By Yeshi Wamishe, Extension Rice Pathologist
Leaf blast (Figure 1) is one of the most important blast disease in rice. It can start early at tillering in vegetative stages. Neck and panicle blast (Figure 2) can cause near 100 percent grain yield loss in severe situations. In 2018, leaf blast started in the first week of June and to date (7/12), it has been reported in 16 counties in Arkansas. However, only a couple of fields reported severe leaf burndown. Below are some selected questions about blast we have received this year and in past that may be timely and useful.
Q1. If I detect leaf blast early in the season and raise flood depth immediately, do I need to spray twice for later neck/panicle blast suppression?
A1. Maintaining flood depth until drainage time helps to increase the field tolerance of the variety. However, if the field has a history of blast and the variety is susceptible, the crop can benefit from the fungicide applications. Remember, well managed fields benefit from fungicide application better. Note also, the 1stapplication is to protect the primary tillers and the 2nd is for secondary tillers. If you opt for only one application, you have to time it right, and also you should expect lesser protection than a field that obtained two applications.
Q2. I have a row rice field planted with a blast susceptible conventional variety. Blast symptoms have not been detected yet. However, half of the field has slope and the upper part often gets dry faster. I also see some dry spots at the field edges. I do not know about the history of the field. It is an open field with no trees around. Do I need to spray as recommended for neck/panicle blast protection?
A2. I would say yes. A susceptible conventional rice is less recommended for row rice due to the risk to blast. Rice blast is a disease where the pathogen’s spores may be blown from distant fields. Besides susceptible rice can get infected at any developmental stage. No detection of blast lesions now does not guarantee no blast at all. The drier areas in the field would be more at risk for blast to start and serve as a hot spot.
Q3. I have a row rice field planted with hybrid rice. Should I scout for blast?
A3. So far, in the U.S.A, we are fortunate to have hybrid rice unaffected by severe blast. However due to the adaptability of the blast pathogen races, scouting is encouraged. However, it doesn’t have to be near as frequent as in blast-prone fields planted with conventional and susceptible varieties.
Q4. I have Provisia rice and it has blast. How bad will the neck blast get? Do I need fungicide protection?
A5. Last year I observed that Provisia rice was susceptible to neck blast in Arkansas. Provisia rice is not a hybrid and should be handled as any blast susceptible conventional rice.
Q6. At what stage of the crop should I start scouting for blast and for how long should I continue scouting?
A6. This year (2018), blast lesions were detected at early tillering before permanent flood was established. Since the pathogen infects susceptible rice at any developmental stage provided conditions for the pathogen are right, scouting should start as early as tillering stages and continue at least until the correct timing for fungicides are determined and the field is treated.
Q7. If I see leaf blast early in the season and treat field with Quadris, do I need to spray twice later in the season to control neck blast?
A7. Unless there is leaf burndown, fungicides are not encouraged to suppress leaf blast. You should keep permanent flood of at least 4 inches; that is, 4 inches in the shallowest part of the paddy. The effective fungicides work best if applied twice for blast. The first application should be made at late boot to beginning panicle tip emergence and the second when panicles are 50-75% out of the boot on most of the main tillers. Higher rates are best. Spraying three times is not recommended most of the time unless the field is severely infested with blast and the infection started early as you said.
Q8. Do I need to worry about blast when I hear reports of its occurrence in LA, TX, or even in AR?
A8. No, you shouldn’t. Your integrated management practice would be beneficial in such a situation. You should manage your fields, particularly fields in blast-prone area (light soil, tree line, low lying etc). The information will keep you alert about the disease and will encourage you to scout diligently.
Q9. If there is no blast in my field but I know my variety is susceptible to blast, do I need to spray fungicide for protection?
A9. Yes and no depending on the circumstances. Flood depth, blast history of the field, pre-flood nitrogen level, favorable weather, blast potential in your field and more should be considered before making your decision on a fungicide application. Note also blast lesions sometimes are not easily identified. Besides, in some varieties neck/panicle blast can be bad without many lesions on leaves. So it is a judgment call.
Q10. Which rice stages are important for neck/panicle protection using fungicides?
A10. Starting from late/full boot to 75% head out. Remember, “head out” is not the same as “heading”. It is referring to what proportion of the head has exerted out from the boot. Once the neck is out of the boot, it is already too late for a fungicide application. Decision should be based on the larger proportion (50% and >) of the primary tillers. At full boot, boots are fully swollen and flag leaves are fully extended. Panicles develop completely and may reach up to 5 inches or more. Note that internode spacing and panicle length are dependent on the variety’s genetic potential and field nutrient and water management. Boot split stage follows a few days after full boot and then panicles start to exert and become visible outside of the stem. It is important to note how far the head is out. The 1stapplication is recommended between late boot to 10% head out and the 2ndapplication from 50% to 75% head out.
Q11. What fungicides are best for blast diseases?
A11. Fungicides that contain trifloxystrobin or azoxystrobin, xemium or flutolanil should work. For detailed information, see Fungicide Timing for Rice Disease, Page 2.