Powdery mildew

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.
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