Host:  Aspen

Symptoms/signs:  Curling, blackening and dieback of the tips of terminal and lateral shoots on young aspen sprouts.

Biology:  Primary infections by Venturia spp. occur in spring on leaf blades or petioles and on young stem tissue. Dark brown to black lesions expand rapidly, causing leaves and shoots to droop, whither, and become brittle. Secondary infections, promoted by wet weather, are initiated throughout the period of shoot elongation by conidia from newly blighted shoots. New shoots frequently grow adjacent to blighted ones and are blighted in turn. Lesions do not extend into woody twigs.

Effects:  In wet seasons, Venturia spp. can kill virtually all terminal shoots in susceptible aspen stands that are regenerating by sprouting. This damage reduces height growth and deforms trees by causing a bend in the stem at the point where a lateral shoot became a new leader following death of the terminal shoot. Successive leaders may be killed during seasons of severe disease. Plants less than 3 meters tall are at greatest risk, and damage becomes negligible as trees attain heights greater than 5 meters.

Similar Insects and Diseases:  This is the only fungus on aspen causing black lesions on leaves, petioles and stems, which curl and form characteristic shepherd’s crooks.

References:  38, 92