Development and Efficacy Evaluation of 5% Metalaxyl·Dimethomorph Fumigant for Managing Phytophthora Leaf Fall Disease...

Rubber tree leaf fall disease, caused by Phytophthora botryosa, leads to severe defoliation and yield loss, yet no fungicides are currently registered for its control. Conventional spray applications often fail in tall rubber plantations due to dense canopies and hydrophobic leaf surfaces that limit coverage. This study developed a fumigant-delivered fungicide formulation designed to address these challenges. Five percent metalaxyl·dimethomorph fumigant (MD FU) was optimized at a 4:1 active ingredient ratio. The final formulation (4% metalaxyl, 1% dimethomorph, 44% KMnO₄, 4.4% starch, 28% wood powder, 9.4% NH₄Cl, and the remainder kaolin) ignited readily and produced dense, upward-diffusing smoke, meeting the national quality standards for fumigant formulations. Laboratory assays showed that 0.04 g a.i. of 5% MD FU completely inhibited the mycelial growth of P. botryosa. In detached leaf inoculation tests, 5% MD FU provided 100% protection against infection and 87% curative efficacy at 0.04 g a.i. of 5% MD FU. A greenhouse trial on potted rubber seedlings demonstrated up to 70.54% disease control at 0.96 g a.i. of 5% MD FU. Finally, a field trial in a mature rubber plantation confirmed effective disease suppression: the highest dosage of 1,200 g/hm² achieved 70.20% disease control, with a low dosage of 400 g/hm² providing 60.14% control, demonstrating the formulation's effectiveness across a range of application rates. The fumigant vehicle efficiently delivered the fungicides into the canopy, resulting in uniform deposition on foliage and significant reduction of leaf disease. These findings confirm that 5% MD FU offers a superior alternative for managing Phytophthora leaf fall in rubber trees.