Monilinia vaccinii-corymbosici, also known as the fungus causing mummy berry, replaces blueberry fruit with a fungal pseudosclerotium, causing massive crop loss. It is an ascomycete pathogen
The earliest symptom is wilting of leaves. Newly-formed pinkish leaves turn rosy brown, and sometimes develop a sheperd’s crook or curl. There is a severe wilting of vegetative and floral shoots, then a brown discoloration or blight. These symptoms may appear some years, but not all, making the disease hard to track at times. Within 72 hours of infection, the whole shoot collapses, with the highest incidence of infected shoots occuring on the lower canopy, since it overhands moist soil and develops apothecia.
During secondary infection, individual flowers are colonized by the fungus. The fruit initially appears healthy, but then begins to discolor with progression of the disease. The berries turn from green to pinkish-purple, then wrinkle. Healthy blueberries, as you probably know, ripen to a dark bluish-purple. Infected berries may also appear swollen at first, then shrivel to about half of normal fruit size, before dropping.
One may identify the fungal overwintering structures (pseudosclerotia) from late summer until early spring on the orchard floor. The pseudosclerotia are hardened masses of fungal hyphae that resist decomposition and allow the fungus to survive over winter. They can be found under fallen blueberry leaves, moss, and other plant debris. After the floral buds break, the pseudosclerotia produces apothecia (fruiting bodies) that release ascospores (sexual spores) and cause primary infection. After primary infection, the fungus produces conidia (secondary asexual spores) in chains that look like a gray mantle on infected plants. The odor is distinctly sweet, because the conidial mat secrete sugars and reflects UV light to attract pollinators.
To identify mummy berry, one may cut open a developing blueberry fruit. If infected, mats of mycelium will be observed in the fruit locules until it forms a pseudosclerotium, when the blueberry tissues shrivel before dropping. The mycelial mat is soft and cream colored, then hardens and becomes more tan/brown. The most mature pseudosclerotia are black..