Causal agent: The fungus Sphaerotheca pannosa,
Synonyms :
- Podosphaera pannosa (de Bary),
- Powdery mildew leucoconium (Desmazières),
- Alphitomorpha pannosa (Wallroth),
- Albigo pannosa (Kuntze),
- Erysiphe pannosa (Fries),
- Sphaerotheca persicae (Woron.),
- Sphaerotheca rosae (Jacz.),
Description :
- Like all powdery mildew , Sphaerotheca pannosa developsits mycelium on the surface of contaminated organs.
- When conditions become favorable, the mycelium sporulates on the surface of the leaf.
- Reproduction and dissemination of the fungus takes place through the formation of conidiophores at the end of which conidia are produced.
- The size of the conidia is around 19-25 X 9-14 µm.
Biology :
- This fungus does not overwinter on apricot or peach and the primary inoculum comes from roses infected in the spring.
- The winter conservation of the fungus takes place on rosebush in the form of mycelium both on the twigs at the base of the thorns and inside the buds.
- This conservation also takes place very rarely through the intermediary of the perithecia which form in the felted clusters of mycelium present on the branches of the rosebush.
- Powdery mildew spores are moved by wind, water, and plant-to-plant contact.
- Once the spores reach the susceptible host tissue, they germinate when there is sufficient free moisture (high humidity, rain or splashing water) and begin to colonize the plant tissue.
- Mycelial development is at its maximum between 11 ° C and 28 ° C.
- The optimum for conidial formation is between 21 ° C and 27 ° C.
- The formation of conidia is zero below 10 ° C and above 27 ° C.
- Germination of conidia is very fast when the humidity level is around 99% but zero below 75%.
Symptoms :
- On fruits, the disease usually appears as round whitish spots a few weeks after petal fall.
- The spots will widen to cover much of the fruit.
- Later, the white mycelium peels off to reveal round, rusty lesions.
- Diseased leaves develop dry brownish patches covered with a white powdery growth.
- Newly infected leaf tissue is dwarf and deformed.
Control :
- Ensure that rose plants near susceptible hosts are treated appropriately to reduce the initial inoculum and the incidence of disease.
- A good size to increase airflow and reduce relative humidity.
- Balanced plant nutrition.
- Chemical control: AGRIKAR STAR at a dose of 50 ml / hl.
