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Managing bacterial panicle blight: Research gives helpful clues
Author: Yeshi Wamishe, Extension Rice Plant Pathologist

“Be proactive” – this is the best approach to beat the unexpected effects of sporadic diseases such as bacterial panicle blight (BPB) of rice.  Looking back, the rice yield and quality losses we had in 2010 and 2011 were enormous.  Having most of the conventional varieties still susceptible to BPB and with no chemical options at hand, we need to be proactive and implement research indications into our integrated disease management approach.  It is possible to be in the same situation again if we fail to be proactive and do not do the right thing ahead of time.

Planting dates

Studies in 2012 indicated that planting early plays a significant role in minimizing bacterial panicle blight of rice (Clues from 2012 studies).  A study in 2013 also showed a similar trend even if the conditions were unfavorable for the disease.  Research plots were planted with artificially inoculated seeds of Bengal (a susceptible variety) and Jupiter (a moderately resistant variety) in the 3rd week of each month in March, April, and May.  Overall, BPB disease was low in March- and April-planted plots compared to those planted in May (Figure 1).  May-planted Bengal showed a 62 percent yield loss when compared to the yield from the April planting.  Likewise, Jupiter showed a 44 percent yield loss. For both varieties, the yield losses were quite significant.  It is good to note that the low yield from the May planting may not be totally due to BPB disease severity.  Yield comparisons were not made with March-planted plots because plots were severely damaged by bird feeding.  The take-home message from this research is: plant early as long as there is a window of time to put seeds in the ground.  Early planting appeared to help the crop escape BPB disease.  Moreover, other diseases such as blast and false smut are also known to be less severe in early planted rice.  Wamishe1

Seeding rate

A test on seeding rate showed higher BPB incidence with higher seeding rates in Jupiter than in Bengal. These two varieties appeared to have differences in tillering capacity.  Bengal tillered much more than Jupiter.  Therefore, plots with a lower seeding rate were as dense as the plots with higher seeding rate.  As a result, there was no substantial treatment effect on BPB disease incidence, yield, or grain quality in Bengal.  However, the treatment effect in Jupiter was substantial.  Note that tillering capacity of a variety can be affected by several factors.  In addition, seeds for both varieties were artificially inoculated.  This means the inoculum amount per plot can be doubled as the seeding rate doubles.  Infected panicles could be maxed out in a susceptible variety because it is already susceptible.  However, a variety with some level of resistance could eventually be overtaken by the inoculum (high pressure) which also the case in severe epidemic years in natural conditions.  The take-home from this research is: as plant populations increase, the plot density can also increase allowing the disease to spread from plant to plant (Figure 3).  Therefore, it will be a judgment call to choose the correct seeding rate based on your soil type and the variety you are using.
Wamishe2 Wamishe3

Water management

A test to compare intermittent flushing against permanent flood on BPB disease showed twice as high BPB disease incidence in flooded compared to water-stressed plots (Figure. 4).  The disease incidence trend of 2013 was in agreement with that of 2012.  However, when it comes to yield, flushed plots had much lower grain yield than the flooded plots.  It is important to note that regardless of the BPB disease incidence, water is crucial for the normal growth of rice crop and is critical particularly during reproductive stages.  The effect of water shortage can be more severe than the disease.  Therefore, adequate water management is required in rice farming. Bacterial panicle blight is a sporadic disease and hard to predict.  The crop should not suffer from water shortage to manage a disease that may or may not prevail.  The total mean water provided in 2013 during the season for the intermittent treatment was 45 percent less than the flooded treatment and this has caused a significant effect on the crop and its yielding capacity.Wamishe4

Use plump seeds for planting

Bacterial panicle blight is largely a seedborne disease. Healthy looking seeds may carry the bacteria.  In our laboratory activity, we tested seeds from infected panicles.  We used density separation technique and separated the chaff and the discolored light-weight seeds from the plump (healthy looking) seeds.  Seeds from each group were plated out on partially-selective medium as shown in Picture 1.  Each group showed positive for the bacteria.  Discarding the chaff and light-weight discolored seeds from planting could help to reduce the disease to some extent.  In doing so, we may reduce the inoculum source that may be incorporated into the soil through non-emerging seeds and seed parts.  This won’t guarantee absence of BPB disease at the end of the season.   Note that plump seeds could carry the bacteria if obtained from infected fields.  However, using plump, healthy looking seeds for planting has another major advantage.  Vigorous seeds produce vigorous seedlings that would better tolerate adverse conditions of the early season including diseases.  Obviously a minimal amount of chaff and light-weight seeds exist in any bag of seed and this should not be a concern.  However, this information is useful if seeds from heavily BPB infected fields are to be used for planting.  Fortunately, the last two years, BPB was minimal in Arkansas commercial fields and also seeds are normally cleaned prior to sale. Therefore, this should not be a concern.

Picture 1. Infected panicles from field tested for BPB. The bacteria were expected in the chaff and discolored seeds. However, healthy looking seeds also tested positive (yellow is the toxin diffusing into the agar from the bacteria).

Picture 1. Infected panicles from field tested for BPB. The bacteria were expected in the chaff and discolored seeds. However, healthy looking seeds also tested positive (yellow is the toxin diffusing into the agar from the bacteria).



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