Determination of in vitro biocontrol efficiences of endophytic and epiphytic bacteria against some citrus fungal pathogens

Citrus (Citrus spp.) species, as a strategic crop, are considered an important fruit due to the economic value of exports and local consumption in Turkey. Fungal diseases such as anthracnose (Colletotrichum spp.), Fusarium wilt and dry root rot (Fusarium spp.) and sour rot (Geotrichum citri-aurantii) cause serious yield and quality losses during cultivation and storage periods. In recent years, priority has been given to researching alternative methods to chemical pesticides with diseases. One of the promising control strategies that show great potential as a reliable environmentally friendly method in the management of such diseases is biological control using antagonist and plant growth promoting bacterial biocontrol agents. Multiple aims of this study were to isolate and identify of endophytic and epiphytic bacterial isolates obtained from healthy citrus trees, to determine their in vitro antagonistic activities against Geotrichum citri-aurantii, Colletotrichum gloeosporioides and Fusarium solani as well as to characterize mechanisms potentially involved in promoting plant growth and antagonistic activities. Total of 48 bacterial isolates were obtained from the rhizosphere and endosphere of healthy trees and identified by MALDI-TOF MS analysis. In dual culture assay, Bacillus vallismortis YGL73ep, Bacillus cereus YGK25en and Pseudomonas chlororapsis YGM82ep isolates were found as the most effective antagonistic bacterial isolates against Geotrichum citri-aurantii (73.33-77.22% inhibition); Bacillus thuringiensis YGT22en, Bacillus vallismortis YGL73ep, Bacillus cereus YGK25en and Pseudomonas chlororaphis YGM82ep against Colletotrichum gloeosporioides (65.56-72.78% inhibition); Bacillus subtilis YGS5en, Bacillus cereus YGK25en and Pseudomonas chlororaphis YGM82ep against Fusarium solani (71.11-66.11% inhibition). Volatile organic compounds (VOCs), produced by the efficient isolates such as Pseudomonas chlororapsis YGM82ep, Bacillus vallismortis 73YGep, Bacillus thuringiensis 22YGen and Bacillus cererus 25YGen, significantly inhibited mycelial growth of tested fungal disease agents in vitro dual culture tests. Among the tested 25 bacterial isolates, 20 isolates were positive for siderophore production, 24 isolates for ammonium production, 19 isolates for protease and 5 isolates for HCN production. Of the 25 bacterial isolates tested, 21 isolates produced varying amounts of IAA in the concentration range of 1.84-69.68 µg/ml, and 7 isolates were found to be capable of solubilising phosphorus. Among the isolates, Pseudomonas koreensis YGL72ep, Pseudomonas chlororaphis YGM82ep and Bacillus simplex YGS12ep produced a relatively large amount of IAA and Rahnella aquatilis YGP63ep, Raoultella ornithinolytica YGP64en ve Pseudomonas koreensis YGL72ep solubilised phosphate. In conclusion, the strong antagonistic and PGP traits displayed by effective isolates of Bacillus spp. and Pseudomonas chlororaphis may be developed as biological control agent in the suppression disease caused by Geotrichum citri-aurantii, Colletotrichum gloeosporioides and Fusarium solani as well as their potentials as a biofertilizer.