EFFECT OF LEAF WETNESS DURATION TEMPERATURE AND INOCULUM CONCENTRATION ON INFECTION OF EVERGREEN AZALEA BY COLLETOTRICHUM ACUTATUM THE CAUSAL AGENT OF ANTHRACNOSE
D. Bertetti, M.L. Gullino, A. Garibaldi
Colletotrichum acutatum is the causal agent of anthracnose of azalea (Rhododendron azalea). This disease, first observed in Italy in 2002 on cv. Palestrina in nurseries located in the Verbano-Cusio-Ossola province of Piedmont (northern Italy), causes serious losses. Although most azalea cultivars grown are resistant to the disease, some of the most popular are very susceptible. In order to understand the effect of environmental parameters on anthracnose, the effect of inoculum density, leaf wetness duration and temperature on the development of the disease was studied. Conidial concentration of 105 and 106 produced the most severe symptoms. The disease was severe at 15°C and 20°C, and severity decreased at temperatures outside this range. At 20°C the pathogen required at least 24 h of leaf wetness to develop significant symptoms, whereas at 15°C it needed extended periods; with 48 h of leaf wetness the pathogen produced 85 to 100% infection of leaf surface. No symptoms developed at 5°C or 30°C. Knowledge of the factors favourable to disease development could help improve management tactics, based on the control of environmental factors.