Purple blotch disease, caused by pathogenic fungi Alternaria porri, is an important foliar disease of shallot that can substantially reduce crop productivity. Biological control using Trichoderma-derived preparations offers a potential alternative to intensive reliance on synthetic fungicides. This study evaluated the effects of different concentrations of a Trichoderma sp. preparation on purple blotch development and yield performance of three shallot varieties under field conditions. The experiment was conducted in Tabanan Regency, Bali, Indonesia, from February to May 2020 using a factorial randomized block design. The treatments consisted of four concentrations of the Trichoderma preparation (0, 20, 40, and 60 mL L-1) and three shallot varieties (Bima Brebes, Bali Karet, and Local Tabanan), with four blocks. Disease development and yield-related parameters were evaluated to determine the responses to Trichoderma concentration, shallot variety, and their interaction. The combination of 40 mL L-1 Trichoderma preparation and Bali Karet was associated with a low purple blotch severity of 11.55%. However, the highest dry bulb yield (14.10 t ha-1) was recorded in the untreated Bali Karet control, whereas the 40 mL L-1 treatment in the same variety produced 13.89 t ha-1. These findings indicate that suppression of purple blotch did not necessarily correspond to a proportional increase in bulb yield and that treatment responses were dependent on shallot variety. Therefore, the effectiveness of the Trichoderma preparation should be interpreted by considering both disease suppression and varietal yield response rather than defining a single concentration as universally optimal.
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