Biological control of three Colletotrichum lindemuthianum races using Pseudomonas chlororaphis PCL1391 and Pseudomonas fluorescens WCS365

Colletotrichum lindemuthianum is the causal agent of anthracnose, one of the most important diseases of bean worldwide. The rhizobacteria Pseudomonas chlororaphis PCL1391 and Pseudomonas fluorescens WCS365, known for their biocontrol ability against the tomato pathogen Fusarium oxysporum f. sp. radicis-lycopersici, were tested in planta against three C. lindemuthianum races. Pseudomonas chlororaphis PCL 1391 in the absence as well as in the presence of the pathogen promoted several plant growth characteristics. The promoting effect was greater regarding certain growth characteristics when this strain was tested in combination with P. fluorescens WCS365. On the contrary, treatment with P. fluorescens WCS365 resulted in minor differentiations in the growth characteristics. Treatment with P. chlororaphis PCL1391 resulted in best biocontrol of anthracnose, while P. fluorescens WCS365 showed no significant difference compared to the positive control. The combined bacterial treatment did not differ from the treatment with P. chlororaphis PCL1391 alone. Colonization experiments under gnotobiotic conditions showed that P. chlororaphis PCL1391 and P. fluorescens WCS365 are both excellent colonizers of bean roots. Their combined treatment resulted in increased total bacterial populations on the root tips and reduction of the P. fluorescens WCS365 population. When tested against the three races of the pathogen in vitro, P. chlororaphis PCL1391 reduced pathogen growth, sporulation, and conidial germinability. Similar results were obtained when both bacteria were used in combination. In contrast, P. fluorescens WCS365 applied alone did not affect any of these characteristics. It was assumed that phenazine-1-carboxamide produced by P. chlororaphis PCL1391 was the crucial factor for the in vitro activity of this strain. This hypothesis was supported by the absence of fungal growth over phenazine-1-carboxamide on a TLC plate seeded with C. lindemuthianum spores. In conclusion, P. chlororaphis PCL1391 alone or combined with P. fluorescens WCS365 can be a potential factor in integrated control systems against bean anthracnose in Greece.