The leaves are most susceptible 16–23 days after unfolding. High humidity favors the development of disease, but infection can occur at  relative humidity as low a 50%. The conidia of the fungus are spread through the air and thus can travel over great distances.
Leaves are most prone to disease 16–23 days after unfurling. High moisture fosters disease growth, although infection can also occur in relatively dry conditions with a humidity level as low as 50%. The fungal spores are airborne and can spread far and wide.