Biocontrollers in the management of yam dry rot nematodes Produtos biocontroladores no manejo de nematoides da casca-preta- do-inhame

One of the main diseases affecting yam crops (Dioscorea spp.) in Brazil is the dry rot caused by Scutellonema bradys, Pratylenchus brachyurus and P. coffeae nematodes. The use of biological control agents is an auspicious procedure which has been tested in order to reduce losses by pathogens. The objective of this work was to evaluate the nematicidal activity in vitro and in vivo of commercial biological products on yam dry rot nematodes. Products based on Trichoderma harzianum (2.0 x 109 conidia mL-1) at dosages of 1.5 and 2.0 L 200 L-1 of water; Bacillus subtilis 20% 1.0 x 1011 cfu g-1 + B. licheniformis 20% 1.0 x 1011 cfu g-1 at 100 and 150 g 100 L-1; B. subtilis 200 g kg-1 + B. licheniformis 200 g kg-1 at 130 and 200 g 100 L-1; combination of rhizobacteria including Bacillus spp. and organic carbon at 5L and 7L 100 L-1; and the control (distilled water), were tested in in vitro assays on S. bradys or Pratylenchus sp. In experiments performed under greenhouse conditions, healthy seed tubers were planted in sterilized soil and thirty days later the soil was infested with a suspension of 1,000 specimens of a mixed population of S. bradys and P. coffeae. Then, after 30 days products based on B. subtilis 20% + B. licheniformis 20% 150 g 100 L-1; T. harzianum 2 L 200 L-1 and rhizobacteria + organic carbon 7 L 100 L-1, at 100 mL per pot, were applied to the soil. Three months after planting, the percentage of sprouting of the seed tubers was evaluated and in the fifth month, the nematode population densities were determined. The sprouting of seed tubers was of 100% in all treatments. Bacillus subtilis 20% + B. licheniformis 20% and T. harzianum caused 89% and 61% mortality in S. bradys respectively, at the highest concentrations. In specimens of Pratylenchus sp., rhizobacteria + organic carbon exhibited 51% and 45% mortality at higher and lower concentrations, respectively. Under greenhouse conditions, B. subtilis 20% + B. licheniformis 20% and rhizobacteria + organic carbon were more effective in reducing nematode population densities, compared to the control.

However, studies on the management of yam dry rot nematodes by the use of these organisms are limited. In view of this, the objectives of this work were to evaluate the in vitro nematicidal effect of biological products on S. bradys and Pratylenchus sp. and the efficiency of in vitro selected products to reduce these nematodes on yam plants.

sp.
Nematode populations were obtained from yam tubers with dry rot symptoms from yam fields in the state of Alagoas. Tuber peels were processed according to Coolen The experimental design used for both assays was a completely randomized design with nine treatments and five replicates. All data were submitted to analysis of variance, and the means were compared by Scott-Knott test at 5% probability.

Greenhouse experiments -management of yam dry rot disease
Two experiments were performed from January to June, 2020, in a greenhouse

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Brazil. Mean maximum and minimum temperature were of 35ºC and 19.1º C (Laboratory of Irrigation and Agrometeorology, CECA/UFAL).
In both experiments, healthy yam (D. cayenensis) tuber seeds from a farm field in the state of Alagoas were used. In the first trial tuber seeds with fresh weight from 52 to 159 g were used, while in the second one the value ranged from 58 to 126 g. Tubers were planted in 3-L plastic pots containing sterilized soil.
Inoculation was achieved with naturally infected yam tubers processed according These results are partially in accordance with the data obtained in the present study, considering the product based on B. subtilis 20% + B. licheniformis 20% caused mortality of 89% in S. bradys and 30% in Pratylenchus sp. at the highest concentration tested (Table   1).

Greenhouse experiments -management of yam dry rot disease
Statistical significance was observed for nematode population densities and reproduction factor regarding the treatments, except for the nematode population in the soil in the first experiment (Table 2). In general, the application of products based on B.
subtilis 20%+ B. licheniformis 20%, T. harzianum and rhizobacteria + organic carbon on the soil, significantly reduced (P≤0.05) nematode populations in the experiment 1, however, in the experiment 2, only B. subtilis 20%+ B. licheniformis 20% and rhizobacteria + organic carbon showed efficiency (Table 3). Tuber sprouting was of 100% in all treatments. The difference observed between experiments is attributed to the different mixed nematode populations considering that in the first experiment S.
bradys was the dominant species whereas in the second one P. coffeae was the prevalent species.
In both experiments low RF values were observed in the control plants (RF<1) ( Table 3). This likely occurred because nematode population was not assessed on tubers, due to their poor development during the experimental evaluation. According to Bridge and Starr (2007)  Studies related to the dry rot management using Trichoderma or Bacillus spp.
were not found in the literature, which difficult a comparison between results. Further research is needed to test the efficiency of these products on yam fields infested by S.
bradys and/or Pratylenchus sp. **Significant at 1% probability by the F test; *significant at 5%; ns no significant; CV.coefficient of variation. PS-Nematode population in the soil (100 cm 3 ); PR -nematode population in roots (1 g); PT -Total nematode population (soil+root); RF -Reproduction factor. Data transformed to Means followed by the same letter do not differ by Scott-Knott test at 5% probability. PS-Nematode population in the soil (100 cm 3 ); PR -Nematode population in roots (1 g); PT -Total nematode population (soil +root); RF -reproduction factor.

CONCLUSIONS
The product based on rhizobacteria + organic carbon exhibited good potential to reduce motility and cause mortality of both S. bradys and Pratylenchus sp. in vitro.
The application of rhizobacteria + organic carbon and B. subtilis 20% + B. licheniformis 20% on soil reduced the population density of yam dry rot nematodes in greenhouse.