Occurrence and functional diversity of bacteria in rhizosphere of citrus trees infested by Tylenchulus semipenetrans in a citrus-growing area of Tunisia

Abstract

Tylenchulus semipenetrans is a major plant-parasitic nematode pest of citrus which readily infests sour orange rootstock Citrus aurantium. In order to examine the bacterial diversity and prevalence in the nematode infested citrus rhizosphere, 282 bacterial strains were isolated from roots of mature citrus trees, from egg masses of T. semipenetrans, and from nematophagous fungi in the orange producing area Cap bon in Tunisia. The rhizosphere of citrus had a high diversity of bacterial strains among the 20 agricultural parcels. According to morphological and biochemical characteristics, sixty bacteria strains were selected to explore the diversity of biological activity. None of the bacterial strains considered in this study have phytopathogenic activity. Each strain was identified and tested as a potential natural antagonist against fungi and against citrus nematode (egg hatching and juvenile mortality). Bacillus cereus, B. mycoides, B. pumilus and B. thuringiensis were the four most abundant species regardless the sampling location. Canonical discriminant analysis grouped the bacteria into five groups of biological activities: Group A, similar to water control, had not specific effect. Group B showed strong effect, and group C caused moderate growth inhibition of the nematophagous fungi Monacrosporium cianopagum CBM. Group C had stronger growth inhibition of the fungus Arthrobotrys conoides RGA than groups B and D. Groups D and E showed a strong activity on nematode egg hatch inhibition.