The fungus Penicillium digitatum is a highly common mold species recognized globally as the cause of green mold on citrus fruits, including oranges, lemons, and grapefruits. This organism is responsible for substantial economic losses in the agricultural industry due to its rapid post-harvest spoilage capabilities. While its presence on food naturally raises concerns about human health, the risk to a generally healthy, non-allergic individual is considered low. Understanding this mold requires separating its aggressive role as a plant pathogen from its comparatively limited interaction with the human body.
Identification and Primary Role in Nature
Penicillium digitatum is easily recognizable as the green, powdery mold that first appears on damaged citrus rinds. This organism is a mesophilic fungus, thriving best in moderate temperatures, with optimal growth occurring around 75°F (24°C). The fungus begins its lifecycle as a white fungal mat, or mycelium, before producing masses of velvety, olive-green asexual spores, which give the mold its characteristic color and texture.
The fungus is a necrotrophic pathogen, meaning it kills its host tissue to obtain nutrients. Its primary ecological role is to decompose post-harvest citrus fruit, where it acts as a wound pathogen. The mold requires a breach in the fruit’s skin, such as a scratch or bruise from handling, to initiate infection. Once established, P. digitatum rapidly spreads, causing the fruit to become soft and watery, leading to the condition known as green rot.
This species is the most destructive post-harvest disease of citrus worldwide, often accounting for up to 90% of citrus fruit losses after harvest if not controlled. Its rapid growth and ability to produce vast numbers of airborne spores allow it to quickly spread from one infected fruit to others nearby. The mold’s natural habitat is the soil of citrus-growing regions.
Assessing Direct Human Health Risks
For humans, the potential health risks from exposure to P. digitatum fall into three primary categories: infection, allergic reaction, and mycotoxin exposure. The infectious risk from this mold is very low for individuals with a normal immune system, as it is generally considered non-pathogenic to humans.
However, like many environmental molds, it can act as an opportunistic pathogen. There have been rare case reports of P. digitatum causing severe infections, such as fatal pneumonia. These cases were observed in individuals who were severely immunocompromised or had underlying health conditions like pulmonary emphysema.
The most common human health issue associated with this mold is an allergic response due to the inhalation of its spores. People who are sensitive to molds may experience typical allergy symptoms like allergic rhinitis, which includes sneezing and a runny nose, upon exposure. In more sensitive individuals, the inhalation of the abundant spores can trigger asthma or, in rare instances, hypersensitivity pneumonitis.
Another concern is the production of mycotoxins, which are toxic compounds produced by some fungi. P. digitatum is specifically associated with the production of citrinin. Citrinin is known to be nephrotoxic, meaning it can damage the kidneys, and has also been studied for potential embryotoxic properties. Although the amounts produced on a single moldy orange are likely small, the potential for toxin penetration into the fruit is a primary reason for discarding contaminated food.
Distinguishing from Medicinal Penicillin
The genus name Penicillium often leads to confusion because it is the same genus that gave the world the antibiotic penicillin. However, P. digitatum is distinct from the species used in medicine.
The life-saving antibiotic penicillin is produced primarily by a different species, Penicillium chrysogenum. This medicinal species possesses a specific genetic pathway that enables it to synthesize the beta-lactam antibiotic compound. In contrast, P. digitatum lacks this complete genetic machinery, meaning it does not produce the beneficial antibacterial compound penicillin.
The two species have entirely different metabolic focuses. One is a well-known spoilage agent that produces decay-related enzymes and minor mycotoxins, and the other is a cultured fungus used in industrial fermentation to manufacture medicine. Therefore, encountering P. digitatum on a piece of fruit should not be interpreted as exposure to a source of beneficial antibiotic.
Prevention and Safe Handling of Contaminated Food
Preventing the growth of P. digitatum in the home begins with proper storage and minimizing fruit damage. Since the fungus requires a wound to begin its infection, handling citrus fruits gently and avoiding cuts or punctures helps maintain the fruit’s natural defense barrier. Storing citrus in a cool, dry place can slow the growth rate of this mesophilic fungus.
If a citrus fruit shows any sign of green mold, it should be discarded immediately to prevent spore spread to other produce. Citrus fruits are considered soft, high-moisture foods, and the mold’s filamentous “roots,” or hyphae, can penetrate deep into the fruit below the visible surface growth. The United States Department of Agriculture (USDA) advises against attempting to cut the mold off, as the contaminated tissue and potential mycotoxins may have spread throughout the entire fruit.
When disposing of moldy fruit, it is advisable to place it directly into a closed bag or container before moving it to an outside trash receptacle. This action helps prevent the dispersal of airborne spores into the kitchen environment, which is important for people with mold allergies. Cleaning the area where the moldy fruit was stored with a mild detergent solution is also recommended to eliminate any residual spores.