Trichoderma asperellum, a potential biological control agent of Stemphylium vesicarium, on onion (Allium cepa L.)

Leaf blight disease caused by Stemphylium vesicarium affects onion production in several countries; in Mexico the disease has scarcely been documented. In this study, we identified S. vesicarium in onion plants from Mexico with symptoms of leaf blight, and evaluated the potential of Trichoderma asperellum as a biological control agent against S. vesicarium. The in vitro antifungal activity of T. asperellum isolates from cultures of mango (Tm), tomato (Tt) and onion (To) were evaluated against S. vesicarium. In dual culture assays, the three T. asperellum isolates inhibited between 54 and 61% of S. vesicarium mycelial growth. Using the 'poisoned food technique' we showed that the cell-free culture filtrate of T. asperellum isolate To inhibited growth of S. vesicarium by 61% and reduced conidia production by 75% compared with controls; culture filtrates from isolates Tt and Tm were also inhibitory but less so than isolate To. When onion plants were inoculated with T. asperellum isolate To the severity of disease symptoms caused by S. vesicarium were reduced compared with control plants. Plants infected with S. vesicarium had 62 and 34% less chlorophyll and carotenoids, respectively than control plants; prior inoculation with T. asperellum ameliorated this reduction in chlorophyll and carotenoid content in S. vesicarium-infected plants; the chlorophyll content was reduced by only 38% and the carotenoids content was similar to that found in control plants. The levels of phenolic compounds in plant tissues changed depending on experimental treatment; inoculation with T. asperellum increased the quantity of phenolic compounds by 97.6% compared with the control but only in onion bulbs. In contrast, inoculation with S. vesicarium increased the quantity of phenolic compounds in leaves and roots by 50 and 88.9%, respectively, compared with the control. These data suggest that T. asperellum isolate To has potential for biological control of S. vesicarium in onion crops.