Evaluation of Trichoderma harzianum, Bacillus subtilis and Aspergillus species efficacy in controlling Pythium ultimum associated with apple seedlings decline in nurseries and their growth promotion effect

Abstract Background Apple decline diseases, responsible for seedlings root and collar rot in nurseries, are an important disease. Different fungal and bacterial antagonists were evaluated to control Pythium ultimum associated with this serious disease using in vitro and in vivo greenhouse assays. Results The in vitro test of ten Aspergillus spp. and ten Trichoderma harzianum isolates showed their efficacy to reduce the radial growth of P. ultimum. The isolates, A. niger A10, A. candidus A5, T. harzianum Tr9 and Tr10, were the most effective with a high inhibition percent that exceeded 50%. The in vivo test of the four most effective antagonists and a strain of Bacillus subtilis (B) showed that the combination of the two Aspergillus isolates (A5 and A10) gave the best result with a decrease in root browning index by 55.55%. Results showed also the efficacy of all tested antagonists and their combinations on the sanitary state index of the inoculated plants except the combination between T. harzianum (Tr10) and B. subtilis (B). The two treatments, T. harzianum (Tr10) and B. subtilis (B), significantly improved the height of inoculated plants by 173.19 and 191.3%, respectively. Regarding the efficacy of antagonists on apple seedlings root weight, A. niger (A10) was the only treatment that significantly increased this parameter by 363.17%. Conclusions A. niger A10, A. candidus A5, T. harzianum Tr9 and Tr10 exhibited the highest value of in vitro inhibition to growth of P. ultimum. The combination of A. niger A10 and A. candidus A5 was the most effective in vivo treatment in reducing the disease severity index. However, T. harzianum (Tr10), B. subtilis (B) and A. niger (A10) revealed to be able to stimulate the apple seedlings growth.