Answer: 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. You can maximize plant vigor by fertilizing and watering, however, watering should be done early in the day to give the foliage a chance to dry before nighttime. It is also helpful to pick and remove affected leaves as soon as they develop. Although not usually necessary, applications of fungicides can be made when new growth emerges in the spring. Among the compounds registered for use are mancozeb and copper compounds. Consult the label for dosage rates and safety precautions. Soapy water will not control this fungal disease. Good luck with your ivy.
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