Powdery mildew is a common plant disease that manifests as a white, powdery fungal growth on the surface of leaves, stems, and sometimes fruit. Caused by various species of fungi in the order Erysiphales, it is one of the most recognizable diseases in gardens and commercial agriculture. Powdery mildew does not actively grow or spread on harvested, dead, or stored produce. The disease only affects the plant while it is actively growing, meaning its progression stops once the tissue is removed from the host plant.
Why Powdery Mildew Needs Living Tissue
The inability of powdery mildew to spread post-harvest is rooted in its classification as an obligate biotrophic parasite. This means the fungus is dependent on living host cells to obtain the nutrients needed to survive and complete its life cycle. The fungus grows primarily on the exterior surface of the plant tissue, but it sends specialized structures, called haustoria, into the host’s epidermal cells.
The haustoria extract sugars and other metabolites from the living plant cells. Once tissue is harvested, the cells begin senescence and death, which cuts off the fungus’s food supply. Without a continuous supply of nutrients, the powdery mildew mycelium cannot grow, and the active infection ceases.
Spore Viability on Harvested Material
While active fungal growth stops upon harvest, the microscopic fungal spores on the surface do not immediately die. Powdery mildew produces two types of spores: asexual conidia, which spread the disease rapidly during the growing season, and sexual chasmothecia (resting structures). The conidia are generally short-lived, often losing viability within a few days to weeks after detachment.
The chasmothecia, which appear as tiny, dark specks, are designed to survive harsh environmental conditions, such as winter. These structures can remain dormant and viable on debris for long periods, but they pose little threat to stored produce. The primary concern is that if debris is improperly disposed of, the spores could overwinter and initiate a new infection on a susceptible crop the following season.
The Real Threat: Secondary Storage Rots
The presence of powdery mildew, even if inactive, increases the vulnerability of harvested produce to other post-harvest pathogens. While powdery mildew cannot spread in storage, the initial infection compromises the plant’s natural defenses. The fungus’s feeding structures damage the epidermis and cuticle, which are the plant’s protective outer layers.
This damage creates microscopic entry points for saprophytic fungi that thrive on stored produce. These secondary invaders, often called storage rots, are the actual cause of spoilage and economic loss. Common storage rot pathogens include Botrytis cinerea (gray mold) and Penicillium species (blue or green mold).
These secondary molds can colonize the weakened tissue and spread rapidly from one piece of fruit to another in a process called “nesting,” especially under high humidity. For example, on grapes, powdery mildew damage can lead to berry splitting, allowing opportunistic fungi to enter and cause bunch rots. The danger of powdery mildew on harvested crops is the entry point it provides for these fast-acting decay organisms.