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28
Jul
2017
Rice disease update: Bacterial panicle blight and beyond
Author: Yeshi Wamishe, Extension Rice Plant Pathologist

No bacterial panicle blight (BPB) has been reported from commercial fields to date (7/28). However, we have started seeing some on conventional rice at RREC (Rice Research and Experiment Center) in observation and experimental plots. In our survey in the past years, we confirmed Burkholderia glumae as the major causal species of bacterial panicle blight in Arkansas. For the fact that this species is mainly seedborne, the sources of inoculum are largely infected seeds.

For disease to occur, three factors are important —the presence of virulent pathogen, a susceptible host and favorable environment. In our informal survey of seed samples, 2% to 10% seeds were found positive for B. glumae. In susceptible rice varieties, even the lower percentages of seeds may bring significant levels of disease pressure under favorable weather conditions. Extended high night temperatures mostly (>75ºF) are considered conducive for BPB development. Boot split to flowering of rice crop are also proven to be the most susceptible stages for infection. We have also proved the role of dew or moisture in enhancing infection. Our preliminary test has also indicated the favorability of tree lines to the disease in elongating dew periods. Rain associated wind at the sensitive stages of the crop has also been observed to help BPB spread. The biggest concern usually is when rice crops get close to flowering and if the extended heat of July is followed by rain. Generally, it has not been easy to predict severity levels of BPB. Weather parameters such as temperatures, moisture and wind being the biggest factors in disease development and spread, lack of knowledge on seed infection levels at or before planting even makes prediction more difficult. However, research has proven the probability of the disease in early planted rice that have passed flowering to have much less BPB than those planted late.

Field Identification of BPB: Symptoms of BPB are easily detected at early stage of grain filling. Go to a fact sheet on BPB to read on typical symptoms of bacterial panicle blight.

To correctly diagnose BPB in rice, rule out other panicle blanking factors listed below before you get into details of BPB.

  1. Environmental factors such as high night temperature: Symptoms spread across the field. Floret blankness nearly uniform within panicles and across the field. No floret discoloration if affect is new. Check other fields in the area planted with the same variety and the same day for comparison.
  2. Drought stress: Compare the rice from drought stressed and unstressed spots of the field.
  3. Neck blast: Look for neck rot, the rachis (panicle branches) should also be tan and dry and you will see no floret discoloration at the base.
  4. Panicle blast: Blanking usually partial.  The rachis are tan dry. No floret discoloration in panicle blast as in BPB.
  5. Wind: Rice plants at field edges or on levee usually get the wind effect. Early wind symptoms make upper half of the kernels tan with deep brown ring and lower greenish. Later the parts will turn tan. Florets that failed to pollinate turn tan faster.
  6. Stem borer: Pull up rice plants with blank heads and see if they pull out easily. Check also for holes formed by larvae, you may sometimes find the larvae. Early stem borer injury shows the closest floret symptom to BPB. Later it tans out completely and then saprophytes grow on it.

All BPB symptoms are not fully blank panicles, see Figure 1. Blank florets may be found across the panicles, see Figure 2 or BPB may occur just on the older part of panicles, see Figure 3, or more at the youngest part of the panicle as in Figure 4. Secondary tillers may get more infection from the primary panicles as in Figure 5. Up right blank florets can easily be identified early or late in the season. Otherwise, with the other types lab test may be required.

Fig. 1. Panicles fully blank are usually seen up right clearly as healthy panicles bend during grain fill.

Fig. 1. Panicles fully blank are usually seen up right clearly as healthy panicles bend during grain fill.

Fig. 2. Infected blank florets may be seen across the panicles

Fig. 2. Infected blank florets may be seen across the panicles

Fig. 3. Infected blank florets may sometimes be more on the older portion of panicles.

Fig. 3. Infected blank florets may sometimes be more on the older portion of panicles.

 

 

 

 

 

 

 

 

 

Fig. 4: Younger panicle florets may be severly affected than the older florets.See the typical green panicle branches one of idetifying features for BPB.

Fig. 4: Younger panicle florets may be severly affected than the older florets.See the typical green panicle branches one of idetifying features for BPB.

Fig. 5.  When it appears few main tiller panicles infected with BPB, panicles from the secondary tillers may have been infected more (open the canopy to see such a situation).

Fig. 5. When it appears few main tiller panicles infected with BPB, panicles from the secondary tillers may have been infected more (open the canopy to see such a situation).

Rice blast

To date (July 28, 2017), although we have received reports of leaf blast from 13 Arkansas counties, namely Woodruff, Monroe, Perry, Pulaski, Lawrence, Clark, St. Francis, Randolph, Jackson, Desha, Clay, Greene and Crittenden, severity levels have not been alarming. Cultivars reported included Jupiter, LaKast, Diamond, Francis, Titan, Diamond, Rex, Roy J and CL151.

Continue scouting for blast and take timely actions to minimize yield risk. Blast is more severe in late planted rice. It is never too late to raise flood levels and keep permanent flood in blast-prone fields cultivated with susceptible varieties. As long as dew period is long in a range of 9 to 14 hours (Figure 6) blast pathogen appears versatile with temperatures. Higher temperatures may slow down the blast disease, however, dew, fog or occasional rain can still help the pathogen to continue spore production.

Fig. 6.  Dew on LaKast at around 10.30 am in the morning and early blast lesions.

Fig. 6. Dew on LaKast at around 10.30 am in the morning and early blast lesions.

Sheath blight: It is more humid this week and sheath blight appears to slowly move up to the upper canopy in susceptible and very susceptible rice (Figure 7). Keep on scouting for sheath blight. A one time rain shower and heat makes a big difference. To read more, view this article written by Dr. Don Groth, Plant Pathologist at LSU go to More Sheath Blight-Potential Reasons.

Fig. 7. Severe sheath blight from natural infection on CL163 at RREC (7/27/17).

Fig. 7. Severe sheath blight from natural infection on CL163 at RREC (7/27/17).

Kernel Smut and False Smut: No kernel or false smut has been reported so far. The false smut fungus is relatively less sensitive to propiconazole fungicides and is more severe in late planted rice. Kernel smut likes hot weather if associated with rain and more sensitive to the fungicides than false smut.

Based on the susceptibility your variety, field history, your management related to water and fertilizations, if you decide to apply protective fungicides for blast, kernel smut and false smut, do it right!  Right means: application of the correct fungicide at the correct timing, correct rate, frequency and volume of water for adequate coverage.  


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