2007 - Texas A&M AgriLIFE Research Center at Beaumont - Texas ...

The late-boot application is most important if there are
leaf lesions caused by blast. The heading application is more
important to protect panicles from spore showers. Blast is
favored by excessive nitrogen fertility, thick stands, lighter
soils and inadequate flooding.
Kernel smut
Kernel smut is a serious disease caused by the fungus
Tilletia barclayanna (Neovossia horrida). The disease
causes the endosperm of the rice grain to be replaced completely
or partially by a black mass of smut spores. Usually
only one to five grains per panicle are infected.
Although yield losses are insignificant, monetary losses
can be very high if the rice cannot be sold or the price is
reduced at the mill. Infested lots of grain often have a dull,
grayish cast caused by the smut spores. Rice lots exceeding
3 percent kernel smut infection presently will not qualify
for government loan.
The disease is not systemic. The smut spores fall to the
soil surface, where they remain dormant until the following
rice crop, or they can be introduced into a field on
the surface of infested rice seed. The smut spores float to
the surface of the irrigation water where they germinate
and produce air-borne spores which infect individual rice
florets. Disease development is favored by frequent light
showers and high relative humidity.
Kernel smut is difficult to control. Field tests indicate
that a late-boot application of Tilt ® or Propimax ® at 4 to
6 fluid ounces per acre reduces the number of smutted
kernels. The semidwarf varieties Lemont and Gulfmont are
less susceptible to the disease than Cocodrie or Cypress.
Heavy nitrogen fertilization favors the disease. A 3-year
crop rotation should help reduce the number of smut spores
present. Do not plant seed contaminated with smut spores.
Sheath blight
Sheath blight, caused by the fungus Rhizoctonia solani,
has rapidly become the most important rice disease in
Texas and probably the second most important rice disease
worldwide.
A change in cultural practices during the 1980s is the
reason for this. The increased use of sheath-blight-susceptible
semidwarf varieties, along with the recommended
high nitrogen fertilization required to obtain their maximum
yield potential, has resulted in much greater losses from
sheath blight. Also, the trend toward shorter crop
rotations has made the disease more troublesome by
allowing the fungus to increase in quantity within fields.
As a result, rice producers have increased their reliance on
fungicides to manage sheath blight.
Cultural control
To most effectively and economically reduce losses
from sheath blight, use an integrated package of management
practices. Some practices may be economical only
where sheath blight is a persistent, significant problem.
Others are recommended in all situations as sound production
practices that will help prevent the buildup of a sheath
blight problem or limit its effects where the problem exists.
Some recommended cultural practices include:
• Avoiding excessive seeding rates, which result in a
very dense canopy that creates a microclimate
favorable to disease development.
• Avoiding excessive rates of nitrogen fertilization,
which increase the severity of the disease.
• Where possible, increase the interval between rice
crops to at least 1 year of rice in every 3 years.
Research has shown that rotations of pasture-pasturerice,
soybean-soybean-rice and rice-soybean-rice had
average incidence of sheath blight of 0.4, 2.7 and 5.4
percent, respectively, at panicle differentiation. In addition,
more sheath blight inoculum for future rice crops
tends to be produced in drilled soybeans than
in row-planted soybeans.
• Controlling grass weeds that can serve as hosts of the
sheath blight fungus. Barnyardgrass, crabgrass and
broadleaf signalgrass are known hosts of the pathogen.
Variety selection
Long-grain rice varieties differ in their susceptibility to
sheath blight. Among those considered very susceptible are
Cocodrie, Gulfmont and Cypress.
Less susceptible (moderately susceptible) are Jefferson,
Saber and most of the medium grain varieties. Taller varieties
tend to sustain less loss than semidwarf varieties.
Chemical control
In many situations, foliar fungicides may be economically
justified for reducing losses from sheath blight if:
• The disease pressure is sufficiently high;
• Susceptible varieties of rice are grown;
• The crop has a high yield potential in the absence of
sheath blight; and
• Environmental conditions are favorable for the disease
to spread to the upper leaves of the rice plant.
It is difficult to estimate the potential severity of sheath
blight in a field in order to determine the economic feasibility
of applying a fungicide. However, with the high costs of
fungicide spray programs and the need to reduce production
costs, estimates should be made.
To estimate the severity of sheath blight infestation,
monitor the field at or shortly after panicle differentiation
(PD) growth stage (See Fig. 3). It may not be necessary
to precisely monitor a field with a recent history of severe
sheath blight that is on a short crop rotation (more than one
rice crop in a 3-year interval).
Figure 3. Panicle differentiation (PD).
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