This widespread plant disease is caused by a fungus (Botrytis species) that is found on most dead plant tissue. The fungus initially attacks foliage and flowers that are weak or dead, causing spotting and sometimes mold. The fuzzy mold that may develop is composed of millions of microscopic spores. Once gray mold has become established on plant debris and weak or dying leaves and flowers, it can invade healthy plant tissue. The fungus is spread by the wind, splashing water, or infected pieces of plant tissue contacting healthy tissue. Cool temperatures and high humidity favor gray mold growth. Crowded plantings, rain, and overhead watering also enhance the spread of the disease. Infection is more of a problem in the spring and fall when temperatures are lower. In warm-winter areas where freezing is rare, gray mold can be a year-round problem.
Spray infected plants with a fungicide containing chlorothalonil at regular intervals of 10 to 14 days, as long as weather is favorable for disease. Remove infected flowers and leaves, and clean up plant debris. Try to avoid wetting the flowers when watering.
Plant Care for Petunia