Saturday, April 13, 2024

Mitigating rice disease in Arkansas

Managing rice diseases with seed treatments and foliar fungicides.

University of Arkansas Division of Agriculture


here are compelling reasons why an integrated approach should be employed to reduce the damage due to diseases in rice or any other crop. As some think, fungicides are not the only options to suppress rice diseases. For a plant disease to occur, three aspects or factors (namely, susceptible host, virulent pathogen and favorable environment) should perfectly align. In a given time, a susceptible host and the pathogen’s infective propagules need to be in contact under a favorable environment to start and proceed the infection process. 

Therefore, when we talk about disease management options, the most important concept is to be able to disrupt the alignment of a pathogen from its host and the favorable environment. If the host is relatively resistant to the disease in question, that disease will be either mild or may be absent regardless of the presence of the virulent pathogen or the environment. 

Similarly, if the environment is disrupted to not favor disease development, there will be mild or no disease regardless of the pathogen’s presence and the susceptibility level of the host plant.  Although weather plays a big role in controlling environment, more goes to field management practices you employ. 

To disrupt the favorable environment, you need to do what you can starting from land preparation for planting, weed and insect suppression to applications of adequate seeding and fertilization rates.  Instead of choosing a susceptible variety, you need to go for a relatively resistant one. It may not be practical to look for a variety that is resistant to multiple disease pathogens. However, a history of your field helps you identify the economically important diseases in your respective rice fields so you can match the variety of your choice with a field history. 

Fungicides are needed to disrupt the activities of fungal pathogens only when required. Often, well-managed rice fields, planted with some levels of disease resistant varieties, may not require fungicide application. However, when fungicide application is required, it can be profitable in effectively managed fields.  

Tips why you should use fungicides on rice

• The disease requires preventative treatment due to a variety’s susceptibility level and field history.

• The field has, or potentially will have, significant disease pressure due to favorable weather, and field management is not sufficiently adequate.

• The specific disease in question definitively and adversely affects milling quality. 

• Pathogen’s propagule accumulated in soil or residue could potentially affect future farming due to poor residue management and land preparation practice employed. 

• There is a potential to ratoon, and the disease may significantly affect the yield potential of the ratooned crop.

Tips why you should not use fungicides on rice

• There is no fungal disease; hence, no need for fungicide application.

• The variety is resistant or moderately resistant. There will be no significant yield loss from the disease.   

• It is already late or too late. No need to apply fungicides because it is past the recommended timing to benefit from the fungicide application. 

• The field is bad with low yield potential (i.e., poor stand and damaged with weeds, insects, etc.); hence, it is not a good candidate for fungicide treatment to make a profit. 

• A peace of mind or “just in case” is not a good reason for one to justify a fungicide application.

Why should you use seed dressing fungicides?

One big reason often given for seed treatment is lower cost compared to foliar application. However, seed treatment with fungicides does not warrant protection from diseases beyond the seedling stage.  

Figure 1: There are some conditions such as low spots that aggravate seed rotting and rice seedling diseases. It can be as serious as shown here.

It is often said that rice is forgiving, and a significant proportion of rice seedlings are likely to survive the harsh conditions of the spring season. Modern rice varieties have a better ability to tiller and fill in available space, compensating for early stand loss later when they get sunshine and warmer weather. 

However, there are some conditions that aggravate seed rotting and rice seedling diseases together with insect damages (Figure 1). Therefore, rice seeds for planting often are treated with fungicides combined with the appropriate insecticides. Nevertheless, seed treatment may last only for two or three weeks in very wet soil conditions. Moreover, fungicide seed treatment is not recommended in water-seeded rice. 

It is more likely the main purpose of seed treatment in rice is for stand establishment.  As long as the purpose of seed treatment is to have adequate seedling stand, the following tips are helpful:

• Seed rotting will be reduced, and seedling emergence will improve when treated with appropriate fungicides and insecticides — using higher rates of seed treatment containing mefenoxam, fludioxonil, metalaxyl, trifloxystrobin, Sedaxane (Vibrance) or any other improved and new seed dressing product — either individually or in combinations of two or more of the fungicides and insecticides is desired.

• Seedling emergence and seedling vigor improve with gibberellic acid seed treatment, particularly when germination capacities of the seeds are weak or seeding rates are low.

• Seeds need to be uniformly covered with the seed treatment products. 

There is more to do beyond seed treatment to have an adequate seedling stand 

• Low areas that puddle in your field need to be corrected for uniform seedling emergence. 

• A germination test before planting is advised to determine the potential seedling emergence. 

• Seeds stored under high moisture and temperature have a greater possibility of losing their viability. Using poor quality seed, particularly seeds stored inadequately for lengthy periods, often germinate poorly.

• Timely correction of soil nutrient deficiencies can maximize crop tolerance to diseases. 

Rice diseases that warrant foliar fungicide treatment

Sometimes, foliar fungicide application is inevitable in some scenarios. However, all rice diseases are not warranted fungicide treatment options. 

Figure 2: A disease threshold needs to be determined before fungicides are applied to manage sheath blight.

Sheath blight is the most prevalent rice disease in Arkansas (Figure 2). Sheath blight epidemics can begin over a period of weeks during the growing season. Therefore, the growth stage when the disease is initiated has significant effects on crop damage and the need for fungicide application. 

Scouting for sheath blight should begin about green ring to ½-inch internode elongation in highly susceptible (VS), susceptible (S) and moderately susceptible (MS) varieties. Fungicide application decisions should not be based on the disease severity at the edge or bottom of a rice field. A disease threshold needs to be determined scouting further into the field in a zigzag pattern. 

The recommended threshold for varieties rated “VS or S” is 35% positive stops and 50% for those at “MS” rating. Visit MP192 chapter 11 for detailed information. Application of fungicides more than once for sheath blight alone is not economical. Time it right or to reduce cost, pair it with other rice diseases. You may mix or use pre-mixed fungicides in such situations.  

Figure 3: Neck and panicle blast late in the rice developmental stages can cause nearly 100% yield loss.

Blast is the most devastating and yield-robbing disease in rice with no timely actions through field management or adequate rate and frequency of fungicides. The pathogen can infect rice throughout a cropping season. It infects leaves, nodes, collars and panicles of rice, and the disease is more threatening to late-planted, susceptible rice. 

Neck and panicle blast are of great concern (Figure 3). The disease is more severe in a field with sandier soil, at river-bottom, surrounded by trees, low in potash and with inadequate irrigation water resources. In such fields, we need to match the right variety in the right field—meaning relatively resistant varieties need to be sought. 

The disease is aggravated by frequent, light rains or conditions that allow long dew periods such as being overcast. Unless too severe, leaf-blast can be suppressed by increasing the flood level, and fungicide application may not be needed at this stage. The flood needs to be held until it is time to drain the field. 

Figure 4: Isolated lesions of leaf blast can coalesce and totally burn the leaves in susceptible rice under favorable conditions to the extent of killing seedlings early in a cropping season.

However, in furrow-irrigated rice, fungicide application may be needed to suppress leaf blast (Figure 4) at seedling stages. Often, two protective applications are recommended to suppress the late-season blast in flooded rice — the first at late boot to boot-split to protect the primary tillers and the second when heads are about two-thirds out in main tillers. The second application is to protect secondary tillers. The time interval between the first and the second application is often between 7-10 days.  

The benefit from fungicide application once the heads/panicles are fully out of boots is minimal and hence, no gain. For fungicide application decisions, a field history, susceptibility of the variety and field management need to be considered.

Figure 5: Kernel smut affects both quality and grain yield. Black pathogen’s spores fill up the endosperm to cause the damage.

Kernel smut and false smut are frequently blamed for quality loss in fields with a history of either or both diseases. Excessive nitrogen fertilization and a field history are considered to be the main culprits for severe incidence of these smuts.

The smuts are unpredictable diseases. They may occur in hot, dry years or warm, wet years. False smut can be more severe on late-planted rice since it is favored more by cooler temperatures than needed for kernel smut. Under favorable conditions, most of the cultivars, including hybrid rice in Arkansas, are susceptible to these smuts. 

Figure 6: False smut galls harvested with rice is a grain quality killer.

False smut appears to be less sensitive to propiconazole fungicides than kernel smut. For this reason, the fungicide rate has been increased to at least a six fluid ounces rate of tilt equivalent. 

Timing is also very important to obtain adequate suppression of these smuts. Often, fungicide applications at early-to-mid boot show better suppression than late applications. Preliminary field tests and observations have shown secondary infection — resulting in high incidences of false smut in secondary tillers in plots that received fungicides that have a greening effect in rice. Both kernel smut (Figure 5) and false smut (Figure 6) cause adverse effects on grain quality. 

Take home message: Although fungicides are best to suppress rice diseases, choosing the right varieties and matching them to your respective fields and field management practices, such as seeding and fertilization rates (particularly N), play crucial roles in rice disease suppression.

Dr. Yeshi Wamishe is an Extension plant/rice pathologist, University of Arkansas Division of Agriculture.

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