Foliar Diseases
FIELD GUIDE TO INSECTS AND DISEASES OF ARIZONA AND NEW MEXICO FORESTS
Shepard's Crook
Venturia tremulae var. grandidentatae M. Morelet

Host:  Aspen

Figure 178. Figure 178. Curling and blackening of aspen shoots from Shepherd's crook.

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

Biology:  Primary infections by Venturia tremulae var grandidentatae occur in spring on leaf blades, petioles, and 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, shepherd's crook can kill virtually all terminal shoots in young susceptible aspen within the same locale. 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 wet favorable summers. 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:  39, 93