Penicillium digitatum was recently identified as a postharvest pathogen of plum. Little is known of this host-pathogen association. Disease occurrence and severity was higher on older fruit. This study aimed to determine the effect of ripening on the infection and colonisation of P. digitatum and P. expansum on plum at a physical (disease incidence/severity, pH and firmness) and molecular (gene expression) level. Storage conditions and inoculum loads were also considered. Disease incidence and severity of P. digitatum was significantly affected by ripeness, cold storage and inoculum load. Both species acidified tissue and advanced host ripening. Host ripening had a small effect on gene expression (P. digitatum: ACCD decreased; P. expansum: pacC and creA increased). A dual mechanism of pH modulation was discussed; higher pH at and beyond lesion borders will facilitate invasion, maceration and colonisation (nutrient uptake and growth) by/during acidification. The pH of lesions was comparable to that of controls. Alkalinisation via accumulation of ammonium/ammonia can be linked to the pathogen’s nitrogen metabolism. Host ripening directly (elicited) or indirectly (ethylene stress) caused by pathogen attack can increase the pH of uncolonised tissue. P. digitatum can be considered an important pathogen of riper fruit often found in long or ill-managed distribution chains. It is still unclear what stimulates (molecular) the opportunistic lifestyle expressed by P. digitatum on plum. There was little to no correlation between gene expression and the increase in disease incidence and severity on riper fruit. Future work should consider the decline of host resistance during ripening.