Chances are, you’ve seen this fungus yourself. Blue Mold Rot is one of the most common fruit fungi around and it often manifests after the fruit has been picked and sold at market. It is the most common destroyer of market apples in the United States.
Although penicillium spp. (or expansum) can appear on many types of seed-core fruits, it’s most prevalent on apples. It usually infects fruits after harvest, often being introduced through poor sanitation or quarantine procedures.
The most obvious symptom that manifests when Blue Mold is present is the premature rotting of the fruit. It usually starts as small brown spots on the apple’s skin, often around a wound or bruise, and spreads quickly. An entire apple can be half-rotted in a few hours and, as we all know, “one rotten apple spoils the whole bunch.”
Penicillium begins its lifecycle as a spore that can sit dormant for some time. There are several species of Blue Mold, each of which has a slightly different growth rate or infection speed. Few can affect fruits with unbroken skin, requiring a bruise or softening from over-ripeness to gain a foothold. Once inside, the mold grows quickly.
The mold has two tools it uses to spread. First, it multiplies quickly, spreading through the infected fruit very quickly. Secondly, it gives off a carcinogen called mycotoxin patulin. This changes the flavor of the fruit should someone eat it, but it also works to compromise fruits around the one rotting, speeding their ripening and resulting in soft skins through which the Penicillium can spread.
Once an apple is well-rotted, spores will appear as grey-white splotches – the hallmark of the penicillium family.
How Blue Mold Rot Impacts the Plant
This disease usually doesn’t harm the tree or plant itself, though it potentially could if infected fruit is still on the vine. It ruins the harvest, however, and in the wild can prevent a tree from propagating if the seeds are prematurely exposed due to the early rot of the apple.
How to Prevent Penicillium
Prevention is all about cleanliness during harvest and proper wax sealing after harvest. Storage methods that avoid bruising and keep the apples at sub-prime temperatures so the fungus can’t spread is also important. Before refrigeration, many farmers would store apples in individual bushels, with bunches of apples kept apart from one another so that if one went bad, it would take no more than one bushel of others with it.
This was effective, in its way, but still meant a lot of loss. Most apples were quickly jarred or pressed instead.
How to Treat Penicillium
Once it sets in, the only effective treatment is to discard and destroy the infected apples. Although some growers spray a fungicide on apples after harvest, it is a rare and often frowned-upon practice.