Time is passing and disease pathogens are racing to catch up with our growing rice crops. It is not always true that all diseases affect our rice crop at the same growth stage. With some, the resistance level for certain diseases could differ with growth stages. The disease pathogens also have their preferences. In general, the resistance level of our varieties, environment, and growth stages of our rice crops are all important factors for a disease type the crop can contract. Both grain yield and quality are highly affected by several of the major diseases unless managed timely and appropriately.
Seedling diseases: Pythium damping off and Rhizoctonia die back: If we planted treated seeds of high quality, these two diseases have a minor effect on seed germination and emergence under “normal” weather conditions and good water management. Nevertheless, it is important to note emergence since symptoms for these diseases are usually difficult to diagnose. Moreover, other factors such as rodents and birds adversely affect crop stand. In 2012, for early planted rice, seed germination and emergence were delayed in some areas due to lack of rain. Certain rice fields also had spotted leaves on young seedlings believed to be due to the cold spell in early April, possibly freezing dew at night, leaf tips were dried out due to the prevailed dry wind, and in a few cases seedlings were affected by grape colaspis. However, injuries from Pythium and Rhizoctonia were not out of the ordinary statewide in early planted rice. With the badly needed rain of May 5th – 6th and flushing, rice seedlings appeared relatively well-established, withstanding the early-season weather and disease associated hardships.
Once emergence is over, familiar yield-robbing rice diseases start during tillering or soon after and are encouraged after pre-flood nitrogen application. If more nitrogen is available beyond the need of the crop itself, pathogens use it for their own development.
Sheath blight can increase in no time with high pre-flood nitrogen rates during stem elongation, panicle initiation, booting and beyond. Sheath blight appears as water-soaked spots on sheaths near the water line starting from tillering. The disease usually develops from fungal structures that float on the flood water. 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. Experiments have shown that sheath blight epidemics in field plots can be initiated artificially at the green ring, panicle differentiation, early boot, and late boot growth stages indicating a susceptible variety can get infection at any stage of the crop development if the environment is favorable. Scouting for sheath blight should begin about green ring to ½ inch internode elongation in highly susceptible varieties.
Rice blast can appear early or late depending on the sources of the disease pathogen, the resistance level of our varieties, and the environment. The pathogen spores (known as conidia) can be carried in the seeds or they may survive in the soil on infected plant debris or can be blown by wind from nearby infected rice fields. Within the same field if the disease starts early, the spores produced on young leaves can infect the necks of panicles (known as secondary infection), however by this time most leaf infection may have “disappeared” into the lower canopy due to growth of the crop or dried up over time and be hard to recognize. In some cases, neck blast occurs without obvious leaf blast symptoms in the field. Blast is favored by long dew periods, cloudy rainy weather, and excessively high pre-flood nitrogen rates. Therefore, start scouting early and pay more attention to the area of the field near the tree line. Keeping a permanent flood (at least four inch depth) will suppress the disease. Apply the appropriate fungicide timely with the right rate, if needed.
Brown spot can occur at any crop development stage but typically shows after the tillering stage. Brown spot is worse if rice suffers potassium or nitrogen deficiency and can be influenced by many other stresses. Cold water areas of rice fields may also suffer more brown spot. In fields with severe potassium deficiency, brown spot can be very serious, attacking kernels as well as leaves. All varieties have some brown spot resistance under proper fertility and other management, and fungicides have not proven economical. Scout for brown spot starting from tillering stage on as it is a good indicator of plant stress, many of which can be corrected to avoid more severe disease and other problems.
Kernel smut and false smut are now considered more serious diseases of rice. In 2011, we observed up to 30 percent grain loss in some fields that had high levels of pre-flood nitrogen. These diseases cannot be scouted for but can be predicted somewhat by field history (smut problems before?; knowledge of preflood N practices = high rates promote both diseases; and knowledge of the variety = some are more prone to severe smut, e.g. Francis, Cocodrie, Cheniere, RoyJ, etc). The fungicide we use is preventative and not curative. Therefore, timely application with the right rate is extremely important to reduce the incidence of rice smuts.
Bacterial Panicle Blight is now one of the most important diseases of rice in the US. In 2010 and 2011 in Arkansas, the disease was widespread and ruined many rice fields. Scouting for bacterial panicle blight is currently not possible. The source of the bacteria could be seeds or crop residues and the disease may be initiated early in the season but symptoms are not seen until heading. Currently we do not have preventive or curative chemistry for this disease. Early planting, balancing appropriate macro- and micro-nutrients nutrients (such as potassium, phosphorus and zinc) and proper water management were associated with lower disease in field surveys.
Overall, rice is a valuable crop and scouting for developing diseases and other problems could mean big money if a problem could be prevented, or corrected in time. Once diseases or other problems have advanced too far, salvage treatments or methods may be less than satisfactory.
As always, contact your local county extension agent if you have questions.