Disease cycle and epidemiology (Apple powdery mildew)

P. leucotricha is an obligate parasite that overwinters on apple as mycelium in dormant buds infected during the previous growing season.

  • Overwintering potential is limited primarily by temperature.  In severe winters infected buds are killed as they are more susceptible to winter cold than healthy buds.
  • Temperatures near –12oC can kill mildew mycelium in buds and leave them to emerge healthy.
  • In UK in recent years the mild winters generally mean that most mildewed buds survive to provide primary inoculum the following season.

Mildewed fruit buds emerge as primary blossom mildew at pink bud.

  • Conidia produced on this mycelium are spread by wind to initiate the new epidemic, to infect developing rosette leaves, flowers and bourse shoots.
  • At the end of flowering primary vegetative mildewed shoots emerge when terminal shoots start growth.
  • Conidia produced on this primary mildew initiate epidemics on the extension growth and start the secondary mildew epidemic which continues throughout the summer, as long as extension growth continues.
  • Young developing fruitlets may also be infected.
  • Leaves are susceptible when young and for only a few days after they emerge.
  • Conidia do not need free water on the leaf to germinate, but do require high humidity 96-100%, although germination can occur at relative humidities as low as 70%.
  • Optimum temperature is 20‑22oC and germination is slow below 4-10oC.  Only limited germination occurs above 30oC.

So, in the UK during summer, provided there is susceptible young leaf tissue, presence of mildew inoculum and it is not raining, every day is a mildew infection day.

  • Under ideal conditions the time from infection to sporulation is around 4-5 days.
  • Mildew can therefore build-up very rapidly in an orchard.
  • Infection of fruit buds takes place within a month of them being formed, before the bud scales suberise, usually in June.
  • The mycelium then remains quiescent in the bud until the following spring.
  • Terminal buds on extension shoots become infected at the end of the summer when extension growth ceases.
  • These remain susceptible to infection for longer than the fruit buds.

Occasionally the sexual state of P. leucotricha occurs as pin-head sized brown/black fruiting bodies among mycelium on infected shoots or leaves.

  • This form has rarely been observed in the UK in orchards and at present is thought to be unimportant in the perennation of the disease.