By Yeshi Wamishe, Extension Rice Pathologist
and T. Gebrmariam, T. Mulaw, S. Belmar, C. Kelsey
In the paradigm of plant pathology three factors are important for disease to occur. This includes the perfect alignment of a susceptible host with a virulent pathogen under favorable environment. Some may associate environment only with weather. Sometimes, specific factors that contribute to plant disease occurrence may even be neglected, particularly when they are related to field management. Bacterial panicle blight (BPB) disease of rice was at its peak in 2010 and 2011 robbing significant yield. We worry whenever our rice goes through extended high night temperatures, which also attributes to chalkiness and panicle blanking, without thinking of the other factors which play role in development and spread of BPB disease of rice.
Since 2012, we have seen less and less BPB in commercial fields planted with conventional rice. Here are some notes from what we learned through our research and observations, which may help predict the occurrence of the disease in 2016 in your field.
A three season study on planting dates showed less BPB in early planted rice. As always said, every season has been different. However, trends of data were consistent across the three seasons. BPB disease was low in plots planted in months of March and April compared to those planted in the third week of May (Figure 1).
Nitrogen Fertilizer and Seeding Rates
Studies in seeding and fertilizer rates have shown higher incidence and severity of bacterial panicle blight in experimental plots with excessive seeding or fertilizer rates (Figure 2 and 3), respectively.
Studies on Distribution and Survival of the Major Causal Agent of BPB
Of 165 rice panicle samples that showed BPB-like symptoms collected in 2015 from PREP and ARPT and URRN experimental fields in 9 counties, 45 were verified to have Burkholderia glumae. None were identified with B. gladioli. Based on this survey, B. glumae appears the predominant species in the counties tested. There was no report of BPB in commercial fields in AR during the sampling season.
B.glumae is mainly seedborne. However, there was no strong evidence for it not to inhabit soil or rice residue. We carried out a few experiments and obtained these results: a). B. glumae survived in greenhouse soil for up to four months with a substantial decrease in population size over time. b). Infected rice residue buried or kept on soil surface in greenhouse tested negative when checked after four months. During these test periods, the greenhouse temperature ranged from 78 to 830 F; c). Artificially infected rice residue with B. glumae or infested soil samples buried or kept on the soil surface in a field during off season tested negative after two months. These experiments indicated B. glumae may not survive to infect the following season rice crop. This suggested seeds as the major or may be sole sources of primary inoculum of BPB disease to Arkansas rice fields.
Preliminary Studies on Seed Samples from “BPB Clean Seed Lots”
Although we did not follow systematic sampling, we have been randomly testing for B. glumae in seed lots. These included conventional rice seeds that were harvested from seemingly BPB clean fields in the past few years, i.e., during the low BPB pressure years. Some seed samples tested positive for B. glumae on culture medium ranging from 1 to 8% indicating the possible presence of the bacteria in seed lots that may serve as inoculum sources in fields.
Observations/tests on Effect of Dew/Mist on Bacterial Panicle Blight Disease
From our observations and experiments, there are clear indications that moisture in the form of dew, mist, fog and possibly rain are important factors in BPB disease epidemic. For instance, experimental plots established in an open field and planted with artificially inoculated seeds revealed nearly 100 percent BPB incidence in a susceptible rice cultivar, Bengal, within a week after the tropical storm “Isaac” passed through in 2012. In another scenario, a natural waterway near the tree line of a field caused elongated dew periods which enhanced severe BPB in Jazzman 2 in 2013 in Lee County. However, the disease was less as you go further away from the area. Such observations and other tests contrasted between dew chamber and dry incubation in our studies strongly supported the role of moisture in enhancing BPB symptoms. Moisture as windy rain and other forms are also important in spreading the bacteria within tillers of a rice plant and/or to neighboring rice plants.
Boot split to flowering developmental stages are our peak times for artificial spray inoculation in our studies. This largely simulates infection and the spread of the bacteria among the tillers of a plant. In the absence of windy rain, even if the disease shows up, spreading to neighboring plants can be limited. In some situations, BPB may not blank all the seeds in a panicle. The level of genetic resistance in a cultivar and the prevailing means of spreading (moisture and windy rain) play a role on how severe the disease may be on a panicle. If a panicle is fully blanked due to BPB, it stays upright and can easily be detected. However for seed producer, seed from partially filled panicles can carry the bacteria from season to season without being noticed and this way the inoculum can be accumulated. Therefore, it is possible to overlook panicles that do not show the typical set of BPB symptoms.
It is known, most of the conventional commercial rice in Arkansas are susceptible to BPB. We now know, it is difficult to predict the absence of BPB in any field. However, we need to worry less for early planted rice in 2016 if we had been proactive with our field management.
Here are some tips that may help identify panicles with BPB disease while monitoring your field. Rule out the symptoms caused by the following before you begin the process for BPB identification.
- Extended high night temperature: Symptoms spread across the field. Floret blankness nearly uniform within panicles and across and the field. Check other fields in the area planted with the same variety and the same day for comparison.
- Drought stress: Compare the rice from drought stressed and unstressed spots of the field.
- Neck blast: Look for neck rot; the rachis (panicle branches) should also be tan dry and you will see no floret discoloration at the base.
- Panicle blast: Blanking usually partial. See if the rachis are tan dry. No floret discoloration.
- 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 the base of floret greenish. Florets that failed to pollinate turn tan faster.
- Stem borer: Pull up rice plants with blank heads and see if they pull out easily. Check also for holes in the stem formed by larvae, you may sometimes find the larvae too. Early stem borer injury shows very similar BPB-like symptoms.
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. For symptom confirmation contact your county agent. No chemical is recommended in the U.S. to manage bacterial panicle blight.