near maturity.
Spores of the leaf rust fungus can't
endure low temperatures. Thus, the fungus overwinters primarily in the
southern states and Mexico. In mild winters, the leaf rust fungus may also
survive within leaves of fall-sown or volunteer wheat in Kentucky. The
typical, orange-red pustules are then produced in early spring.
Spore produced on wheat grown in the
southern states and Mexico are carried northward into Kentucky by the wind.
They settle on the wheat plants and, when moisture is present, germinate
and infect within 6 to 8 hours. Once established, the fungus may produce
a new generation of spores every 7 to 14 days if moisture is prevalent
and temperatures are favorable (59 to 77 degrees F). The disease continues
to spread from plant to plant and from field to field by windblown spores
until the crop matures. Thus, rust has the ability to "explode" on susceptible
varieties if weather conditions remain favorable after the disease begins
to develop. The migration of spores is reversed in the fall when spores
are blown from the northern states southward, where they infect winter
Like most cereal rusts, the leaf rust
fungus is specialized into numerous physiologic races. More than 150 races
are known to exist. A wheat variety may be immune to certain physiologic
races of leaf rust, moderately resistant to other races, and completely
susceptible to still others. No wheat variety is highly resistant or immune
to all known races. Luckily, only a few races are abundant and widely distributed
in Kentucky during any one year.
As new, virulent races of rust develop,
wheats that were formerly resistant may become susceptible, or at least
moderately susceptible. For example, the soft winter wheat varietiey Caldwell
at one time had fairly good resistance to leaf rust; it is now rated as
moderately susceptible and may someday be rated as very susceptible to
leaf rust. To meet this challenge, wheat breeders and plant pathologists
are working constantly to incorporate resistance to an increasing number
of races into new crosses, selections and varieties. The battle between
the rust fungi and the wheat breeders and plant pathologists is a continuous
1.Sow wheat varieties with at least
moderate resistance to leaf rust.
2.Sow winter wheat after the Hessian
fly-free date for yur locality and at the recommended rate. Plant seed
into fertile, well-prepared soil. In situations where moderate or greater
amounts of nitrogen have been applied to wheat without adding sufficient
potassium and phosphorus (as determined by a soil test), the possibility
of severe rust attack increases. Following the recommendations given in
the soil test report should make it possible to increase yield without
increasing the susceptibility of the crop to leaf rust.
3.Treat the crop with a foliar fungicide
if warrented by sufficient leaf rust pressure.
Only apply a foliar fungicide if:
•the yield potential of the crop is
sufficient to offset the cost of the fungicide plus application costs;
•the wheat variety is at least moderately
susceptible to leaf rust and field scouting of the crop indicates that
rust is active, the crop has not yet passed the soft dough stage of grain
development and leaf rust is not yet severe on the top two leaves throughout
A well-equipped ground rig is the best
means of applying fungicides to the crop. Proper application requires twenty
to thirty gallons of water per acre; this is especially important when
protectant fungicides such as mancozeb are used. Use of less water, although
tempting, frequently results in poor disease control because of poor fungicide
distribution and coverage. Protectant fungicides MUST be applied BEFORE
significant infection occurs. Thus, timing of the fungicide application
is critical and must be coordinated with frequent field scouting to keep
at least one step ahead of the disease. Systemic fungicides, such as propiconazole
(i.e., Tilt), are taken into the plant and can eliminate young infections
as well as protect tissue from new infections. Nonetheless, timing of application
is still very important with systemic fungicides.
While waiting too long to apply a fungicide
is a major concern, applying them too far in advance of a leaf rust epidemic
is also a problem. Protectant materials are only active for a period of
7-10 days following application. In addition, leaves that emerge following
application will be completely unprotected. Systemic products are active
for longer periods (14-21 days), but it is still very possible to "run
out" of product prior to the time when significant infection occurs. The
key to avoiding this problem rests with field scouting. Specifically, regular
observation of leaf rust susceptible crops will give a pretty good indication
as to if and when fungicides are needed. At a minimum, scouting will allow
you to determine with little difficulty when fungicides are not needed
because of little or no disease pressure.
The proper time of application varys
from area to area and season to season. This is because of yearly variations
of leaf rust overwintering in kentucky, movement of spores into Kentucky
from the southern states, and the weather. The main goal of fungicide use
to control leaf rust is to keep the flag leaf as disease-free as possible
until after the kernels have filled (mid dough stages).
If possible, leave a nonsprayed strip
in your field if you decide to use fungicides. After harvest, compare yields
and test weights between the sprayed and nonsprayed areas. This will give
you the means to assess the value of the spray relative to disease control
and its economic impact. Always read and follow label instructions prior
to making any fungicide applications.