Phosphite Compounds Suppress Anthracnose in Soybean Seeds Infected by Colletotrichum truncatum, Stimulate Growth and Defense Mechanisms

Soybean is one of the main agricultural commodities, and its productivity is limited by several diseases, such as anthracnose, which is caused by a complex of fungal species, with Colletotrichum truncatum being the most prevalent. Management is mainly carried out through chemical seed treatment. However, a reduction in the sensitivity of C. truncatum to fungicides was observed. Therefore, it is extremely important to search for products that are effective in controlling the disease. The objectives of this study were to evaluate the efficacy of commercial formulations of Cu, K, Mn and Zn phosphites in the treatment of soybean seeds infected by C. truncatum and their ability to induce defense mechanisms against the fungus. For this purpose, seeds inoculated with C. truncatum were treated with phosphites and a fungicide (carbendazim + thiram). The seeds were subjected to germination, health and vigor tests. Fungal toxicity and the ability of phosphites to induce defense through the activities of catalase, peroxidase and superoxide dismutase enzymes, as well as the levels of lignin and total soluble phenols, were also evaluated. Mn and Zn phosphites were toxic to C. truncatum and effective in controlling anthracnose in infected soybean seeds, showing no difference in relation to the fungicide. Mn phosphite induced the production of CAT, POX and lignin, while Zn phosphite increased the production of CAT and POX. These results demonstrate the effectiveness of Mn and Zn phosphites in controlling anthracnose in infected soybean seeds, their direct toxic action and their ability to induce defense mechanisms.