The fungus is endemic in SK, MN, and NS, but likely occurs in North America wherever its host is common. Epidemics have been observed on the weed in field crops in the prairie provinces, but these are sporadic and occur too late in the season to provide effective weed control. However, C.g.m. provides effective and selective control in a number of field crops when applied as a mycoherbicide, provided conditions are suitable for initial and subsequent infection. Preliminary classical' releases of the pathogen in orchards and pastures in NS have also resulted in excellent control. These turn greyish in the center with an almost black margin.
Lesions often coalesce and girdle the stems or petioles resulting in wilting above the lesion.
Mallow leaves and flowers may also become infected and develop wilted and necrotic zones, and ultimately the plant dies. It reproduces and spreads only by spores which germinate under suitable conditions within 20 h to form black appressoria with hyphae that penetrate the epidermal cells. Secondary hyphae are then formed which colonize and destroy the plant tissue. Hyphal stroma forms beneath the cuticle of necrotic tissue and gives rise to aceruli in which conidia are produced on conidiophores. These can cause a number of secondary disease cycles depending on conditions.