By Yeshi Wamishe, Extension Rice Pathologist
According to Dr. Jarrod Hardke, Extension Rice Agronomist, over 70% of Arkansas rice is headed, most in the southern Arkansas. Of these fields, most are ready for harvest or close to it. However, the rain has moved in and it appears will continue for several days.
False smut: With the rain, temperatures have also gone down. These conditions may encourage false smut. I observed (8/7) a high level of false smut in CL151 planted on April 12 in test plots at RREC (Rice Research and Extension Center near Stuttgart (Figure 1). The plots were provided with 20% more nitrogen fertilizer above the recommended rate. Excessive nitrogen fertilizer was added to encourage diseases. False smut is a late season disease favored by a wet and cooler environment in susceptible rice. It is severe in fields with history of the disease that received excessive nitrogen fertilization. The false smut pathogen is relatively less sensitive to the protective fungicides than the kernel smut pathogen. Protective fungicides (Propiconazole) have been proven to protect only 50% to 75% of the crop if applied with the correct rate (at least 6 oz/acre of tilt or tilt equivalent), correct timing (between early to mid-boot) and adequate coverage (at least 5 GPA). Both false smut and kernel smut are very important diseases that contribute a paramount loss in grain quality and grain yield. Most rice cultivars including hybrids are susceptible to false smut.
Kernel smut: If rice kernels are already infected, endosperms get fully or partially replaced by pathogen spores. Wet conditions allow the kernels to swell and spores to ooze out (Figure 2). If kernels are free of infection, quality loss would be from wetting and drying effect as harvest is delayed by rain. Warm and wet conditions often favor the kernel smut disease in rice.
Sheath blight: Sheath blight disease of rice is widespread in Arkansas and is estimated to be found in more than 50% of rice fields. It is more severe in susceptible semi-dwarf long grain rice varieties. With hot, dry conditions in July, sheath blight disease was moving very slow. In test plots at RREC where plots were artificially inoculated we have observed sheath blight moving up (8/8) in the last few days (Figure 3). The disease affects both sheath and leaves and progresses quickly under favorable conditions. Therefore, it is advisable to continue scouting both headed and late rice. Rice past milk stage may possibly outrun the disease. Remember the PHI before deciding to apply fungicide. For strobilurin fungicides the PHI is about a month. PHI (Pre-harvest interval) is the number of days from treatment to harvest. To slow down sheath blight disease for up to two weeks, 6 to 8 oz/acre rate may be used. Remember, sheath blight at later stages of the crop is more to protect stem strength against lodging than getting a grain yield advantage. Generally, as long as the upper two or three leaves including the flag are not threatened, there is no need to apply fungicides.
Blast: Last year (2016), joint blast (Figure 4) was seen in some headed rice fields after continuous rain that started at the end of July. Blast spores that had rested on the leaves or leaf collars appeared to be washed and percolated with water between leaf sheath and stem. The spores got trapped at stem joints and similar to neck blast, they germinated and penetrated in joint tissues leading to joint blast and then lodging. To check for joint blast, we remove the flag leaf sheath carefully and look for greyish rot at joints. In hybrid rice we have observed similar joint lesions. Instead of blast spores, we only isolated spores of Culvularia spp. To date, in 2017, leaf blast has been reported in 14 Arkansas counties. A few cases of neck blast have also been reported recently.
Sooty molds and other weak pathogenic and saprophytic microorganisms: Sooty molds (Figure 5) affect the appearance of rough rice and hence, lower the quality of bran. Sooty mold symptoms are caused by opportunistic fungi that colonize rice panicles under wet, humid weather. These molds are often severe in lodged rice or in rice where harvest is delayed. Sooty molds are superficial and, therefore, milled rice is not affected. Often kernel surfaces get covered and black spores are seen on leaves. Sooty molds can sometimes be confused with kernel smut. To separate sooty molds from kernel smut, remove the hull and see if the dark color is superficial or internal. If a kernel has sooty molds, once the hull is removed the kernel should be whole and clean. If kernel smut, the kernel should be partially or completely filled with black spores. Both sooty molds and kernel smut problems may result in black, discolored equipment during harvest. However, heavy kernel smut fields will have severe yield losses and will cause darker discoloration on the combine harvester. Exceptionally high yields and intense kernel smut do not go together. Sooty molds may sometimes be associated with high yield.
If harvest is delayed due to wet conditions, infection from Fusarium spp. may be seen (Figure 6). Also, other microorganisms that cause various kernel discoloration may be favored as in (Figure 7). Neck or panicle blast-looking symptoms as in (Figure 8) may also be seen where most grains in panicles are full unlike neck blast where panicles are blank and upright (Figure 9).
Summary: When harvest is delayed due to rain, real pathogens, opportunistic pathogens and saprophytic microorganisms, in isolation or collectively, work against grain yield and quality.