Rice Diseases are not Forgiving to Rice

by Yeshi Wamishe, Extension Plant Pathologist - August 11, 2022

It is said often that rice is forgiving, but diseases are not forgiving to rice. While the weather was hot and dry, rice grew well without significant disease pressure in the 2022 season. There is not much concern now for rice that is in grain filling. However, some rice diseases are still of concern with late planted rice. For disease to occur, it all depends on the presence of the pathogen, susceptibility of varieties, and favorable environmental conditions. A disease occurs when virulent pathogen propagules are aligned with a susceptible variety under a favorable environment that includes weather and field management. 

With late planting, you may be applying more nitrogen fertilizer to enhance plant growth through the short crop season. The excess nitrogen enhances the development of not only the crop but also the pathogens. Excess nitrogen also makes the plants more lush and prone to pathogen attack. As the season gets warmer and plants go into reproductive stages, water consumption by the crop increases. When water is limited, it creates stress on the crop and the crop becomes prone to pathogen attack. As weather is perpetually dynamic, the combination of dry- hot or wet-warm, foggy-warm, or other situations create favorable conditions for some pathogen groups to survive and reproduce. It is important to note that reproductive stages are generally the most susceptible times for most diseases in crop development.

Sheath blight

Sheath blight appeared to move faster in our artificially inoculated experimental plots at Rice Research and Extension Center (RREC) in the last two weeks (Figure 1) with the humid conditions and ideal temperatures of the upper 80s and lower 90s ºF. It is wise to scout your rice fields that are late planted and still a little behind from grain fill. Make sure at least the upper three leaves, including the flag leaf, are not threatened by the disease.     

Figure 1. Sheath blight reached to the top of the canopy in the last two weeks from height 3 in a 0-9 scale

Blast

Blast is one of the diseases of primary concern in late-planted rice. The pathogen attacks leaves, nodes (Figure 2), leaf collars, and panicles of rice plants. If a susceptible variety is planted late in a field with a history, blast is likely to prevail. These fields include those with sandier soils, river-bottom areas where dew is prominent, surrounded by trees, low in potash, and with inadequate irrigation capacity. So far, we have just a few cases of leaf blast in Arkansas in the 2022 season. However, a few reports do not mean the disease won’t prevail. Therefore, it is important to be proactive if you know your field and variety are prone to rice blast. 

Figure 2. Neck blast shows after the panicles are completely out of the boot. This is already too late to treat with fungicides. Neck blast can blank panicles completely.

Kernel Smut and False Smut

Kernel smut (Figure 3) and false smut (Figure 4) are mostly blamed for quality loss in susceptible varieties and fields with a history. Yield impact is also tremendous from kernel smut. Just like blast, kernel smut and false smut are unpredictable diseases. Under favorable conditions, most of our cultivars are susceptible to smuts. Kernel smut often prevails in warm and wet seasons. However, false smut can often infect rice either in hot-dry or warm and wet seasons as long as heads are still green in a cooler August or after. The pathogen of false smut favors lower temperatures than the kernel smut pathogen. False smut appears to be less sensitive to triazole fungicides than kernel smut. For this reason, the fungicide rate has been increased to at least 6 fl oz propiconazole equivalent rate. Fungicide application is time sensitive ranging from early to mid-boot. Disease suppression levels using fungicides decline as the rate is lowered and application timing is delayed. 

Figure 3. Severe kernel smut can affect both quality and yield.

Figure 4. False smut affects quality of rice more than yield.

Bacterial panicle blight

Bacterial panicle blight (Figure 5) can be a serious problem in late-planted rice since the temperatures remain adequate and moisture helps the bacteria to spread. However, in recent years it has been of low concern in well-managed fields planted with clean seeds.

Figure 5. Bacterial panicle blight blanks panicles and, if severe, can affect yield up to 60%.