Colonization of strains Bacillus subtilis B5423-R and Pseudomonas flourescens Pf7-14 associated with healthy and sheath blight in rice plants

Pseudomonas flourescens strain Pf7-14 and Bacillus subtilis strain B 5423 effectively suppress rice sheath blight both in greenhouse and in field, both were thought to be potential biological control agents. The average population sizes of strain Pf7-14, a natural resistant strain to nalidixic acid, and strain B 54213-R, a spontaneous rifampicin-resistant mutant strain of B5423, to colonize on rice leaves and stems were monitored under greenhouse condition. The results showed that the colonization of both strains on leaves or stems were positively correlated with bacterial concentration when both were applied at rice maximum tiller stage. That of Pf7-14 leaves was higher than that of B5423-R, furthermore, Pf7-14 survived longer on leaves than B5423-R when both were applied at the same concentrations. That of Pf7-14 on stems consistently declined with time regardless of applied concentrations. In contrast, that of B5423-R dropped with time when lower concentration (4.0 x 10 E7 cfu/ml or less) as applied, whereas the average population sizes remained constant or increased slightly from milky to yellow-ripe stages after higher concentrations of the bacterium (2 x 10 E8 cfu/ml or more) was applied. The difference of the two strains to colonize on healthy and diseased (sheath blight lesion) on rice stems depended on the percentage of lesion area accounted for sampled total stems area. When the percentage is below 8%, the difference was not obvious. When the percentage was 20-35%, the average Pf7-14 population size on diseased stems was about 6 times lower than that on healthy stems at one day after its application, and that on diseased stems was about 2 times lower than that on healthy stems at 14 days after its application. The difference showed decreasing trend with time. While that of B5423-R was about 2 times lower on diseased stems than that on healthy stems at one day after its application, thereafter, that of B5423-R on diseased stems constantly increased as compared with that on healthy stems, and it was 6 times higher on diseased stems than on healthy stems at 14 days after its application. At lesion leaves segments (with 70-90% lesion area), the average population sizes of strain Pf-14 were 3-10 times higher than that on healthy leaves segments at 1 to 7 days after bacterial application. The results also demonstrated that there existed intense competition between introduced biological agents and native bacteria on sheath blight lesion stems or leaves