|
|
Plant Diseases
Rice Diseases in Arkansas
Management of Sheath Blight and Blast in Arkansas
Sheath Blast •
Blast
Sheath Blight
This important disease of rice is very widespread in Arkansas, easily found
in 50-66% of rice fields randomly surveyed in 1993 and 1994 and at damaging
levels in most long term rice fields growing highly susceptible semidwarf long
grain rice varieties. Actual yield loss to sheath blight appears to have
lessened somewhat the past decade with the development of more tolerant
varieties and improved cultural practices employed by many farmers, however,
yield losses of 5-15% are still fairly common in many fields. Losses of 50% and
greater, reported in the 1980's on Lebonnet and other very susceptible
varieties, are now only rarely observed and are confined to smaller areas of
fields. Before 1970, sheath blight was only a curiosity in Arkansas, but with
the introduction of shorter and more susceptible varieties during the 1970's
-1980's, the increased use of nitrogen fertilizer for higher grain yields on
these varieties, alternate year rice rotations, and reduced tillage practices,
sheath blight rapidly became the most widely established rice disease in the
state. Sheath blight is more consistent but much less dramatic than blast,
causing routine problems and losses in many fields each year. Weather can affect
the severity of the disease with hot, humid years like 1991, 1993, and 1995
resulting in greater damage state-wide than years with cooler summers like 1992,
1994, and 1996.
Symptoms - Sheath blight is usually not noticed until late tillering to
midseason (panicle differentiation). Early symptoms include oval sheath spots
(lesions) at or just above the water line, often at the junction of the leaf and
sheath. Early lesions are pale green to off-white with a narrow purple-brown or
brown border, usually 2" or less wide and 1-2" long on most varieties. Lesions
may join as the disease moves up the plant. Both sheaths and leaves are commonly
attacked and killed as the disease grows upward. Other sheath spot diseases
sometimes confused with sheath blight include black sheath rot (Gaeumannomyces
graminis var graminis), stem rot (Sclerotium oryzae), aggregate sheath spot (Rhizoctonia
oryzae-sativae), and bordered sheath spot (Rhizoctonia oryzae) (Fig.1). The
first two diseases have black or gray-black lesions on the leaf sheaths.
Aggregate sheath spot lesions are very similar to sheath blight lesions but have
a narrow, brown vertical line in the center of the lesion, do not tend to
overlap, and the disease rarely attacks the leaf blades. Bordered sheath spot
lesions also tend to be separate and distinct, be more brown in color, have wide
dark brown borders, and usually only appear on the sheaths (Fig.1).
Sheath blight can grow from plant to plant, either across narrow stretches of
water or from leaves touching other leaves. The disease may move 1" per day up
the plant under ideal conditions of cloudy weather, high temperatures (82 - 90
F), and high humidity (95%+) in the rice canopy. Sclerotia of the fungus are
formed on the sheaths and leaves as early as a week after leaf and sheath
lesions are seen, but are more typically observed on infected rice in the boot
to heading stages. Sclerotia are initially white and slightly fuzzy, rapidly
turning brown with an irregular shape. They average about 1/8" in diameter,
resembling tiny potatoes with the side next to the plant being flat or concave,
and eventually fall to the ground where they survive until the next host crop.
On current varieties, the bottom portion of panicles on heavily infected tillers
will blank if the disease destroys the flag leaf before grain fill is complete.
Sheath Blight can infect the panicle under severe conditions, destroying all
grain fill, and stick the panicle together into a vertical "spike" with its
hyphae (microscopic fungal threads).
Cause - Sheath blight is caused by the fungus Rhizoctonia solani
AG1-1A, also called Thanatephorus cucumeris. This fungus is very widespread in
agricultural crops, but exists as many forms under this one name. The form
attacking rice can also attack soybeans, causing aerial blight, and persist on
other summer crops like corn or grain sorghum. It can also attack many grassy
weeds in and around fields under the right environmental conditions. While it
can infect these other hosts, the fungus causes the most damage to rice. R.
solani overwinters in the soil as long-lived and tough sclerotia, and grows on
infected plants as microscopic hyphae (threads). It also forms a sexual stage on
rice, which appears as a thin white layer on the lower plant stems that looks
like frost, shortly after midseason and then disappears. Little is known about
the role of this stage in nature.
Disease Cycle - Sheath blight is a modified single cycle disease. This
means that the fungus infects a plant and does not produce a new generation on
that plant which will attack other plants the same season. This differs from
blast, a multiple cycle disease, which infects then produces several generations
of spores, that in turn infect other plants in the field the same season. Sheath
blight grows from plant to plant, however, so an initial infection can spread
short distances (usually 3 ft. or less) in the same growing season. Thus, sheath
blight is the type of disease that tends to build up via accumulated sclerotia
in the same fields or areas of fields, attacking the plants in these areas each
time rice is planted, but usually does not spread to uninfected field regions
during a single growing season.
Sheath blight starts when a sclerotium or a piece of infected plant stem from a
previous season floats to the surface of the flood water and comes into contact
with a rice stem (Fig.2). The fungus grows out onto the sheath at the flood line
and begins growing upwards on the plant, occasionally penetrating the sheath and
causing a lesion or spot, and to nearby plants in the row.. As the rice plant
shifts to the reproductive stage (panicle differentiation or midseason) the
disease becomes more aggressive, causing larger lesions and damage and growing
upwards more rapidly. The fungus eventually reaches the upper leaves or panicles
(blows out the top) resulting in damaged "circles" or patches up to several feet
in diameter in the field. These are readily observed from the combine during
harvest and are often congregated in lower ends of fields or near levees and
field edges where sclerotia and infected plant debris are congregated over time
by rainwater between crops or flushing and initial flooding of each rice crop.
Factors that increase the disease include consistent use of highly susceptible
semidwarf long-grain rice varieties, short rotations (rice each year or every
other year), overuse of nitrogen fertilizer, and reduced tillage practices that
encourages the survival of the fungus.
Management - There is no "silver bullet" control method for sheath
blight. Control is largely based on adopting a rice production system that
allows one to "live with" the disease, reducing the inevitable loss as much as
possible. We recommend a three-pronged attack against the disease: 1) growing
more tolerant varieties; 2) using cultural practices not favorable to the
disease; and 3) fungicides when disease levels exceed a threshold level in a
particular field. Tolerant varieties are the first option that should be
considered. While all varieties are somewhat susceptible to damage under the
right conditions, taller, open-canopied types have much less yield loss from
sheath blight than the semidwarf varieties. This is probably due to several
factors, but the simple fact is that damage is mostly due to the disease
destroying the upper two leaves before grain fill is complete - and this is
harder for sheath blight to accomplish on a 40-42" tall variety than on one 36"
tall or less. The taller types also tend to have a more open canopy for more
sunlight penetration and lower humidity than the shorter versions, slowing the
disease. Also, long grain varieties are more susceptible than medium grain
varieties, so if medium grains can be grown on historical sheath blight fields,
so much the better. Many county agents, consultants and farmers have recognized
the value of rotating tolerant varieties on sheath blight problem fields for
several years now and, when combined with other appropriate cultural practices,
this has resulted in more consistent and higher rice yields than before, without
the need for fungicides.
Cultural practices greatly influence long and short term sheath blight
problems. Rice grown each year or every other year will quickly develop more
widespread and severe sheath blight than when grown every third year. This is
because the fungus does not survive at high levels more than two years in the
soil without a susceptible host, whether as sclerotia or infected debris. On the
other hand, once introduced into a field, some of the fungus - however small the
percentage - will survive long periods and is almost impossible to eliminate
solely with rotation out of rice. Seed producers routinely rotate out of rice
for two years to control red rice, and their fields have much less sheath blight
than commercial rice fields in every other year production. Reduced tillage also
increases sheath blight in the long term because the fungus survives better when
infected debris and sclerotia remain undisturbed between crops. Both tillage and
burning destroy at least some of the fungus in infested fields and while the
disease may still be a noticeable problem, it could be even worse without these
practices. During the growing season, two cultural practices greatly influence
sheath blight. The first is thickness of stand and the other is the amount of
nitrogen fertilizer used on the crop. Most modern rice varieties tiller very
well and do not need to be planted all that thick in order to have enough
tillers for maximum yield potential. Research has consistently shown that an
initial uniform stand of 15 - 20 plants per square foot (9-12 per row ft on 7"
drill spacings or 13-17 per row ft for 10" drill spacings) results in the most
consistent and highest yields. Lower or higher populations than this are less
consistent and have increased risks either from more weed and insect problems
(low populations) or disease problems (high populations) and will more often
result in reduced yields. In the case of sheath blight, high plant populations
result in less vigorous tillers due to plant competition and a more dense canopy
earlier, which favors the disease through increased humidity and less air
circulation.
Excess nitrogen fertilizer is the single biggest cultural practice favoring
more severe sheath blight during the growing season. This can often be observed
at the edges of fields where the disease is much higher on plants where nitrogen
is "lapped" than in areas receiving a normal nitrogen rate. Sheath blight
appears to explode upward on plants receiving too much nitrogen fertilizer
because plants are more lush and free nitrogen in the cells can be used by the
fungus for rapid growth. While it is critical to use adequate nitrogen on modern
cultivars to obtain high yields, higher rates than recommended for the variety,
especially during tiller elongation, can mean much more damage. While less is
known about the effect of other nutrients on sheath blight, it is likely that
potassium deficient rice plants are more susceptible to the disease - as is the
case for other rice diseases like stem rot and brown spot. It is always wise to
soil test frequently in today's intensive cropping systems and fertilize
accordingly.
Fungicides are the last weapon in our sheath blight control arsenal and have
tended to be the weakest link historically. Consistent control and especially
economic return from the use of currently registered fungicides has been very
difficult to demonstrate in commercial fields. This is due to several factors
including the patchy nature of the disease in most fields, limited effectiveness
of currently registered fungicides at labeled rates, difficulty in applying the
fungicides evenly in the rice canopy with airplanes, and the high cost. The
patchy nature of the disease can be easily seen in aerial photographs and often
shows the disease congregated in lower ends of fields or near edges. When
considering a fungicide, this means that a large portion of a field may have few
or no infected plants while a smaller area may have many. The point is, a farmer
pays to treat the whole field, but only the infected areas will get any benefit.
Common sense says that this 'patchiness' makes it very difficult for the
fungicide to pay for itself, much less result in profitable control, unless
sheath blight is widespread in the field. Another characteristic of the disease
is that it causes yield loss only if it is able to destroy the upper two leaves
of infected plants before grain fill is complete. All tillers can be infected at
midseason, but if the disease does not move to the upper leaves, little or no
loss can be measured. In years where temperature and humidity are lower after
tiller elongation starts, such as in 1994 and 1996, most fungicide applications
for sheath blight were probably wasted, since weather slowed the disease
dramatically. The above factors mean that a farmer must make the best guess
possible as to whether or not a fungicide application will be profitable, so it
is critical to scout the fields to be treated. Not only is it important to
scout, but it is essential to scout effectively. With a disease like sheath
blight - and given the size of modern rice farms and the limited number of
people available to scout - scouting properly is difficult. The number of
samples needed and man-hours required are just too great. We recommend the
following system that is barely adequate but better than nothing. Scouting a
rice field should be done in a zigzag fashion through the central and largest
part of the field. Lower ends and edges should be scouted separately. We
recommend the positive stop method because it is faster and easier to use than
other methods while still providing useful information. Our definition of a
positive stop is any infected tillers within a 3 ft long section of rice that
you have bent over to inspect at or after midseason. The 3 ft section can be a
row length, if drilled, or merely a 3 ft length bent over if broadcast. A
"tee-stick" made of PVC pipe with a 3 ft horizontal length and 4 ft handle is of
great help in scouting. Construction is simple and cheap and every rice county
agent or rice consultant can advise on how to make them. When scouting, the
human eye will naturally try to focus on diseased areas in the field, because
these stand out from healthy rice. For this reason, we recommend walking a
zigzag pattern stopping every 50 steps to check for sheath blight. When walking,
it is important not to look for the disease so the eyes should be focused in the
distance. At 50 steps, stop and bend over a random length of rice and look at
that 3 ft section and only that section. If any sheath blight is found, count
the stop as positive. Make 40 stops minimum in a normal rice field of 40 acres
or more and keep up with the number of positive stops. Divide the number of
positive stops by the total number of stops to determine the percent positive
stops for the field. On susceptible semidwarf varieties like Cypress, Lemont,
Jefferson, Jodon, and Lacassine, we suggest not treating unless the field has at
least 35% positive stops in the scouted area. Even then, a decision to actually
spray should be delayed until the disease is moving aggressively up the plant
and threatening the upper 2-3 leaves. While we recommend beginning scouting at
panicle differentiation or mid-season, it is usually better to wait to apply a
fungicide until the upper leaves are being threatened by the disease. In other
words, if you have enough positive stops to treat, wait until the disease has
moved up the plant to the flag minus 2 leaf on average. The idea is that,
considering everything, it is better to protect the upper canopy as long as
possible at the least expense. Since the average cost for a single fungicide
application is $20 - $35 per acre, it is imperative to do it right.
A new fungicide, called Quadris, will be available on a limited basis this
year under a Section 18 emergency exemption. Quadris is more effective at lower
rates than the other currently registered fungicides. A single 0.2 lb/ac active
ingredient rate (12.3 fl oz of Quadris Flowable per acre) suppressed sheath
blight vertical development between 21 and 30 days in 1996 field trials. No
surfactant was used and there is no reason to expect improved control with one
at this time. The average length of suppression appeared to be 24 or so days.
This should protect the upper canopy in most cases through much of the grain
fill period if applied at 10-14 days after midseason, which is a normal
recommended timing for a single application of a fungicide to suppress sheath
blight. The addition of this fungicide should result in improved chemical
control of this difficult disease. However, the economics of its use have not
been determined, and no matter how effective, there are many fields that will
not need to be treated. Simply put, if there is not enough sheath blight in a
field to justify treatment or if weather does not favor vertical development,
fungicides will not be profitable. Several other fungicides are currently
registered and we have listed some of the rates used and their relative
effectiveness as determined in field experiments. Recommended treatments are in
bold.
|
Sheath Blight Fungicides for 1997 |
|
Fungicide |
Rate/A
(Product) |
No. of Applications |
Effective-ness
(1-10)*** |
Projected Cost/A (with
airplane) |
|
Quadris* Flowable |
12.3 fl oz |
2 |
9 |
$66-72 |
|
Quadris* Flowable |
6.15 fl oz |
2 |
8 |
$37-42 |
|
Quadris* Flowable |
12.3 fl oz |
1 |
8 |
$32-35 |
|
Moncut** 50WP |
1 lb |
2 |
8 |
$48-58 |
|
Moncut** 50WP |
.7 lb |
2 |
7.5 |
$38-42 |
|
Moncut** 50WP |
1.4 lb |
1 |
7 |
$36-40 |
|
Moncut** 50WP |
1 lb |
1 |
6 |
$25-30 |
|
Tilt 428C |
10 fl oz |
1 |
6 |
$22-25 |
|
Benlate |
1.5 lb |
2 |
6 |
$46-50 |
|
Benlate 50WP |
1.5 lb |
1 |
5 |
$23-28 |
|
Tilt fb Benlate |
10 fl oz fb 1.5 lb |
1 fb 1 |
6 |
$45-53 |
|
Rovral R |
1 pt |
2 |
6 |
$44-50 |
* Quadris is available only on a Section 18 emergency exemption in 1997. Full
registration on rice is expected in 1998.
** Commercial field data on Moncut is limited. Effectiveness has varied somewhat
in plot studies, especially on single application rates. Wheat cannot be planted
after rice treated with Moncut.
*** Rating system where 1 = useless and 10 = very effective.
Read and follow the fungicide label.
Blast
This is the other major rice disease in Arkansas, causing sometimes dramatic
yield losses on susceptible varieties, altering what a farmer can or cannot
grow, and costing large amounts in breeding for resistance and other control
options. Blast is important world-wide and known since ancient times as a
devastating problem in Asia. It is traditionally a bigger problem on lighter
soils with poor irrigation and less a problem in water-seeded or properly
irrigated rice.
Symptoms - Blast is a foliar disease, attacking above-ground plant parts but
most commonly found on leaves (leaf and collar blast) or the panicles (neck and
panicle blast). On the leaves, very early lesions (spots) are off-white to
gray-green in color with a dark green border and only 1/16 - 1/8" across. These
spots rapidly turn gray or gray-white in the center with a brown to reddish
brown border and typically enlarge to 1/2" - 1" long by 1/8 - 1/4" wide. They
are often wider in the middle than at the ends and diamond-shaped, although they
may appear as narrow oval-shaped or irregular on some varieties. On highly
susceptible varieties, such as the California types, lesions may be several
inches long or the entire leaf may die within a week. About a week after
infection, the fungus sporulates within the lesion producing a dark, gray
powdery appearance, especially in the early mornings or on the underside of the
lesion during the day. It is very common for blast to attack the base of the
flag leaf (collar blast), prematurely killing it, and infecting the sub-panicle
node as it exserts through this infected area. Other leaves on the plant can
also be afflicted with collar blast, especially on certain varieties like
LaGrue. Lower nodes can also be infected, turning dark gray or black, and
resulting in lodging. At heading, blast attacks the node just below the panicle
(neck blast) killing the entire panicle before grain fill is complete. This
sub-panicle node is darkened and often covered with spores of the fungus and the
panicle will remain upright and bleach off-white, resulting in a characteristic
"white-head" symptom when seen from a distance. In some cases, blast attacks
only parts of the panicle, killing individual spikelets or flowers.
Cause - Blast is caused by the fungus, Pyricularia grisea (formerly
Pyricularia oryzae). The fungus has a sexual state known as Magnaporthe grisea,
which has only been observed in the laboratory. The fungus commonly produces
three-celled, bowling pin - shaped microscopic spores, called conidia, that are
moved by wind and blowing rain (Fig.4).
Disease Cycle - As far as is known, the rice blast fungus only attacks
rice and has no other plant host. Closely-related forms of the blast fungus
infect wild grasses and some turfgrasses but do not infect rice. The rice blast
fungus survives between rice crops in infested rice residue or on seed. Although
not precisely known in Arkansas, seedborne blast probably helps begin potential
epidemics in some circumstances, but not always (Fig.4). Because the spores are
airborne, movement between fields during the season is common and rapid. Once
blast is established on plants, spores spread the disease within a field
throughout the season. Infection is influenced by many factors. Leaf
wetness(free water) is essential so rainy periods increase blast infection as do
long dew periods. The latter is more common in shaded field areas near tree
lines and can result in early epidemics in these areas. Plants that are
drought-stressed are especially susceptible to blast, and for this reason,
epidemics are often associated with field areas that dry easily or have been
poorly irrigated. Slightly cooler than normal temperatures and cloudy, rainy
weather favor blast.
Management - Control of blast in Arkansas relies most heavily on two
factors, resistant varieties and irrigation. Resistant varieties are readily
available from our breeding program and should be grown if blast is a potential
problem in the area. Most of our strongly resistant varieties use 1 or 2
resistance genes obtained from Katy, a long-grain cultivar released in 1989.
Unfortunately, the type of resistant varieties available can lead to development
of new and damaging races of the blast fungus, especially if planted on large
acreages. A new race known as IE-1k was discovered in 1993 and is capable of
attacking Katy, Kaybonnet, Drew and other varieties. So far, this race has been
only a curiosity but increasing acreage of Kaybonnet and Drew may lead to it
becoming a major problem.
Proper flood depth is a critical factor in managing blast. On moderately
resistant and moderately susceptible varieties, blast damage can be prevented or
greatly reduced by proper irrigation. A flood depth of at least 2" and
preferably 4" must be maintained from green ring through heading for this to be
most effective. Even a temporary loss of flood caused by inadequate pumping
capacity (trying to water too large a rice field, for example) or diverting
water for soybeans can be disastrous with regard to blast. If blast lesions are
found in a field, a deep flood should be maintained if at all possible, as this
will reduce the disease potential and improve fungicide results when they are
used. Where conditions favor blast late in the season, both flood depth and
fungicides will most likely be needed.
Other cultural practices that can reduce blast damage include use of adequate
but not excessive nitrogen fertilizer rates, use of cleaned, high quality seed,
adequate soil coverage of planted seed, and destruction of previous rice
residue.
As before, the final alternative in controlling blast is the use of foliar
fungicides. Benlate has been the only registered blast fungicide for many years.
We have no minimum threshold levels for deciding whether or not to spray - it is
largely a judgement call. If blast lesions are observed on leaves prior to
heading, a single application of 1 lb/ac should be made at late boot ( when at
least 50% of the main tillers are splitting the boot or have the very tips of
the panicles beginning to emerge) (Fig. 5). An additional 1 lb/ac application
should be made about a week later if conditions favor blast or blast pressure is
severe. This second application should be made when at least 50% of the main
tillers have the panicles about 3/4 of the way out of the boot (Fig. 5). The new
fungicide, Quadris, available this year under a Section 18 exemption has been
shown to be somewhat more effective than Benlate in University tests but is
still very dependent on proper application rate and timing. The above timing of
application should be used and Quadris should be applied at .2 lb ai/ac for each
application (12.3 fl oz of Quadris Flowable per treatment) for blast. This is a
very expensive treatment ($33-35 per application) and many growers may find
themselves debating the necessity of using this fungicide or making the second
application. Benlate can be used as the second application, although we have
limited information on the effectiveness of this combination. The first
application is usually the most critical, although past data has shown both to
be needed, especially when disease pressure is moderate or above. Consult the
local county extension agent for advice on particular field situations.
As always, producers should follow all label directions when using fungicides
- whether for blast, sheath blight, or any other disease.
Rick Cartwright, Extension Plant Pathologist
Fleet Lee, Professor of Plant Pathology
Back to Rice Diseases
Back to Plant Diseases |
