Evaluating response variables to the reversion of the ‘engurruñadera del cacao’ through the control of Aceria reyesi (Nuzzaci) (Acari: Eriophyidae) with abamectin and pruning

Abstract

The cacao bud mite, Aceria reyesi (Nuzzaci), has been identified as the causal agent of the ‘engurruñadera del cacao’, characterized by the apparent accumulation of stipules on the buds, followed by die-back, with premature fall of leaves, shortening of internodes and proliferation of lateral branches, or witches’-broom, resulting in production losses or even in the death of plants. Our objective was to evaluate four response variables (numbers of leaves/plant, stipules/bud, internodes/branch and branches/plant) that could be indicative of a possible reversion of the ‘engurruñadera’ after the application of the acaricide abamectin and/or pruning. Two experiments were carried out between November 2020 and April 2021 in a greenhouse at UESC, with cacao seedlings of the genetic group ‘Forastero’ (cacao ‘Common’) infested with A. reyesi and showing typical symptoms of the ‘engurruñadera’. The first experiment was conducted with 4 treatments and 8 replicates, with the application of (1) Abamectin + pruning, (2) Abamectin without pruning, (3) Water + pruning and (4) Water without pruning (control). Sprays were carried out fortnightly with an Abamex® solution (18 g/L of abamectin, in a concentration of 2 mL/L of distilled water + neutral detergent). Pruning was performed only once, at the beginning of the experiment, by removing the apical 10 centimeters of all plant branches. Response variables were evaluated one day before the application of the treatments (Day 0), and afterward (~Days 75 and 150 after the first evaluation, respectively). The second experiment was conducted with 2 treatments and 12 replicates, with the application of (1) Abamectin (treated) and (2) Water (control). Pulverizations and evaluations were carried out as in the first experiment. The number of leaves/plant and internodes/branch were the only response variables that indicated some reversion of the ‘engurruñadera’ in both experiments among treated and control groups, although only 150 days after the application of abamectin and/or pruning. The number of branches/plant showed a significant increase from 75 days after the treatments with pruning, and no difference was found among treated and control groups in terms of numbers of stipules/bud in both experiments. We concluded that the numbers of leaves/plant and internodes/branch seem to be adequate monitoring tools to indicate the reversion of the ‘engurruñadera’ after the application of abamectin and/or pruning. Pruning was effective in the reversion of symptoms of the ‘engurruñadera’ on cacao seedlings as well as the abamectin application of a 2 mL dose of Abamex® per liter of water fortnightly.