Colonization of cacao seedlings by Trichoderma stromaticum, a mycoparasite of the witches’ broom pathogen, and its influence on plant growth and resistance

Trichoderma stromaticum, a mycoparasite of the cacao witches‘ broom pathogen Moniliophthora perniciosa, is being used in Bahia, Brazil to control the disease under field conditions. The endophytic potential of this biocontrol agent was studied in both shoot and root tissues of sterile and nonsterile cacao and bean seedlings. Agar plate and light and electron microscopy studies showed that T. stromaticum is able to colonize extensively both cacao and bean plants grown under sterile conditions. However, colonization was lower when plants were grown under nonsterile conditions, especially in the shoot tissues. Recovery of T. stromaticum from field-grown trees showed that isolates belonging to genetic group II are more persistent (>120 days) as endophytes than isolates from group I. Endophytic colonization of cacao plants by T. stromaticum did not result in plant growth promotion nor induced resistance against M. perniciosa on seedlings that had been treated 30 days prior to application of the pathogen. These results were confirmed by Northern blot studies, where the fungus was unable to alter the expression of selected genes involved in plant defense such as ChiB, a putative class VII chitinase, Glu-1, a putative endo-1,4-β-glucanase, Caf-1, a putative caffeine synthase, and Per-1 apoplastic quiacol peroxidase, genes involved in the regulation of plant growth TcORFX-1(fw2.2-like) and TcLhca-1(photosystem I 24 kDa protein) involved in energy production. This study indicates that induced resistance and growth promotion are not responsible for the activity of T. stromaticum in the biocontrol of the witches’ broom pathogen.