Erwinia amylovora can be spread by rain, wind, insects, and infested pruning equipment. Primary infection occurs when the pathogen comes into contact with a susceptible host under favorable environmental conditions. Pathogens penentrate the surface of a plant directly or enter through wounds or natural openings. The bitter rot fungi, Colletotrichum spp., generally penetrate the apple fruit directly whereas the white rot fungus, Botryosphaeria dothidea , enters through cracks in the cuticle or through lenticels on healthy twigs. Growth and development of a pathogen usually occurs on or within infected plant tissue.
Fungi grow and spread within their host by means of mycelium.
Most fungi produce spore-producing structures such as pycnidia or perithecia on or within infected tissue. These structures give rise to secondary inoculum which eventually causes additional, or secondary, infections during the season. Bacteria spread by rapidly increasing in numbers. Each pathogen has it's own optimal conditions for growth and development which usually coincide with optimal condition for infection. Secondary infection results from spores or cells produced following primary infection or from other secondary infections.