In vitro studies in the effect of some chemicals on the growth and sporification of Penicillium digitatum and P. italicum.

Penicillium digitatum and P. italicum are the pathogens most frequently affecting Citrus fruits after harvest. In recent years the main problem for the citrus industry has been the outbreak of fungicide resistant biotypes. The limited number of registered fungicides and the lasting selection pressure in the packinghouses should be considered as the main reason of this phenomenon. To prevent and/or delay this process, alternative or integrated decay control strategies must be adapted. In this direction we studied the behaviour of P. digitatum and P. italicum when subjected to different concentrations of methanol (MeOH) and dimethyl sulfoxide (DMSO). The experiments were performed with 9 cm Petri dishes containing PDA amended with 0, 5, 10, 20, 30, 40 or 50 L/mL of each of the two tested chemicals. Freshly prepared conidia (100 conidia/ plate) of the two pathogens were sown onto the media and then put into an incubation chamber at 20 °C up to 5 days. Daily, the formed colonies (cfu), the colony diameter and the degree of sporification was monitored. Additionally, Scanning Electron Microscopy (SEM) observations were performed in order to shed light on the effect of these chemicals on P. digitatum and P. italicum growth. Concerning the cfu value a decrease for both pathogens occured as the concentration of the chemicals was raised, but the inhibition was greater for P. italicum. When MeOH concentration was raised from 0 to 5, 10, 20, 30, 40 and 50 L/mL, the cfu values of P. digitatum decreased from 50 to 38, 37, 33, 28, 16 and 0, while those of P. italicum from 14 to 9, 9, 8, 6, 4 and 3 , respectively. The two chemicals differently affected the growth rate of the pathogens and, compared to the control, only MeOH inhibited significantly the colony expansion. With respect to the degree of sporification the pathogens behaved similarly with significant differences between to the tested chemical. Indeed, with MeOH, sporification took place earlier (starting after 24-36 h of incubation) compared to the control (60 h of incubation), while, a complete inhibition occurred during the whole experiment with DMSO concentrations higher than 0.5 L/mL. SEM observations evidenced changes in the pathogen growth behaviour and structure. The importance to fasten or inhibit the sporification will be discussed and data of preliminary in vivo experiments will be presented.