Chemical control (Apple powdery mildew)

The main means of control in the UK are fungicide sprays combined with cultural control.

  • Fungicides recommended for control of mildew have protectant, eradicant and anti sporulant properties.
  • Generally, frequent sprays are required between green cluster and the end of shoot growth in the summer.
  • In some seasons, particularly after dry summers, terminal buds restart growth after harvest.  Additional sprays may then be needed later in the season.
  • Fungicide programmes should be based on products from different chemical groups to avoid the development of strains of mildew with reduced sensitivity to fungicides.
  • Generally choice of fungicide pre-bloom and up to petal fall is governed by the need for scab control, which usually has precedence, and the need for an eradicant/anti sporulant fungicide to control primary mildew.
  • Usually a DMI fungicide such as myclobutanil (Systhane 20EW), mefentrifluconazole (Vayo) or penconazole (Topas, Topenco) is used for this purpose.
  • After petal fall, during extension growth, other fungicides (see Download list in main Control section on previous page) may be used as an alternatives to DMI fungicides.
  • Potassium bicarbonate can act as an eradicant or suppress sporulation of powdery mildew and may assist in mildew control. It has no protectant action. Because of its eradicant properties, it is useful early in the season to reduce existing levels of the disease. When used with products which have protectant properties, protection can be maintained into the season.
  • If mildew is being managed according to Butt and Barlow method or ADEM, then the alternative products listed above may be substituted for a DMI product or one with eradicant properties, where the mildew incidence has increased.

Spray application, spray volume, fungicide dose and mildew control

Control of powdery mildew is dependent on good spray cover. Both spray volume and fungicide dose influence spray cover.  Trials conducted over several years in the 1980s both in the UK and abroad reached the following main conclusions:

  • The average deposit of pesticide was proportional to the rate applied, but the distribution of the pesticide was dependent on the spray volume.
  • Low volumes tended to give a lower percentage cover and a more variable deposit.
  • In general the cover achieved with 50 l/ha appeared to be low, whereas that achieved with 100 l/ha and above was satisfactory.
  • Therefore, based on these results, the mildew control achieved using 50 l/ha and reduced fungicide dose would be poorer than that with medium spray volumes (500 l/ha) or the full dose.
  • These results were confirmed in orchard trials over several seasons in the1980s, where sprays applied at ULV (50 l/ha) and reduced fungicide dose resulted in poorer control of mildew compared to MV (500 l/ha) or LV (100-150 l/ha).
  • However the overall incidence of mildew was low, even in untreated plots, so that the control achieved at ULV was acceptable commercially.
  • None of the replicated trials were continued for long enough to demonstrate the cumulative effects of the poorer control over several seasons.
  • However, these trials do show that there is scope for reducing both spray volume and fungicide dose for mildew control, which can result in significant savings in fungicide inputs.
  • It is, however, essential that this approach is combined with careful monitoring and disease management, as outlined above, so that increases in mildew incidence can be responded to rapidly by adjusting fungicide dose and/or spray volume.

Avoiding fungicide resistance

  • Research has indicated that localised populations of powdery mildew may be less sensitive to DMI fungicides.
  • However, in most cases failure to control powdery mildew is due to a combination of factors.
  • Fungicide products currently recommended for control of powdery mildew fall into a narrow range of chemical groups.
  • Therefore it is important to use products from different chemical groups in a programme to minimise the risk of resistance and maximise fungicide efficacy .