Inoculum size and intraspecific interactions affects Penicillium expansum growth and patulin accumulation in apples.

Penicillium expansum is the most important fungal species causing spoilage in cold stored apples. P. expansum produces patulin, a toxic secondary metabolite. The aim of this work was to study the changes in growth parameters and patulin accumulation that may occur when two different isolates of P. expansum germinate and develop in the same wound. The effect of the inoculum size was also studied. For that purpose, apples were point inoculated with two different isolates of P. expansum at two different conidia concentrations. Both isolates were also inoculated together. Apples were cold stored at 1 degrees C or incubated at 20 degrees C until lesion diameter reached 2.5 cm. Patulin from decayed tissue was analysed at the end of cold storage/incubation. It seems that intraspecific interactions occur. The interactions resulted in slower growth in which patulin content was lower. A competition for resources followed by either a "combat-like" interaction may be involved in this. Higher inoculum size led to shorter lag phase, which reflects the importance of the hygiene in packinghouses facilities. The inoculum size also affected growth rate and patulin accumulation, which suggests that interactions between individuals of the same strain may occur. Competition for resources and a combat-like interaction may take place. In vitro studies should be carried out in order to assess the patterns of the interactions observed.