Symptoms: The most common fungal disease affecting the blossoms and fruit of almonds, apricots, cherries, peaches and plums. Brown rot (Monilinia fructicola) overwinters in mummified fruit (on the tree and on the ground) and infected twigs. The disease first infects blossoms in spring and grows back into the small branches to cause cankers that can kill stems. Large numbers of flower-bearing stems are killed when the disease is severe. Dead flowers often remain attached into the summer. Developing or mature fruits show circular or brown spots that spread rapidly over the surface and light gray masses of spores are produced on the rotted areas. Rotted tissue remains relatively firm and dry.
Brown rot spores are carried by wind, rain and insects to the open or unopened blossoms and young shoots. If water is present — either from rain, dew or watering — on the flower surface the spores can germinate and penetrate the plant. Disease development takes place over a wide temperature range and flowers can be infected from the time buds open until petals fall. Under favorable conditions, brown rot spreads quickly and an entire crop can be completely destroyed on the tree.
Control: Choose resistant varieties whenever possible. Prompt removal and destruction of infected plant parts helps breaks the life cycle of the disease in individual trees and small orchards, and may be sufficient to keep brown rot below damaging levels. It is also very important to rake up and remove any fallen fruit or debris from under trees. Prune trees occasionally to improve air circulation and water at their base to keep from wetting blossoms, foliage and fruit.
Sulfur or copper powders/ sprays should be applied weekly, to infected trees starting when the blossoms are just beginning to open and continuing throughout the growing season. These organic fungicides will not kill the disease, but prevent the fungal spores from germinating.