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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 the fungus.

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 research plots.

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 rice production.

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 productivity.

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.

To summarize:
• 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 introduced races.

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.

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