Determination of long-term effects of consecutive effective soil solarization with vesicular arbuscular mycorrhizal (VAM) on white rot disease (Sclerotium cepivorum Berk.) and yield of onion

The aim of this study was to increase the yield in the fields solarized two years earlier (2007) by reducing the white rot disease in onion resulted from Sclerotium cepivorum caused by Vesicular Arbuscular Mycorrhizal (VAM), Glomus intraradices fungus. Study was conducted in two different experiments (the first experiment where tomato was grown after solarization; the second experiment where lettuce and pepper in previous growing seasons were grown after solarization). Pearl bulb onions were grown with four repetitions according to split-split plot design (2008). In the experiments, the consecutive effects of solarization were considered as main and of sub-VAM and S. cepivorum as mini plots. With the artificial inoculation of S. cepivorum, sufficient disease symptoms were not detected in the onion bulb; however, it reduced the thickness of leaves and stems. And for these two development parameters, results were found significant (P<0.01). Bulb onion yield decreased by 11% in the first experiment and 16% in the second experiment with the S. cepivorum inoculation. The effect of solarization on yield in the second year was higher than in the third year. In the second consecutive season (experiment 1), both of shallot and bulb onion yield increased by 25%, and in the third season (experiment 2), yield increased by 18% for bulb onion. Leaf and stem thickness increased with VAM, but decreased by S. cepivorum. Results were found to be significant (P<0.01) for both the experiments. Consequently, thanks to VAM, product yield increased in both the experiments, and the increments were determined as 22% for shallot; as 25% for bulb onion yield in the second experiment. The effects of solarization combined with VAM increased bulb onion yield by 50%, compared to without both of them. Among the combinations, the highest yield was achieved with the Sol-VAM-non-S. cepivorum application in comparison to non-solarized-non-VAM-S. cepivorum, through this application provided an increase of 54.41% for the first and 91.13% for the second experiment. In conclusion, the effect of solarization, gradually became less effective, in the second and third production seasons. Development of onion alternating in these seasons and loss of yield caused by S. cepivorum could be reduced with artificial VAM inoculation. However, this effect must have depended on the relation between plant species and mycorrhizal colonization in earlier seasons.