Interaction of Colletotrichum coccodes and Verticillium dahliae in pepper plants

Wilting of sweet pepper plants is leading to significant yield losses in organic cultivation in glasshouses in the Netherlands. Verticillium dahliae was consistently isolated from affected plants and is known to cause wilting of plants. While sampling sweet pepper plants, root discoloration and damage of the root cortex were observed. Colletotrichum coccodes was isolated from affected roots. To study the co-occurrence and interaction of both pathogens, sweet pepper plants with, and without wilting symptoms were collected from the glasshouse. V. dahliae was only isolated from plants with wilting symptoms, while C. coccodes was also found on symptomless plants. Single or combined inoculations with V. dahliae and C. coccodes were performed on pepper seedlings to study the pathogenicity and the interaction of both pathogens. Symptom development was evaluated and fungal colonization was measured in the roots and stem with real-time PCR. V. dahliae induced stunted growth, while C. coccodes did not induce symptoms on the shoot. C. coccodes reduced root weight when plants grew under suboptimal conditions but under optimal conditions for plant growth, C. coccodes reduced V. dahliae colonization and symptom development. In conclusion, V. dahliae is the causal agent of wilting of pepper plants and C. coccodes is a weak pathogen, with antagonistic or neutral effects on symptom development and colonization by V. dahliae. This work can contribute to the understanding of soilborne diseases and their interaction with each other.