Biological Control Potential of Spore-forming Plant Growth-Promoting Rhizobacteria Suppressing Meloidogyne incognita on Cotton and Heterodera glycines on Soybean

Abstract

The objective of this study was to screen a library of PGPR strains to determine activity to plant-parasitic nematodes with the ultimate goal of identifying new PGPR strains that could be developed into biological nematicide products. Initially a rapid assay was needed to distinguish between live and dead second stage juveniles (J2) of H. glycines and M. incognita. Once the assay was developed, PGPR strains were evaluated in vitro and selected for further evaluation in greenhouse, microplot, and field conditions. Three sodium solutions, sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), and sodium hydroxide (NaOH) were evaluated to distinguish between viable live and dead H. glycines and M. incognita J2. The sodium solutions applied to the live J2 stimulated the J2 to twist their bodies in a curling shape and increased movement activity. Optimum movement of H. glycines was observed with the application of 1 µl of Na2CO3 (pH =10) added to the 100 µl suspension. M. incognita J2 responded best to 1 µl of NaOH (pH =10) added to the 100 µl suspension. Movement of the nematodes was observed immediately and for up to 30 minutes after application. The 669 PGPR strains were evaluated for the potential of mortality to M. incognita J2 in vitro and for nematode management in greenhouse, microplot, and field trials. Results indicated that the mortality of M. incognita J2 by the PGPR strains ranged from 0.0% to 100% with an average of 39%. Among the PGPR strains examined, 33.5% caused more than 50% mortality of M. incognita J2. In subsequent trials, B. velezensis strain Bve2 reduced M. incognita eggs per gram of cotton root in the greenhouse trials at 45 days after planting (DAP). Bacillus mojavensis strain Bmo3, B. velezensis strain Bve2, B. subtilis subsp. subtilis strain Bsssu3, and the Mixture 2 (Abamectin + Bve2 + Bal13) suppressed M. incognita eggs per gram of root in the microplot at 45 DAP. Bacillus velezensis strains Bve2 and Bve12 also increased seed cotton yield in the microplot and field trials. Overall, results indicate that B. velezensis strains Bve2 and Bve12, B. mojavensis strain Bmo3, and the Mixture 2 have potential to reduce M. incognita population density and to enhance growth of cotton when applied as in-furrow spray at planting. The 670 PGPR strains were evaluated for the mortality of H. glycines J2 in vitro and for reducing nematode population density on soybean in greenhouse, microplot, and field trials. The major group causing mortality to H. glycines in vitro was the genus Bacillus that consisted of 91.6% of the total 670 PGPR strains evaluated. The subsequent greenhouse, microplot, and field trials indicated that B. velezensis strain Bve2 consistently reduced H. glycines cyst population density at 60 DAP. Bacillus mojavensis strain Bmo3 suppressed H. glycines cyst and total H. glycines population density under greenhouse conditions. Bacillus safensis strain Bsa27 and Mixture 1 (Bve2 + Bal13) reduced H. glycines cyst population density at 60 DAP in the field trials. Bacillus subtilis subsp. subtilis strains Bsssu2 and Bsssu3, and B. velezensis strain Bve12 increased early soybean growth including plant height and plant biomass in the greenhouse trials. Bacillus altitudinis strain Bal13 increased early plant growth on soybean in the greenhouse and microplot trials. Mixture 2 (Abamectin + Bve2 + Bal13) increased early plant growth in the microplot trials at 60 DAP, and also enhanced soybean yield at harvest in the field trials.