Apple Powdery Mildew (Podosphaera leucotricha)

Powdery mildew is one of the most important diseases of apple in the UK, reducing yield and quality on susceptible varieties. All the main UK culinary and dessert varieties are susceptible, especially Cox and Jonagold.

The lifecycle and epidemiology are straightforward.  The fungus overwinters as mycelium in fruit buds or vegetative buds which emerge as primary mildew i.e. mildewed blossoms at pink bud or mildewed shoot tips at petal fall.

During spring and summer mildew spreads from the primary mildew sources to developing shoots (secondary mildew epidemic) and under favourable conditions can infect leaves and produce sporing mildew colonies in about 4‑5 days.

Mildew colonises fruit buds in early summer (about June) and colonises vegetative buds at the end of extension growth in late summer, where it remains quiescent until the following spring.

Symptoms are readily recognised on shoot tips, leaves, blossoms and fruit.

Mildew inoculum level is the key factor in determining the seasonal epidemic.  Managing the mildew epidemic through careful monitoring  of disease incidence is essential to rationalise fungicide use and to check that control measures are effective.

Control

Once primary mildew levels are high, effective control becomes difficult. Therefore, control strategies depend on maintaining primary mildew at a low level.

Where primary mildew levels are high, prompt physical removal of mildewed blossoms or shoots may be the only effective way to reduce inoculum levels.

Mildew is always present in the orchard and routine chemical control measures are usually needed from around green cluster until vegetative growth ceases, and occasionally post-harvest if terminal buds re-grow.

The objective of mildew management is to adopt a flexible approach in which fungicide dose, spray interval and spray volume are adjusted to match the levels of epidemic activity.

• Primary mildew is assessed on blossoms at pink bud and vegetative shoots at petal fall.  Primary mildew levels of <1% mildewed blossoms or shoots are satisfactory.  Levels >2% indicate a problem. Assessment of primary mildew provides information on the success of last season’s control programme and indicates the problem for the current season.
• Assessment of secondary mildew from petal fall to the end of extension growth gives a measure of the seasonal mildew epidemic activity.
• Decisions on fungicide use are based on the secondary mildew incidence, growth stage and weather. For example, where the mildew incidence is <10% mildewed shoots, the shoot growth is slow and the weather cool and rainy, there is scope to reduce fungicide inputs by reducing the dose or extending the spray interval.
• This approach to mildew management can be further improved by incorporating information on mildew risk derived from the forecasting program PODEM contained in  ADEM.  This can be used to predict the likely future mildew risk, e.g. a period of warm humid weather will indicate a higher mildew risk, whereas a cool wet weather forecast will indicate a lower risk. This will make spray decisions more robust.

Fungicide choice  is listed in the ‘Download’ table below.

• Generally, fungicide choice up to petal fall is dictated by scab control, but should include fungicides active against mildew which have eradicant action to reduce primary mildew.
• During summer, a range of mildew fungicides with different modes of action should be alternated to avoid fungicide resistance.
• Potassium bicarbonate can act as an eradicant or suppress sporing of powdery mildew and may assist in mildew control. It has no protectant action. Because of its eradicant properties, it can be useful early in the season to reduce existing levels of the disease. When used with products that have protectant properties, protection can be maintained into the season.

Fungicide choice can also be influenced by diffuse browning disorder (DBD).

• This is a physiological disorder of Cox and Cox-type apples that develops during storage.
• Research has linked development of the disorder to intensive use of triazole fungicides especially Topas, particularly in the post petal fall period in summer.
• Therefore for Cox apples intended for long term storage, current information suggests that the use of the triazole fungicides should be restricted to the early part of the season only and not after May.

The success of the mildew management system and reducing fungicide inputs without jeopardising disease control is dependent on regular monitoring of secondary mildew, so that changes in epidemic can be responded to and primary mildew build-up avoided.

Integrated approaches to control

AHDB funded Project TF 223 (Improving integrated pest and disease management in tree fruit) to identify alternative products to conventional fungicides which might achieve similar levels of powdery mildew control in apple, allowing growers to rely less upon chemical fungicides. A number of products were identified with ability to improve tree health and increase the tree’s ability to resist disease and these were investigated in an integrated disease management system.

Control in organic orchards

This is based on a combination of cultural measures  and fungicide use where possible, but sulphur is the only fungicide active against powdery mildew permitted for use in organic production.
There may be some potential in the future for biological control.

Fungicides for the control of apple powdery mildew – efficacy factors – Download

Fungicides approved for use on apple which are recommended to control apple powdery mildew or offer some incidental control when applied for other diseases – safety factors

s = safe; d = dangerous; h = harmful; ir = irritating, a = may cause allergic reaction, t = toxic

PH = post harvest; Pre bb = pre-bud burst, sm=statutory minimum of 5 m for broadcast air assisted sprayers

u=uncategorised/unclassified/unspecified, c=closed cab required for air assisted sprayers

Apple powdery mildew - additional information