At least 11 species of Penicillium have been isolated from naturally infected apples with Penicillium type rots but P. expansum is by far the most common and economically important species.
- Penicillium species known to cause apple rots can be isolated from most orchard soil, but the disease is rare in the orchard except occasionally on fruit that has fallen to the ground.
- The fungus can be found on decaying flower parts at blossom time and can also be isolated occasionally from the cores of fruitlets collected in the orchard.
- Most fruit infections occur during harvesting of fruit when airborne or waterborne spores enter wounds on fruit damaged during harvesting and handling.
- The fungi survive on decayed mummified fruit or fruit bits stuck on bins or lying around the storage or packhouse area.
- However, most wound infections in storage result from waterborne spores in post-harvest drench solutions or in water flumes used to float fruit onto packing lines.
- The drenches and water flumes become contaminated from dirty bins or dirty fruit from the orchard.
- Pear is also susceptible to Penicillium rot which also causes rots of other fruits such as grape and strawberry.