Sphaeroteca Pannosa

The powdery mildew affects leaves, steams, buds and flowers. In the beginning it forms a white tender web coat, which becomes powdery later. Considerably affected leaves curdle, become brown, dry and fall early.

The rose affected by the powdery mildew reduces the number of offshoots by 40–50%, leaves by 10%, growth amount by 10–12%.

Biology. Life cycle.

The agent of disease is an ascigerous fungus (ascomycete) — Spaerotheca pannosa from the family Erysiphales. This parasite develops in two stages — the conidial and the ascigerous. The white coating formed on affected plants consists of a mycelium and conidia. The conidia are ellipsoidal, single-celled, pallid. Their size is 17–30 × 9–17 µm (micrometers). They are formed in chains on short conidiophormes. In the summer ascigerous stage arises and the coating becomes thicker and assumes a dark-brown color. This dark-brown film consists of the mycelium and small fruit bodies of ascigerous stage. The fungus survives winter in infected shoots and organic wastes. The maturation of ascospores in the fruit bodies takes place after overwintering in the spring only. After the arrival of favorable conditions in the spring the out-throw of the ascospores from the fruit bodies occurs and plant infection begins.

The fungus mainly infects young plant tissues. The first signs of the disease emerge on plants in 12–14 days after infection. It is easy to diagnose the disease by a definitive powdery coating, which consists of the mycelium of the fungus. There are several factors which are favorable for disease development: high air humidity, low temperature (12–15°C), the abrupt fluctuations of temperatures, the appearance of condensate (in a greenhouse), the surplus of nitrogen, the deficit of calcium and potassium in the nutrition of plants, bad illumination, short day-light. (7–8 hours per day).