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. Pseudomonas chlororaphis PCL1391 and P. fluorescens WCS365, 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. Treatment with P. chlororaphis PCL1391 resulted in best biocontrol of anthracnose. The combined bacterial treatment did not differ from the treatment with P. chlororaphis PCL1391 alone. Colonization experiments showed that P. chlororaphis PCL1391 and P. fluorescens WCS365 are excellent colonizers of bean roots. However, their combined treatment resulted in increased concentration of the total bacterial population on the root tips and reduction of the P. fluorescens WCS365 population. In vitro experiments showed that Pseudomonas chlororaphis PCL1391 reduced pathogen's growth, sporulation and conidial germinability. Similar results were obtained when both bacteria were used in combination. In contrast, P. fluorescens WCS365 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 confirmed by absence of fungal growth over phenazine-1-carboxamide on a TLC plate seeded with C. lindemuthianum spores.