Bacterial selection for biological control of plant disease: criterion determination and validation

Abstract

This study aimed to evaluate the biocontrol potential of bacteria isolated from different plant species and soils. The production of compounds related to phytopathogen biocontrol and/or promotion of plant growth in bacterial isolates was evaluated by measuring the production of antimicrobial compounds (ammonia and antibiosis) and hydrolytic enzymes (amylases, lipases, proteases, and chitinases) and phosphate solubilization. Of the 1219 bacterial isolates, 92% produced one or more of the eight compounds evaluated, but only 1% of the isolates produced all the compounds. Proteolytic activity was most frequently observed among the bacterial isolates. Among the compounds which often determine the success of biocontrol, 43% produced compounds which inhibit mycelial growth of Monilinia fructicola, but only 11% hydrolyzed chitin. Bacteria from different plant species (rhizosphere or phylloplane) exhibited differences in the ability to produce the compounds evaluated. Most bacterial isolates with biocontrol potential were isolated from rhizospheric soil. The most efficient bacteria (producing at least five compounds related to phytopathogen biocontrol and/or plant growth), 86 in total, were evaluated for their biocontrol potential by observing their ability to kill juvenile Mesocriconema xenoplax. Thus, we clearly observed that bacteria that produced more compounds related to phytopathogen biocontrol and/or plant growth had a higher efficacy for nematode biocontrol, which validated the selection strategy used.

Keywords: Antibiosis; Mesocriconema xenoplax; Obligate parasites; Perennial plants; Rhizobacteria.

Fig. 1

Percent bacteria producing compounds related to biocontrol and/or promoting growth depending on the total isolates number. (A) AMI, amylase; AMO, ammonia; ANT, antibiotics (halos type I – ≤10 mm; II – ≥11 and ≤20 mm; and III – ≥21 mm); FOS, phosphate solubilization; GEL, proteases medium gelatin; LIT, proteinase medium Litmus^®; CHI, chitinase; TWN, lipases medium Tween 80. (B) Groups formed as the isolation origin (Ficus carica, Gramineae, Leguminoseae, others, Liliaceae, Prunus persica, soil, Tagetes).

Fig. 2

Percentage of isolates (total = 1219) grouped as reaction intensity: (i) chitinolytic; (ii) lipolytic; (iii) proteolytic on gelatin medium; (iv) proteolytic Litmus^® milk medium; (v) ammonia-production; (vi) amylase-production; (vii) antibiotic against Monilinia fructicola; (viii) phosphate-solubilization.

Fig. 3

Percentage of isolates with positive reaction grouped as isolation niche (phylloplane, rhizosphere and soil): (i) individually chitinolytic (135); (ii) lipolytic (604); (iii) proteolytic on gelatin medium (839); (iv) proteolytic medium Litmus^® milk (630); (v) ammonia-producers (531); (vi) amylolytic (443); (vii) antibiotics producing (529); (viii) phosphate solubilizers (530). Values between brackets represent the number of isolates with positive results.