Infection at the junction of the leaf blade and sheath results in the typical brown "collar rot" symptom. A severe collar infection may cause the leaf to die completely. When collar rot kills the flag or second-to-last leaf it may have a significant impact on yield.
Neck rot and panicle blast
Infections just below the panicle, usually at the neck node, cause a typical "neck rot"
or "rotten neck blast" symptom that can be very injurious to
the crop. If neck rot occurs early, the entire panicle may die prematurely,
leaving it white and completely blank. Later infections may cause incomplete
grain filling and poor milling quality. Other parts of the panicle including
panicle branches and glumes may also be infected. Panicle lesions are usually
brown, but may also be black.
Stem nodes may be attacked causing
the complete death of the stem above the infection. Diseased nodes are
brown or black.
Collar rot, neck rot and node infections were commonly
observed on California rice in 1996. Lesions on panicle branches and glumes
were also observed, but less frequently. Leaf symptoms were observed, but
were not confirmed as leaf blast.
Rice blast is caused by the fungus
Pyricularia grisea. Shortly after the fungus infects and produces a lesion
on rice, fungal strands called conidiophores grow out of the diseased rice
tissue and produce spores called conidia. These conidia
are dispersed in the air and under favorable conditions may cause new infections.
Disease Cycle and Development
The blast fungus can overwinter
from one season to the next on diseased crop residue and seed. Weeds have
been shown to be alternate hosts for the disease in the greenhouse, but
their role in nature is unclear. Of these overwintering sources, rice straw
and stubble are probably the most important.
Multiple Disease Cycles
A disease cycle begins
when a blast spore infects and produces a lesion on the rice plant and
ends when the fungus sporulates and disperses many new airborne spores.
When conditions are favorable, a single cycle can be completed in about
a week. In addition, a single lesion can produce hundreds to thousands
of spores in one night and may produce them for more than 20 days. Under
favorable moisture and temperature conditions, the fungus can go through
many disease cycles and produce a tremendous load of spores by the end
of the season. This high inoculum level can be very injurious to a susceptible
rice crop. The disease may progress through several phases starting with
leaf blast and followed by collar, panicle and node blast. Leaf
blast usually increases early in the season, then declines later in the
season as leaves become less susceptible.
Environmental Conditions Favoring Blast
is favored by long periods of free moisture, high humidity, little or no
wind at night and night temperatures between 63-73°F. Leaf wetness from
dew or other sources is required for infection. Spores are produced and
released under high relative humidity (RH) conditions, with no spore production
below 89% RH. Sporulation increases with increasing RH above 93%. The optimum
temperature for spore germination, lesion formation and sporulation is
77-82°F. Lesions produce spores for longer periods at 61-77°F than at 82°F.
It is clear that weather in the Sacramento Valley in 1996 was very conducive
for blast. More normal weather should be less conducive.
Other Factors Favoring Blast
Blast is favored
by excessive nitrogen fertilization, aerobic soils and drought stress.
High nitrogen rates and nitrate nitrogen increase rice susceptibility to
the disease. Ammonium nitrogen is converted to nitrate when fields are
drained and aerated. This may explain why rice is more susceptible on nonflooded
aerated soil. Extended drain periods encourage the disease by aerating
the soil, converting ammonium to nitrate and by causing drought stress
Successful control of blast usually
requires an integrated management program including the use of resistant
varieties, cultural practices and chemical control. Since resistant varieties
may be many years away, cultural practices must be used and chemical controls
sought. Important blast control methods are discussed below.
If possible, destroy diseased straw
and stubble by burning. This is an important control measure, but will
not provide complete control by itself. Burning diseased crop residue will
reduce overwintering inoculum in a given field and region, but will not
protect the field from other inoculum sources.
Use clean seed whenever possible.
Use proper seed sampling and testing to identify and avoid the use of blast
infested seed. Using clean seed will help control the disease, but will
not protect fields from other inoculum sources. Seed treatment to eliminate
blast from seed is under study at UC Davis.
Avoid using excess nitrogen
fertilizer, since it encourages the disease. Use the optimum nitrogen rate
for maximum yield, but no more. Fertilizer spills and overlaps must be
avoided since these areas enhance disease development and increase inoculum
levels for the surrounding area. Nitrate nitrogen is more conducive to
disease development than ammonium nitrogen.
Water seeding is recommended
to reduce or eliminate disease transmission from seed to seedlings. Drill
seedzing is not recommended, since it allows seed transmission, nitrate
formation and may result in drought stress.
Continuous flooding is recommended
to limit blast development. Avoid field drainage, especially for extended
periods, since it allows the formation of nitrate and may cause drought
stress. Also, drainage immediately after planting (less than 2 days) may
allow some seed to seedling disease transmission. Since shallow water favors
the disease more than deep water, moderate water (4-5 inches) and deep
water (6-8 inches) are suggested for early and late season, respectively.
No fungicides are currently
available in California to control blast. In the southern U.S., benomyl
(Benlate®) is registered to control the disease. Typically, two applications
are made just before and after heading. These treatments reduce disease
severity, but do not provide complete control. Quadris®, a new unregistered
fungicide from Zeneca, has efficacy on blast and other rice diseases. Studies
in the southern U.S. show it to be as good or better than benomyl.
California varieties are
susceptible to blast and M-201 appears to be the most susceptible. It will
take much time and effort to develop resistant varieties and they may not
always be completely effective. Never-the-less, they are an important,
inexpensive way to control the disease. Plans to screen and breed for resistant
varieties are in progress.