NBLS gains importance in Texas.
Remember the “disease triangle”
when thinking about rice disease
management. The three points of the
triangle are the disease-causing
pathogen, the host and the environment. All three interact to
determine the incidence and severity of the disease.
My first plant pathology course at UC Davis was held in
a huge lecture hall. The professor, who was in the front of
the lecture hall, briefly removed (no more than three seconds)
the lid of a petri dish that contained a mature fungus
(mushroom-like). At the far back of this huge lecture hall,
he had placed a single open petri dish with growth media for
Amazingly, after one week, this dish was totally infected
with the fungus! So, the fungus had the ability to eject
spores throughout the entire lecture hall for a very short
period of time – about three seconds! This demonstrates
the three points of the triangle – spore ejection served as an important
dispersal mechanism for the fungus, the media in the petri dish served
as the host and the closed classroom served as the environment. The
same principles apply to rice fields.
I want to emphasize one of the points of the disease triangle –
environment – which may be especially important this year in Texas.
Planting in the Texas Rice Belt (particularly the eastern portion) has
been delayed this year because of greater-than-normal rainfall during the late winter and spring. This means more rice will be heading
later in the season when temperatures are higher, which may translate
to more panicle blight than in past years.
Panicle blight can be caused by pathogenic bacteria, but some
forms of panicle blight are not associated with pathogenic bacteria.
There are no “silver bullets” for controlling panicle blight. However,
disease susceptibility varies among varieties. For instance, most
hybrids are moderately resistant to bacterial panicle blight. Table 16 on page 36 of the 2012 Texas Rice Production Guidelines lists disease
reactions of many currently planted cultivars.
To view these guidelines, go to http://beaumont.tamu.edu/eLibrary/ Bulletins/2012_Rice_Production_Guidelines.pdf.
Panicle blight also is associated with a mite – panicle rice mite –
which is native to Central America where the mite attacks the developing
panicle to help cause panicle blanking. The panicle rice mite was
under a U.S. federal quarantine, but this quarantine has been lifted.
However, each rice-producing state can impose its own restrictions
regarding the possible movement and spread of this mite.
So far, this mite has not been problematic in the United States –
except in greenhouses. However, research and Extension scientists
continue to diligently watch for this potential pest. Personally, whenever
I am in a field showing signs of panicle blight, I inspect heads for
the panicle rice mite. So far, I have found none.
Another increasingly important disease in Texas is narrow brown
leaf spot (NBLS), which attacks both main and ratoon crops. The
photo at left shows NBLS on ratoon rice. Note the linear lesions on
the foliage and the brown sheath below the flag leaf. This disease is
caused by a fungus that attacks leaves, sheaths, internodes, panicle
branches and glumes. On leaf blades, narrow, brown lesions are
symptomatic. “Neck blight” can also occur on the internode below the
panicle. Yield losses as high as 40 percent have been documented in
Fungicides containing propiconazole control NBLS, but timing
of application is critical. NBLS is often associated with low nitrogen
levels in the plant. In general, stress in the main crop can carry over
to the ratoon crop. So, controlling diseases, insects and weeds in the
main crop can benefit the ratoon crop.
Dr. Shane Zhou, Texas AgriLife rice pathologist, co-authored this
article with Dr. Mo Way.
A step backward?
In the summer of 2010, several farmers,
mostly in an area about 15 miles north of
the Rice Research Station in Crowley,
experienced what they felt was fungicide
failure. They had applied Quadris and/or Quilt and still had significant
sheath blight. Theories were floated around mostly involving
uncertainty about application timing or source of the fungicide.
The season ended without any real determination of cause.
In 2011, these growers made absolutely sure they applied good
quality fungicide at the correct time and rates. And sheath blight
was severe. Pathologists Dr. Don Groth and Clayton Hollier and
scientists from Syngenta were consulted. Samples were obtained
from second crop rice (it was well past first crop harvest time) and
soybeans in the area. Remember, aerial blight in soybeans is
caused by Rhizoctonia solani as is sheath blight in rice. Samples
were submitted to Syngenta’s laboratory that had reference samples
collected prior to widespread use of Quadris.
Quadris still worked fine on the reference samples, while some of the samples “tolerated” the fungicide. More testing followed.
In several samples, resistance to azoxystrobin, the active ingredient
in Quadris and one of the ingredients of Quilt, was confirmed.
Whether this means the fungus will be resistant to trifloxystrobin,
the active ingredient in Gem and one component of Stratego,
remains to be seen. However, from a scientific standpoint, it is logical
to expect that the fungus might be resistant to it too because
of the similar chemistry of the compounds.
This caught the scientific community off-guard because this
fungus does not change easily like the rust fungi in wheat and
other highly variable fungi. Everyone felt Rhizoctonia was so
stable we would not have to deal with resistance issues for a long
time. A spontaneous change – a mutation – occurred and that
changed everything. This is devastating news to growers in that
area. The introduction of Quadris, and later Gem, changed attitudes
toward fungicide use in rice. Farmers were now able to manage
disease, which improved yield and quality. After these products
were released, I had a difficult time getting some farmers NOT to
apply fungicide. A Section 18 Emergency Use label for another
fungicide that has activity on sheath blight and aerial blight has
been submitted. By the time this article is published, it may have
been granted, and folks will have had a chance to try some. Without
something to control sheath blight, we will return to the prestrobilurin
days and the correspondingly lower yields we had
back then. It will truly be a step backward.
‘Feet in the field’
Scouting is the most important tool in making
the right fungicide decision. With rice diseases,
every year is different and every cultivar
is different. As a result, there is no one-size-fitsall
disease control program. Having a routine
scouting program will help you make the right decision on when to
apply a fungicide.
Diseases like sheath blight will always be a problem because the
aquatic environment is perfect for diseases. However, frequent rainfall,
heavy dews and mild temperatures can significantly increase
the severity of sheath blight and blast. Having feet in the field will help
you know the presence and severity of these key diseases.
Cultivars all differ in their ability to tolerate diseases. Cocodrie,
CL111, CL151 and CL162 are rated either susceptible or very susceptible
to sheath blight. From mid-season on, these cultivars need to
be monitored closely for sheath blight.
If sheath blight is a problem shortly after mid-season, I generally
recommend applying 6 to 9 fl oz/A of Quadris and making another application (either Quilt Xcel or Stratego) at the late-boot timing.
Rex and hybrids, such as XL 723, Clearfield XL745 and Clearfield
XL729, are only moderately susceptible to sheath blight. Therefore,
a fungicide application will be on a case-by-case basis.
Fungicide rates for sheath blight control will depend on how long
you need to protect the crop. If you are applying a fungicide in the preboot
timing, a higher fungicide rate will be needed to protect the
crop through heading. As you get closer to heading, a lower rate
may be used since the length of residual control needed will be less.
Timing is key
With fungicides having very little diversity
in their modes of action, the increasing risk of
pathogens developing resistance, the weather changes that aggravate
disease development and the susceptibility of conventional cultivars
to diseases make management increasingly difficult in modern
Diseases in rice are major causes of lower profit through yield
and quality losses of an estimated 10 percent annually, on average.
Fields suffering particular epidemics like sheath blight, blast or bacterial
panicle blight may incur 25 to 100 percent losses. If rice cultivars
are not resistant to certain diseases, profit and production are complicated
by the need to use expensive fungicides or cultural practices.
When it comes to selecting rice varieties for planting, our observations
support hybrid rice as having the best disease resistance
today. For fields with known and severe disease history, hybrids
would likely be a good choice.
Clearfield or conventional pure-line varieties, like CL151, CL111,
CL152, CL142AR, Cheniere, Cocodrie, Wells, Francis, Rex, Taggart
and Roy J, among others, would be most appropriate for highly productive,
less-risky fields planted early and managed well with respect
to fertilization and irrigation under today’s conditions. Even if it
means planting more than one variety, planting the right variety
based on the field’s problem is indispensable to increasing
Certain cultural practices heavily influence disease in rice fields.
One is application of too much pre-flood nitrogen (N). In such a
practice, more nitrogen will be available beyond the crop’s need and
would then be available for the pathogens. Fields receiving too much
pre-flood nitrogen suffer more from sheath blight, blast, kernel smut
and false smut, and probably other diseases as well.
A new soil test for nitrogen, developed by University of Arkansas
Division of Agriculture scientists, called N-ST*R (N-STAR) has
excellent potential to improve yields on crops grown in silt loam
soil while reducing the cost of nitrogen and likely the severity of
many diseases. This test determines nitrogen levels in the soil and provides
a more accurate estimate of actual fertilizer need for the crop in
that field. With urea prices in the $800-per-ton range currently, this test could be just what the doctor ordered.
In addition, striking the right balance in plant nutrition is crucial to
lower disease severity. Additional potassium can reduce the severity
of many diseases, including stem rot, brown spot and likely sheath
blight and bacterial panicle blight, among others. In general, balanced
fertility provides rice plants increased resistance to disease
and other stresses. Therefore, it is very important on many soils to
obtain a recent soil test and follow the recommendations for fertilization
closely with respect to K.
When it comes to planting, managing weeds, irrigation, fertility
management and fungicide application, timing has become more
and more important. Unlike most seasons in the past, over 95 percent
of rice in Arkansas has been planted in the months of March and
April of 2012, and we believe that is a good thing because early
planting provides adequate time for rice plants to develop maximum
yield potential and better escape many late-season diseases, including
blast, narrow brown leaf spot and the smuts.
Effective and timely irrigation of rice minimizes plant stress and
stress-related diseases, like blast and bacterial panicle blight. Timing
and rate of preventative fungicide applications for smuts is critical,
with a minimum of 6 fl oz of “Tilt” equivalent per acre applied
between early and mid-booting for best results these days. Timing is
also critical for fungicide applications to minimize blast and sheath
blight, and the local county Extension agent can provide information
in this regard.
• Pick the right variety for the field, based on disease history.
• Plant early.
• Apply potassium according to a recent soil test.
• Use timely and proper irrigation to minimize blast disease.
• Apply the right amount of N, especially at pre-flood timing.
• If using fungicides, know the disease and apply the right rate at
the right time for best results.
Comprehensive plan for rice blast
After a historically dry winter, California
once again experienced spring rains that have
resulted in localized delays in ground preparation
and planting. Fields with lighter textured soils and areas that
received little rainfall through the end of April were being flooded and
seeded by the first of May, but many fields will be delayed slightly.
2011 was the second consecutive year with heavier-than-normal rice
blast disease pressure in California. Not only were occurrence and
severity higher-than-normal, but rice blast was observed in fields far
from areas where this disease has been endemic since 1996. Significant
amounts of rice blast were observed in Butte County where
this disease normally affects few fields and also in Yuba County
where this disease has not been previously observed.
Conditions during the 2010 and 2011 crop years included late
planting and moderate summer temperatures and are believed to
have provided an environment that was favorable to disease development
over a larger geographical area.
Currently, M-208 is the only commercial California rice variety with
specific resistance to the IG-1 race of the rice blast pathogen first
identified in 1996. However, during the 2010 and 2011 seasons, we
observed a very limited number of rice blast lesions on M-208 indicating
that resistance is breaking down in this variety. Preliminary
investigations indicate that the fungal isolates collected from M-208
have evolved to overcome resistance rather than being newly
M-104 and M-205 are the least tolerant, widely grown commercial
California varieties. In addition to host resistance and environmental
factors, several cultural factors may significantly influence the susceptibility
or tolerance of rice plants to rice blast disease.
It is essential that all factors be considered when planning a comprehensive,
effective integrated rice blast management program.
Drill seeding and draining for stand establishment or herbicide applications
in water-seeded systems increase the risk of infection and
susceptibility to rice blast disease.
From an irrigation standpoint, maintaining a deep continuous flood
is one of the best management options for minimizing the risk associated
with rice blast disease. Excessive nitrogen and potassium deficiency
are also known to increase incidence and severity of rice blast
disease. Managing the rice crop to avoid these plant stresses as well
as others is a significant and often overlooked tool for minimizing risks
associated with rice blast disease.
A comprehensive rice blast management plan will consider all of
these individual factors and achieve greater success in minimizing conditions
that favor greater disease severity and associated losses.