Gray Spots on Petunias

Cause

• Petunias are susceptible to the fungal disease gray mold caused by Botrytis cinerea. The disease is among the most common disorder of bedding plants, cites the University of Minnesota Extension. Other common hosts of the fungus include carnation, begonia, geranium, zinnia and marigold. Improper cultural management is a frequent cause of disorder.

Symptoms

• The growth of the causal fungus is favored by periods of high humidity and low temperatures ranging between 45 to 60 degrees F. The disease is characterized with the appearance of water-soaked lesions on the foliage that gradually assume gray to brown shades. The spots or lesions often have darker borders. With disease progression, the spots get larger in size and spread to other plant areas, including the blooms. Early signs of infection on blooms are water-soaked brown flecks.

Damage

• Plants infected with gray mold start to rot rapidly. The disease infects the stalks and causes total collapse. Unopened buds rot and cankers on stems weaken the plant. The entire crown is often rotted. Under periods of high humidity, the fungus starts to produce reproductive spores seen in the form of fuzzy gray growth on the infected areas. Newly emerged seedlings die from infection.

Management

• Grow plants in well-ventilated areas and provide adequate space between plants. Prune and dispose of affected areas, and keep growing area clear of debris and fallen foliage. Remove heavily damaged plants completely from root. Light symptoms of gray mold are often easily managed by improving cultural conditions. Recommended fungicides include the use of neem oil, chlorothalonil or Bacillus subitlis.