Integrated management of sheath blight of rice by fertilizers, fungicides, and plant growth-promoting rhizobacteria
Type of Degreedissertation
Entomology and Plant Pathology
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Sheath blight (ShB) is a soilborne disease causing major economic losses to rice cultivation. The disease is caused by a soil living basidiomycote fungal pathogen, Rhizoctonia solani Kuhn. Reliable and effective disease management strategies are needed for managing rice ShB disease. Most of the prevalent disease control methods are focused against the pathogen directly and have been moderately successful. Lack of durable sheath blight resistant rice varieties and environmental concerns about chemical usage have led to developing sustainable control methods using microorganisms. Advancements in biological control have led to identification and development of plant growth promoting rhizobacteria (PGPR) with plant and root growth stimulating ability. Besides, PGPR induce pathogen suppression by antagonism, competition for space and essential nutrients, and initiation of systemic resistance (ISR). The research objectives of this study were to 1) Screen strains of Bacillus spp. for biocontrol potential against multiple isolates of Rhizoctonia solani collected from diverse rice growing locations, 2) Evaluate combined efficacy of Bacillus subtilis (AP 301) and Azoxystrobin in managing rice sheath blight caused by Rhizoctonia solani under greenhouse conditions, and 3) Evaluate the combined effect of PGPR and fertilizers (NK) applied at different rates for the inhibition of sheath blight disease of rice under controlled environment. In the first experiment, nine PGPR strains (AP 301, AP 52, AP 7, AP 136, AP 295, AP 305, AP 188, AP 294, and AP 209) were screened for in vitro antagonistic effect on R. solani and for in vivo plant growth promotion potential. Three R. solani isolates collected from Arkansas, Mississippi, and Texas were used throughout the experiment. In vitro studies indicated that all the nine bacterial isolates inhibited R. solani mycelial growth by forming inhibition zones ranging from 0.3 to 4 mm. The most effective isolates were AP 301, AP 305, and AP 52 based on the in vitro mycelia and sclerotia inhibition tests against three isolates of R. solani. Nine bacterial strains were subjected to in vitro detached leaf assay. The majority of the strains had a lower percent lesion spread when compared with control. AP 301 isolate resulted in 65.72% lower R. solani lesion spread when compared to control. In vivo rice seed assay was performed for evaluating bacterial isolates’ plant growth promoting properties. Isolates AP 294 and AP 301 showed increased plant growth by 26.5 and 25.5%, respectively, over control treatment. The second study is to evaluate the combined efficacy of PGPR (AP 301) and fungicide against rice ShB. Strain AP 301 was evaluated at concentrations of 0, 103, 106, 109, 1011 CFU/ml (5 factors) in combination with azoxystrobin at 0, 396, 793, 1189, 1585, 1982, and at the recommended rate of 2,378 ppm (7 levels). Overall, azoxystrobin at the recommended rate (R), when used in conjunction with any of the concentrations of strain AP 301, resulted in complete reduction of ShB lesions (0% severity by RLH). Also, the results from other treatments tested suggest that combined application of B. subtilis AP 301 (at 109 CFU/ml) and Azoxystrobin @ 1189 of recommended rate) is an ideal dose of PGPR and fungicide in controlling ShB disease. The last study is focused on PGPR and fertilizer compatibility applied for inhibition of rice ShB. PGPR at 1x109 CFU/ml density was blended with different rates of nitrogen (N) and potassium (K) to evaluate their effect on ShB disease spread and their subsequent effect on rice yields. Pot culture experiments were treated with high, low, and recommended rates of fertilizers to determine the optimum dose of PGPR and fertilizer under controlled conditions. PGPR combined with N fertilizer applied at half the recommended rate resulted in lowest disease lesion spread up to 2.83±0.15 mm and 2.33±0.16 mm for summer and fall experiments, respectively. For summer experiment, the treatment of PGPR combined with N applied at half recommended rate produced higher yields (23.12±0.33 g) than that of treatments applied using higher N rates. However, application of treatments consisting of PGPR and high rates of N fertilizer has produced lower yields ranging from 20.55±0.30 and 17.94±0.89 g. The experiment was repeated again in fall 2012, and our treatments showed a similar trend of disease and grain weight results. In conclusion, PGPR, when applied with lower rates of N fertilizer, had significant effect on ShB disease spread in comparison to treatments with higher rates. PGPR in conjunction with different N fertilizer rates had a significant influence on rice grain yields.