Several species of Gerbera daisies exist, and all of them are susceptible to the same types of diseases. These diseases can affect the appearance of the plants, but they can be fatal as well. They are also contagious and can be easily spread to other plants, most often by the wind.
Grey mildew is a fungus that attacks a large variety of plants--the Gerbera daisy being one most affected. It lives in the soil and thrives in damp weather. Leaf spot is a fungus that can do a great deal of damage to the leaves. White powdery mildew is a fungus that appears on the leaves, petioles--the stalk that joins the leaf to the stems--and the flower heads.
Grey mildew, or Botrytis cinerea, develops as a gray-brown fuzz and silver-gray spores on infected tissue during times of high humidity and bad air circulation. When the spores disperse, they look like dust flying off the plant. Leaf spot is a fungus that appears as circular purple-brown spots with irregular edges, which will grow larger. White powdery mildew appears on the leaves, petioles and flower heads. It starts out white and turns purplish brown.
No fool-proof way exists to prevent the Gerbera daisies from getting fungal diseases, but you can make it less likely. Plant them far enough apart to insure good air circulation. Do not water the top of the plants; water just around the base. Water is absorbed through the roots, not the leaves, and soaking the leaves, flowers and wood gives the fungus a place to grow.
Fungal diseases thrive in moist, hot conditions. They can winter in the soil or in infected plant tissue and attack the new growth in the spring when it is most vulnerable. Plants that have open cuts or that have been stressed by drastic changes in their environment are the most susceptible.
Grey mold will destroy the flowers from the inside out. In the most serious cases, leaf spot will cause the leaves to drop off. White powdery mildew causes the leaves to become small and curled with the feel of leather. The flowers will not open, and eventually the infected parts will dry up and die
For gray mildew, remove the dead or damaged flowers and burn all the debris. If burning is not allowed in your area, bury it under at least a foot of soil. You can treat leaf spot with fungicides. You can also use fungicides on white powdery mildew, but the product must specifically say it is effective on Gerbera daisies. Begin treatment as soon as the first spots appear. In the fall, gather and destroy all of the fallen leaves.
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