SEPTORIA DISEASES OF WHEAT
Donald E. Hershman
Year in and year out, diseases of wheat
caused by the fungi Septoria tritici and S. nodorum are among
the most troublesome to Kentucky wheat producers. In fact, diseases caused
by Septoria spp. are economically important in nearly all wheat-producing
areas of the world, destroying nearly 2% of the world's wheat annually.
S. tritici is principally a
foliar pathogen, but it can infect wheat heads, while S. nodorum commonly
affects both foliage and heads. Both fungi can, and frequently do, occur
within the same field and on the same plants. The diseases they cause are
often difficult to distinguish from one another without the aid of a good
hand lens and, frequently, microscopic examination of diseased tissue.
Consequently, the foliar diseases are often referred to as the septoria
leaf blotch complex or septoria complex. When one or both fungi infect
the heads, the disease is called glume blotch.
Yield reductions resulting from infection
by the Septoria fungi can approach 10-20% where fungicides are not used
for disease control. Reduced grain yield and test weights are the result
of reduced tillering of plants, reduced seed set, poor grain fill and loss
of shriveled grain with chaff during harvest. Losses are greatest when
epidemics develop prior to heading for septoria leaf blotch complex and
in the early stages of head development for glume blotch.
Other grass crops are also affected
by Septoria spp. S. tritici can attack triticale and rye
during disease-favorable conditions and can also be found on barley and
certain oat species. S. nodorum can cause serious losses in triticale
and barley and infects leaves of rye, but not the heads. The grasses Poa
and Agrostis spp. are also susceptible, but to only a limited extent.
Symptoms & Signs
Lesions of S. tritici start
as small yellow flecks, usually on lower leaves that are in contact with
the soil. Flecks expand into red-brown rectangular lesions 1/16 - 1/8 in.
wide by 1/4 - 3/4 in. long, with regular to irregular margins. Lesions
will have numerous black pycnidia dispersed throughout. Pycnidia are spore-producing
structures of the fungus which, in the case of S. tritici, are visible
to the naked eye. Pycnidia are most noticeable following a period of dew
S. nodorum infections start
as small dark brown to black flecks which eventually expand into lens-shaped
lesions, 1/8 - 1/4 in. wide by 1/4 - 3/4 in. long. Lesions are usually
surrounded by a distinct yellow halo and are initially red-brown, but may
develop gray-brown centers as they age. Lesions will have pycnidia, but
these are difficult to see except by using a 20X lens and looking at lesions
by holding diseased leaves up to the light. The difficulty in seeing pycnidia
is the result of their light color and the fact that they are buried in
the leaf tissue rather than being superficially imbedded, as is the case
with S. tritici.
Pycnidia of S. nodorum are honey-brown
and are almost perfect circles. Mature pycnidia will have a dark brown
hole (ostiole) in the center and may appear dark brown around the edges.
In contrast, pycnidia of S. tritici are black, appear as slightly
compressed circles (but of varying shapes) and are generally about one-third
smaller than pycnidia of S. nodorum. Individual lesions frequently
have pycnidia of both fungi present.
Lesions of one or both Septoria
fungi can rapidly expand and kill portions of leaves or entire leaves.
Lesions frequently begin from the middle to the tips of leaves. This is
especially true when leaves are damaged due to freezing temperatures or
some other physical or chemical factor.
When one or both Septoria fungi
invade the heads of wheat, the disease is known as glume blotch. Infected
glumes will show irregular gray-brown lesions with a purplish border. The
infection starts at the tips of the glumes and works its way toward the
base. Pycnidia, a diagnostic sign of the disease, will be visible in the
lesions using a 10X lens. Entire heads may become dark, and kernels from
diseased heads are usually shriveled and lightweight.
When attempting to diagnose glume blotch
in the field, take care not to confuse this disease with genetic discoloration
of heads of some wheat varieties, with superficial molds growing on over-mature
wheat, or with black chaff, a bacterial disease. The latter disease is
characterized as a purplish streaking of the veins at the tips of glumes
which is quite distinct from the glume lesions of glume blotch.
As previously stated, septoria leaf
blotch complex is the result of two distinct diseases caused by two different
species of Septoria. The two diseases may occur individually, especially
in either early or late season, but they frequently occur on the same plant
at some point in the season and form a complex. Infection by S. tritici
is highly temperature-dependent and requires rather cool, wet conditions
in order to occur. If conditions are favorable, infection will occur at
any stage of plant development. However, because of the temperature and
moisture requirements, infections by S. tritici are most common
in early to mid-season when temperatures are coolest. Infections usually
start in the lowest portions of plants and move upward until, and if, high
temperatures become limiting.
In contrast, infections by S. nodorum
occur over a wide temperature range but are highly dependent on plant stage
of development. Specifically, infections tend to become most evident about
the time that plants are in the "boot" stage (before head emergence) and
get increasingly more severe as the season progresses. Foliar infections
serve as a springboard to head infection, but serious glume blotch can
occur with only light to moderate foliar infection. This is due to the
fact that infectious spores of Septoria are readily dispersed within the
crop canopy by wind and splashing rain.
The fungi survive from year to year
(overwinter) in diseased wheat straw of previous crops, volunteer wheat,
diseased seed, and other susceptible grasses. In seed, the fungi can remain
viable for a year or more. In infested straw, the fungi can remain viable
for as long as three years. Overwintered sources of the fungi provide spores
(inoculum) for infection of the next wheat crop grown. Spores that are
produced from overwintered sources, as well as secondary spores produced
in active lesions, are relatively resistant to adverse (dry) conditions
because of a protective coating. Spores of S. tritici can remain
dormant for months at temperatures of 37-50°F, and those of S. nodorum
can tolerate temperatures above this range. Thus, spores can be produced,
blown, or splashed to wheat and remain dormant on plant tissue until conditions
become favorable for infection.
Infection requires at least 6 hrs of
wetness (up to 16 hrs for S. nodorum), and secondary spores are
produced in lesions within 10-20 days. Development of septoria leaf blotch
complex is optimal between 59-77°, but can occur between 41-95°F.
Activity of S. tritici is favored by temperatures between 59-68°F,
whereas S. nodorum is most active at 68-81°. Because of these
specific temperature and moisture requirements, wet, windy weather favors
epidemics of leaf blotch, while dry weather tends to slow or completely
halt disease development.
Crop Rotation and Tillage
Because the Septoria fungi overwinter
in infested wheat residue, any practice that reduces the amount of residue
in a field may also help reduce the number of Septoria spores available
to infect the next wheat crop. Generally, cropping sequences in which wheat
or other cereals appear no sooner than once every third year will help
to reduce the quantity of residue in a field that may harbor Septoria.
In addition, plowing under wheat residue after wheat harvest or following
harvest of double-crop soybeans will:
1.help to speed residue decomposition
2.limit the dispersal of Septoria
spores from the residue before decomposition. Farms and fields in which
wheat is planted every other year, especially where wheat and other crops
are planted using no-till or reduced tillage methods, are at the highest
risk for infection by Septoria.
Because spore production and infection
by Septoria are highly dependent on moisture, planting wheat in
rows, rather than broadcasting seed, and using adequate, but not excessive,
levels of nitrogen may help in reducing infection by Septoria. These
practices somewhat limit canopy density which, in turn, promote air circulation
and light penetration into the crop canopy. As a result, leaves tend to
dry out more rapidly, providing less favorable conditions for infection
With the knowledge that Septoria
can be brought in with seed, it is important to plant only high quality,
certified seed. As opposed to "bin run" seed and many other seed sources,
certified seed will be well-cleaned and generally free of diseased and
infested seed. Treating seed with fungicides containing carboxin or thiram,
among others, may also help in reducing levels and activity of Septoria
under field conditions.
Wheat varieties differ widely in susceptibilty
to both species of Septoria. Differences in susceptibility also
exist relative to foliage and head infection by the same species of Septoria.
Generally, resistance to septoria leaf blotch complex is less developed
than is resistance to glume blotch. In addition, no varieties have a high
level of resistance to either leaf or glume blotch. Nonetheless, where
Septoria has been a problem, it is important to plant varieties with the
highest available level of resistance to foliar and/or glume infection,
depending on the disease history of a farm.
For a listing of disease reactions
of the most common wheat varieties grown in Kentucky, consult the Kentucky
Small Grains Variety Trial bulletin, published by the University of Kentucky's
Department of Agronomy, and available at your local county Extension office.
When varieties susceptible or moderately
susceptible to septoria leaf blotch complex and/or glume blotch are grown,
foliar fungicides, when properly applied, can slow or even halt disease
development. Fungicides are most effective when applied in the early stages
of an epidemic. Waiting too late to apply fungicides may negate their value
altogether. On the other hand, applying fungicides too early in disease
development may also be of limited or no value. Timing of fungicide application,
thus, plays an important role in determining the level of disease control.
Other factors to consider when using
foliar fungicide are the use of appropriate rate of fungicide, using spray
additives when necessary, and achieving good coverage of plants with fungicide.
Protective fungicides, such as the
mancozeb products, must be applied before infection occurs. These products
will have no effect on infections that have already occurred. Systemic
products, such as propiconazole (i.e., Tilt®) are able to eradicate
infections to a certain degree. They also reduce the secondary development
of Septoria after infections are well-established. Both mancozeb
and propiconazole will give good results against Septoria when used properly
and according to label directions.
Fungicide use decisions are complex
and involve many factors such as crop yield potential, variety, crop stage
of development, disease development, current and forecasted weather, and
production practices, among many others. For help in making fungicide use
decisions, consult PPA-36 Fungicide Use in High Yielding Wheat available
at your county Extension office.
Depending on the season, more than
one fungicide application may be necessary in order to achieve control
of septoria leaf blotch complex and glume blotch. However, when producers
are unwilling to make more than a single application in a season, it is
usually best to make that application in the early heading stages rather
than at flag leaf expansion.
While this approach allows early disease
to develop, late-season disease will be controlled. This almost always
results in higher crop yields than in the converse situation where early
disease is controlled, but late-season Septoria is allowed to develop
unchecked. The advantage, however, may not be as great as where two fungicide
applications were made. The risk to this approach is in waiting too long
to make the application and allowing too much damage to occur before acting.
The main goal in using fungicides is
to keep the top two leaves of plants (and heads) relatively free of disease.
Once the upper leaves or heads become heavily infected and are showing
symptoms, yields will have already been seriously affected and fungicides
will be of no value. Thus, when scouting wheat, a decision to spray is
usually based on disease development on the second (F-I) and third (F-2)
leaves down on the plant. If the F-I leaf is showing 0-5% Septoria
and/or the F-2 leaf 10-20%, then fungicide use should be considered as
a means of protecting the upper leaves and heads.
For glume blotch control, best results
are achieved when sprays are delayed until partial head emergence through
the stage at which the heads are completely emerged. However, it would
be unwise to delay making a fungicide application until this time if the
upper two leaves are being threatened before the heads completely emerge.
This is a judgment call which requires some experience and regular field
It must be stated that, even where
fungicides are used properly to control Septoria and other foliar diseases,
serious yield loss can still occur due to the development of other diseases
for which fungicides offer no control, or because of the development of
other yield-limiting situations. Thus, fungicide use is not a panacea,
but it must be considered as a major part of wheat production if the highest
economic yields are to be reached.
Where trade names are used, no endorsement is intended, nor criticism
implied of similar products not named.