Métodos de inoculação e quantificação de Xanthomonas vasicola pv. vasculorum, desempenho de produtos no controle da doença e ativação de mecanismos de defesa no milho

Abstract

Maize foliar diseases have potential to reduce the production of this grain, such as bacterial leaf streak, caused by the bacterium Xanthomonas vasicola pv. vasculorum (Xvv), for which there are few studies available in the literature. This work aimed to evaluate methods of inoculation and quantification of Xvv, verify the performance of products in controlling the disease and in activating defense mechanisms in maize. The experiments were carried out in a greenhouse with a randomized block design and in the laboratory with a completely randomized design, both with five replications. To evaluate the Xvv bacteria inoculation methods, the hybrid P4285 VYHR was inoculated by leaf spraying, in hydathodes, stalk wounds, leaf wounds, deposition of the inoculum within the whorl and control (non-inoculated). Then, the Xvv inoculum amount experiment was conducted with treatments 0, 106, 107, 108 and 109 colony forming units (CFU) mL-1. To evaluate the antibiotic effect of the All Crim®, Bion®, Curative®, tea tree essential oil, Serenade®, Status® and Stop® products, an in vitro test was implemented to determine the highest control dose and, subsequently, the test was carried out in vivo. All assays were performed in duplicates, in vitro the presence and size of the bacterial inhibition zone were evaluated and in vivo the area under the disease progress curve (AUDPC) was evaluated. The biochemical analysis experiment was carried out in a randomized block design in a 4x7 factorial scheme (four treatments: All Crim®, Curative®, Bion® and control, and seven collection times: 0, 24, 48, 72, 96, 120 and 144 hours), with three replicates, evaluating peroxidase (POX), polyphenoloxidase (PPO) and phenylalanine ammonia-lyase (PAL) activity. The inoculation methods that provided the highest AUDPC were leaf spraying and inoculum within the whorl, and the inoculation via hydathodes was not promising for the appearance of symptoms. The experiment to evaluate bacterial concentrations showed a quadratic behavior with a maximum point at 1x108 CFU mL-1, providing the highest AUDPC. The All Crim®, Curative® and Bion® products did not show a halo of inhibition of bacterial growth at all doses used, in contrast, the products Status®, essential oil of tea tree and Serenade® showed a halo of inhibition at the lowest dose up to highest used in the experiment. The Stop®, in turn, presented a quadratic effect. In the in vivo test, All Crim®, Curative®, Bion® products showed the lowest AUDPC both in the third (treated and inoculated) and in the fourth (only inoculated) maize leaf, presenting a local and systemic behavior. Increase in POX, PPO and PAL activity was observed with All Crim®, Curative® and Bion® treatments. These results indicate the potential of inducing resistance in maize as a complement to control bacterial leaf streak, suggesting further studies, such as the activation of other defense enzymes and evaluation of maize behavior with other product doses.