By Yeshi Wamishe, Extension Rice Pathologist
After the hot and dry weather for several weeks, in the last couple of weeks we got rain without a substantial decrease in day or night temperatures. Although it appeared good for rice crop, these conditions seemed to favor the development of bacterial panicle blight and sheath blight.
We already got a couple of reports on bacterial panicle blight from commercial rice fields and also sheath blight picking up in several fields. We also are seeing more of these two recently in fields at Rice Research and Experiment Center.
Bacterial panicle blight (BPB) of rice: The bacteria that cause panicle blight disease of rice are mostly seedborne and follow the rice canopy vertically until heading with or without the presence of rain. However, rain, particularly windy rain, plays a significant role in the spread of the bacteria and in the intensity (Figure 1) of the typical symptoms assuming the presence of infected plants in the field is a susceptible variety. To read more on what we learned through our observations and research in the past few years, go to http://www.arkansas-crops.com/2016/07/22/bacterial-panicle-research/. To identify bacterial panicle blight go to: https://www.uaex.edu/publications/pdf/FSA-7580.pdf and http://www.arkansas-crops.com/wp-content/uploads/2011/12/Panicle-Blight-Small.pdf. Since there are other factors that can cause similar symptoms to BPB, for symptom confirmation contact your county agent.
Chemicals are not used in the management of bacterial panicle blight. Most conventional rice varieties are susceptible to BPB. Hybrids and Jupiter have better resistance. Knowing the presence or absence of BPB in your seed production field may be indicative of how much inoculum will go back to fields next year in addition to the yield and quality loss it may currently cause.
Sheath blight disease of rice can easily be managed with commercially available fungicides in Arkansas (Refer to MP154). In the dry, hot weeks of July, sheath blight was creeping slowly. As a result fungicide application was relatively low in early planted rice. However, in a short time under the prevailing high heat and humidity, it is picking up both vertically and horizontally (Figure 2). Therefore, it is important to scout and take timely action. Fungicides are not recommended for sheath blight after the heads started filling. However, it is your judgement call to apply fungicides at a lower rate (6 to 8 oz) in rice prone to lodging after heading. Please take the 28 days pre-harvest index (PHI) into consideration before you decide on a late fungicide application for sheath blight. More than one fungicide application to manage sheath blight has not proved profitable in previous studies.
Blast: The frequent rain we are having these days can also be favorable for blast disease development. Considering the versatility of the blast pathogen to different environmental conditions and its survival mechanisms, we need to be diligent on its management in late rice. Timing and frequency of fungicide application have been always important.
Blast is an unpredictable disease. Varietal resistance can sometimes be tricky due to the race changes and field management. Neck blast can cause near 100 percent grain yield loss (Figure 3). Due to the airborne nature of the blast pathogen, the absence of leaf blast early in the season does not guarantee absence of neck or panicle blast later in a season. Although we have not learned enough on each commercial variety, we have a few examples of varietal response to blast. Roy J for instance, is a kind of variety that may not show leaf blast earlier in the season, but it is often very susceptible for neck blast. Jupiter is very susceptible for leaf blast but tolerates neck blast better. While CL151 is susceptible both for leaf and neck blast. Therefore, we need to be ahead of the pathogen about timing of fungicide application. Once the panicle necks are fully out of the boots, we cannot go back and correct fungicide application timing. It is already too late!