Banca de DEFESA: Karoliny de Almeida Souza

"Nematicidal action of Bacillus methylotrophicus on Meloidogyne enterelobii and physiological changes induced by the nematode in guava trees treated with the bionematicide".

biological control, photosynthesis, root-knot nematode, Psidium guajava, rhizobacteria.

Photosynthesis, stomatal conductance, and transpiration are plant physiological activities that influence the productive potential of crops. The interference of root-knot nematodes in plant physiology is still poorly studied, as well as the capacity of rhizobacteria to mitigate physiological damage caused by nematodes. The lack of alternative methods of control to the nematode Meloidogyne enterolobii hinders efficient and sustainable management friendly to the environment. The objectives of this study were to evaluate in vitro the effect of a bionematicide, with the active ingredient Bacillus methylotrophicus, on the eggs and second stage juveniles (J2s) of M. enterolobii; and to monitor changes in the physiology of guava plants inoculated with M. enterolobii and treated with the bionematicide over time. In vitro experiments were conducted on M. enterolobii eggs and J2s. The treatments included concentrations of 1%, 10%, 25%, 50%, and 70% of the bionematicide, with water as control treatment. The experimental design was completely randomized with six replicates and two repetitions over time. The treatments with eggs were evaluated at 2, 4, 6, 8, and 10 days after incubation by counting the number of damaged eggs. The treatments with juveniles were evaluated at intervals of 24 to 96 hours by counting the number of dead juveniles. The monitoring of guava tree physiological activity was assessed in greenhouse and field conditions. The treatments included: 1) non-inoculated and non-treated guava cv. Pedro Sato (control); 2) non-inoculated and non-treated guava cv. Paluma guava tree (control); 3) guava 'Pedro Sato' treated with B. met and noninoculated; 4) 'Paluma' treated with B. met and non-inoculated; 5) Paluma' inoculated and non-treated with B. met; 6) 'Pedro Sato' inoculated and non-treated with B. met; 7) 'Pedro Sato' inoculated and treated with B. met, and 8) 'Paluma' inoculated and treated with B. met. The treatments were arranged in a completely randomized design (DIC) and randomized block design (DBC) in the greenhouse and field, respectively. Four applications of the bionematicide were made, at doses of 3 mL/L of water per plant in the greenhouse and 1 mL/L of water per plant in the field experiment. Between the second and third application, the plants were inoculated with 5000 eggs and eventual juveniles of M. enterolobii in the greenhouse, and 6000 eggs and eventual juveniles of the nematodes in the field. In the greenhouse, seven evaluations of physiological parameters of the guava trees (stomatal conductance, water use efficiency, photosynthesis, and transpiration) and six evaluations of total chlorophyll content were performed throughout cycle of the guava plant development. At 132 days after inoculation (DAI), the roots were weighed, and the gall index, egg mass index, and reproduction factor were determined. In the field, ten evaluations of physiological parameters of the guava trees (stomatal conductance, water use efficiency, carboxylation efficiency, photosynthesis, and transpiration) were performed, and productivity was estimated through harvest and subsequent fruit weighing. In vitro, damage to the eggs increased for all treatments, with no difference between doses after six days of exposure. J2 mortality increased up to 48 hours in the lower doses (1%, 10%, and 25%), remaining close to 100% in the higher doses (50% and 70%), with no difference between doses after 48 hours of exposure. In the greenhouse, changes in physiological parameters were concentrated in the first 44 DAI. Meloidogyne enterolobii and B. met combined reduced stomatal conductance and transpiration at 26 and 44 DAI, respectively. Photosynthesis was lower at 26 DAI in treatments that received both nematode and bacterium in combination. Meloidogyne enterolobii and B. met, either isolated or combined, showed reduced water use efficiency at 26 and 44 DAI. Gall index, egg mass index, and reproduction factor were higher in treatments that received the nematode. In the field, there was no difference over time for physiological variables for all treatments, and there was no pattern in the changes caused by isolated or combined M. enterolobii and B. met. Meloidogyne enterolobii reduced stomatal conductance in eight evaluations, water use efficiency and photosynthesis in three evaluations, and transpiration in seven evaluations. The only increase conditioned by the nematode was observed in water use efficiency at 369 DAI. Bacillus methylotrophicus reduced photosynthesis at 421 DAI but increased it at 527 DAI; at 318 and 369 DAI, there was an increase in water use efficiency. Meloidogyne enterolobii and B. met combined reduced stomatal conductance in eight evaluations and transpiration in four evaluations. Reduction in water use efficiency and photosynthesis was observed at 421 DAI. Meloidogyne enterolobii and B. met combined increased water use efficiency at 369 DAI and carboxylation efficiency at 224 and 465 DAI. The application of B. met did not mitigate the negative effects of M. enterolobii on the physiological activities of the guava trees, and isolated or combined M. enterolobii and B. met did not alter guava tree productivity. Bacillus methylotrophicus caused damage to the eggs and mortality of M. enterolobii J2. Under controlled conditions, M. enterolobii and B. met combined reduced stomatal conductance, water use efficiency, photosynthesis, and transpiration of guava trees during the initial phase of fruit development. Isolated M. enterolobii and B. met reduced water use efficiency at 26 and 44 DAI. The nematological parameters were not influenced by the bacterium. In the field, the absence of alterations may be due to the influence of environmental factors, inoculum density, bacterium application method, and combinations of nematode and bacterium application over time. Reduction in physiological parameters by the nematode was dominant. The application of B. met did not mitigate the negative effects of the nematode on the physiological activity of the guava trees. Isolated or combined M. enterolobii and B. met did not alter guava productivity.