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October 21, 2004
Northern
corn leaf blight, part I: considerations in planning for 2005
by Paul Vincelli,
Kentucky Pest News
Northern leaf blight (NLB) was more severe in
Kentucky this past season than in any since I joined the faculty
at UK in early 1990. The cool, wet, cloudy weather than
prevailed during most of the 2004 season in many areas played a
major role in disease development. Wet weather with temperatures
in the range of 64-81oF favors infection and spore production by
the fungus that causes NLB (called Setosphaeria turcica, but
also known as Exserohilum turcicum and Helminthosporium
turcicum). Furthermore, extended cloud cover favors the disease
by making plants of a susceptible corn hybrid much more
susceptible to disease activity than when growing under sunny
conditions.
However, my concern is that the unusual
weather was only part of the picture, as there are some
indications of a possible trend of increasing pressure from NLB.
A respected and highly experienced corn pathologist from a
neighboring, major corn-producing state reported to me that the
incidence of this disease has increased over the past 4-5 years.
Records from the UK Plant Diagnostic Laboratories are certainly
consistent with this. The weather this past season certainly
played a role in the severe outbreaks observed, as explained
above. However, the question is, are adequate levels of
resistance to NLB available in the commercial hybrids being sown
in the region? This question is especially critical given the
high inoculum levels that are now present on many farms. At this
time we don't know the reason for the apparent increased
severity of NLB over the past several years. It could be due to
a new race of the fungus. Alternatively, it could be due to an
unintended erosion of partial resistance in the currently
available hybrids to NLB as breeders focused on other
priorities. It may simply be due to prevailing weather patterns.
This issue is currently being researched by pathologists in the
Midwest; I'll know more in a few months.
Some Biology of NLB
Symptoms of NLB are elliptical, grayish-green or tan lesions 1
to 6 inches long with smooth margins. The large ones are typical
for a susceptible hybrid growing under cool, cloudy, wet
conditions. During damp weather, greenish-black fungal
sporulation is produced in lesions. Older leaves are affected
first. Severely affected leaves can be killed when lesions
coalesce.
The fungus survives in undecomposed corn
residue. Spores are spread by air currents. Severe yield loss
can occur when upper leaves become blighted during early grain
fill. Strains of the fungus also infect sorghum, johnsongrass,
and sudangrass, although strains that attack these plants do not
attack corn.
Factors That Can Favor NLB Development
- Cool, wet, cloudy weather, as mentioned
above.
- Reduced tillage, since the fungus
survives in undecomposed leaves of diseased corn.
- Continuous corn, which favors a buildup
of inoculum by repeatedly planting a host.
- Substantial late-season growth of
volunteer corn after harvest. If a corn crop was harvested
early enough to allow some late-season growth of volunteer
plants from spilled kernels, these plants can serve as a
food base for further increase of inoculum in the field,
setting the stage for higher disease pressure next year.
- Late-planted crops can be exposed at a
relatively young age to spore clouds coming from
earlier-planted crops, resulting in more yield loss than in
an earlier crop.
- Irrigation, since this provides the
humidity and leaf wetness that favors the disease.
Management
Producers should consider the level of susceptibility to NLB for
all hybrids to be planted in 2005, but especially in or near
fields under conservation tillage where NLB occurred this past
season. The law of averages suggests that next season is not
likely to be as cool, cloudy, and wet as this past season, in
which case many producers would "dodge a bullet". However, with
the high inoculum levels that are present in certain areas,
especially in western Kentucky, a repeat of the same kind of
weather could result in destructive epidemics on susceptible
varieties in some localities.
There are two types of resistance to NLB:
complete resistance (more or less) and partial resistance.
In hybrids with (nearly) complete resistance,
lesions form which are yellow and limited in size, and
sporulation by the fungus is very limited. These are hybrids
with one or more Ht genes. For example, on hybrids carrying an
Ht1, Ht2, or Ht3 resistance gene, long, yellow to tan lesions
with wavy margins and no sporulation are observed on leaves
infected with S. turcica. These lesions are a resistance
reaction and can be easily confused with Stewart's wilt.
More commonly, corn hybrids have partial
resistance. In that case, NLB can still develop on the hybrid
but it usually does so more slowly that on a fully susceptible
variety.
Under many circumstances, a moderate to high
level of partial resistance would be sufficient to control the
disease. However, high to very high disease pressure, a hybrid
with complete resistance would often outyield a partially
resistant hybrid. Consider the factors discussed in the
preceding section to decide which fields might need a
substantial level of resistance to NLB in 2005.
Conclusion
We don't know what the weather will be in 2005. However,
inoculum levels of NLB are high in some areas of Kentucky.
Producers should carefully consider planting corn hybrids with
moderate to high levels of resistance to NLB in 2005.
Thanks to Bill Meacham of Pioneer Hi-Bred for
observations of the association of volunteer corn with NLB.
For information about corn pests,
visit
"Insect Management Recommendations".
October 28, 2004
Northern
corn leaf blight, part II: selecting resistant hybrids
by Paul Vincelli,
Kentucky Pest News
In the previous issue of Kentucky Pest News, I wrote about a
potentially serious threat of Northern corn leaf
blight in 2005. To understand the potential risk, I'll frame the
situation in the disease triangle, the most fundamental concept
in plant pathology. What the disease triangle tells us is that
three components are necessary for plant disease to develop: a
causal agent (such as a pathogenic microorganism) must interact
with a susceptible host plant in an environment that favors
disease development. Like a three-legged stool, it one leg of
this triangle is missing, the disease triangle "falls down" and
no disease develops. When all three are present, disease
develops. So consider the Northern leaf blight (NLB) situation
from that standpoint.
The causal agent (=pathogen)
The fungus that causes NLB (Setosphaeria turcica, but also known
as Exserohilum turcicum and Helminthosporium turcicum), is
present in the region in many fields at higher levels now than
they have been in several decades. I base this statement on
diagnostic records, scattered field observations, discussions
with industry representatives, and observations from Ohio, with
a long history of study of NLB. The good news is that there is
no indication of a new race of this fungus. Race 0 & 1 are known
to be widespread in the region, and thus far, tests by
colleagues have indicated no races beyond those, although tests
are continuing. This suggests that hybrids with genes such as
Ht2, Ht3, and HtN will perform as expected, providing very good
levels of disease control. (Note that these genes do allow the
development of varied types of NLB lesions. However, in hybrids
with one of these genes, overall leaf damage is greatly reduced
compared to susceptible hybrids.) While the race situation is
good news, the overall level of inoculum is still high in many
areas, particularly in areas of western Kentucky.
The host plant
Dr. Pat Lipps from The Ohio State University has conducted
studies recently to determine susceptibility of
a wide range of corn hybrids to NLB. His studies, which he
shared with me since my previous article, suggest
that the levels of partial resistance among many–perhaps the
majority of--hybrids without Ht genes is quite low.
This is worrisome. Work continues on the question of how much
partial resistance exists in the corn hybrids on
the market, but the data from Dr. Lipps certainly raise flags,
given the high inoculum levels that are now
present on many farms.
The environment
The presence of an infectious agent on a susceptible plant does
not necessarily lead to disease. The
environmental conditions that favor disease development must
also be present; otherwise, no disease develops.
As stated above, many farms have high inoculum levels, and there
are indications that quite a few hybrids on the market are
moderately to highly susceptible to NLB. That is not enough to
lead to an epidemic, but producers are at the mercy of the
environment. Will we have a repeat of the cool, cloudy, wet
weather that predominated in 2004? Of course, the odds are that
we will not. However, weird things happen on Earth, and a repeat
of that weather pattern is within the realm of possibilities. If
we do see a repeat, things could get "ugly" in many corn fields.
Conclusion
While sowing fear is not my usual educational style, I am
very concerned about the potential risk for serious
damage from NLB next season. The key here is that this is
potential risk. High levels of NLB inoculum are
present on many farms, and there are indications that many
hybrids in use are rather susceptible. Thus, we
have two "legs" of the disease triangle. Will we have the third?
We don't know if the weather in the 2005 season will favor
disease development. Only time will tell. In the meantime, I
encourage producers to pay close attention to the level of NLB
resistance in the hybrids they choose for next year,
particularly for use in or near fields that had serious damage
this past season. |
