DECOMPOSITION OF MEDICAGO SATIVA RESIDUES AFFECTS PHYTOTOXICITY FUNGAL GROWTH AND SOILBORNE PATHOGEN DISEASES
G. Bonanomi, V. Antignani, E. Barile, V. Lanzotti, F. Scala
Disease problems resulting from replanting alfalfa (Medicago sativa, MS) have been associated both to autotoxicity of undecomposed plant residues and activity of soil-borne fungal pathogens such as Pythium spp. and Rhizoctonia solani. However, the interaction between MS residues and soil-borne pathogens has not been previously investigated. In this work we studied the effects of the decomposition process of MS residues, in aerobic and anaerobic conditions on: (i) the growth of MS seedlings; (ii) the growth of 17 fungal species selected among saprophytic, biological control agents, foliar and soil-borne pathogens and (iii) the outcome of the plant-pathogen interactions. MS chemical changes during decomposition were characterized by 13C-CPMAS- NMR spectroscopy. Autotoxicity and phytotoxicity of MS extracts rapidly decreased during decomposition, in both laboratory and litterbag in aerobic conditions, but increased sharply in anaerobic conditions. Undecomposed MS extracts positively affected fungal growth, while extracts from MS residues decomposed both in aerobic and anaerobic conditions showed an inhibitory effect. MS seedling damping-off caused by Pythium ultimum and R. solani increased when soil was amended with MS residues, suggesting that availability of more nutrients and energy sources may be important in increasing disease severity. 13C-CPMAS-NMR analysis revealed that dramatic chemical changes occurred during decomposition. O-bearing carbon compounds, mainly associated with sugars, decreased steadily, while aliphatic compounds increased during decomposition. It was concluded that there is a consistent relationship among MS chemical changes, its autotoxicity and the effects on fungal growth.