Arbuscular mycorrhizal fungi (AMF) and members of the genus Trichoderma have emerged as promising
groups of microbial inoculants that can induce plant growth and resistance to disease. This study aimed
at investigating the potential of AMF and a strain (PR11) of Trichoderma asperellum to promote cacao
growth and induce resistance against Phytophthora megakarya. Cacao seedlings were either noninoculated,
or inoculated with the saprophytic fungus T. asperellum and/or a mixture of two different
mycorrhizal fungi, Gigaspora margarita and Acaulospora tuberculata. Eighteen weeks after planting,
a series of morphological as well as biochemical changes, which are considered to be part of the plant
defense response, were measured after a challenge inoculation of the leaves with zoospores of P. megakarya.
Inoculation with AMF and T. asperellum alone was essential for the promotion of plant growth.
Significant increase in plant height, root and shoot fresh weights, as well as phosphorous uptake was
recorded in comparison to non-inoculated control plants. However, dual inoculation of cacao seedlings
with T. asperellum and AMF did not always positively benefit the plants. Leaf inoculation showed variation
among the treatments, with the lowest disease index (highest level of resistance) recorded in
plants inoculated with either AMF or T. asperellum only. This came along with a high synthesis of amino
acids and phenolic compounds in both healthy and infected leaves, suggesting that these metabolites are
implicated in disease resistance