Boston Ivy (Parthenocissus) Leaf Spot

The fungus Guignardia bidwellii f. sp. parthenocissi causes distinct angular, tan, spots to form on Boston ivy leaves in the spring and summer. A dark brown halo or margin along the edge of the spot is usually present. Within the tan areas, small black dots (fungal fruiting structures where new spores develop) can be observed soon after the spot reaches full size. Branch dieback occurs if the disease is very severe. A close relative of this fungus attacks grapes and causes the disease called black rot during which a leaf spot and berry rot occurs.

The fungus overwinters on the plants. In the spring and summer when moisture has been on the leaves for an hour or longer, infection occurs. Symptoms appear in within a week of infection and new spores begin to form about one week after that. The new spores can cause more spots to form during that same growing season of wet weather persists.

Management

When watering Boston ivy, irrigate in a manner that keeps leaf surfaces dry. If there has been a leaf wetting period of several hours, apply a fungicide within 48 hours.