Powdery mildew of apples

Note Number: AG0161
Published: December 1999
Updated: August 2010

Powdery mildew of apples, caused by the fungus Podosphaera leucotricha, affects leaves, buds, shoots and fruits, and forms a dense white fungal growth (mycelium) on the host tissue. The disease stunts the growth of trees and is found wherever apples are grown. It has also been reported on pear and quince although damage on these hosts is rarely seen in Australia.

Symptoms

Figure 1. Dormant shoot infected with powdery mildew

Dormant shoots that were heavily infected in the previous growing season are covered with dense white mycelium, and the terminal bud is pinched and shrivelled (Figure 1). Other shoots may look normal but the fungal mycelium may over-winter in otherwise healthy-looking fruit or leaf buds.

Primary mildew infection results from the growth of infected, over-wintered leaf or fruit buds. These buds may be killed, or they may grow abnormally; leaves become narrow, brittle, curled and covered with a white powdery layer, while flowers may be stunted and fail to develop (Figure 2). Secondary mildew infections may appear as a powdery mottling on either side of the leaves.Early fruit infection causes a web-like russet on the skin that may be difficult to distinguish from early spray damage. Less commonly, fruit may be distorted and partly covered with a white powdery coating of spores (Figure 3).

Economic importance

Figure 2. Primary mildew of apple blossom and leaves resulting from bud infection in the previous season

Powdery mildew can be a problem in all Victorian apple districts where adequate control measures are neglected. Highly susceptible cultivars include Jonathan, Bonza, Jonagold, Elstar, Pink Lady, Lady William and Gala. Powdery mildew is favoured by dry conditions, unlike apple scab, which is worst in districts of high rainfall.

Losses attributed to powdery mildew are hard to quantify. They include poor tree growth, dead buds, reduced leaf area, reduced shoot growth and lowered quality of fruit. Heavy infection that continues over several seasons may reduce yields by up to 80 per cent.

Disease cycle

The fungus overwinters in fruit and leaf buds, and the first powdery mildew symptoms occur on flower trusses and shoots that emerge from infected buds in spring. Spores from these infected leaves give rise to secondary mildew infections on the leaves of shoots which are rapidly extending throughout spring and early summer. Spores from all sources infect newly-formed buds in leaf axils. Fruit buds on spurs are susceptible to infection by powdery mildew for about one month, between pink bud and petal fall. Leaf buds are susceptible to infection for about one month after they appear in the axils of leaves on extending laterals. However, the terminal buds on laterals can be infected throughout the period of lateral growth. Infected buds are generally not killed but provide the main source of primary infection in the following season.

Control measures

Figure 3. Fruit infected with powdery mildew showing the white powdery coating of spores

Powdery mildew may be controlled by removing infected buds, by modifying the environment so that it is less favourable to infection and by spraying to protect buds from infection. However, there are several features of powdery mildew that make control difficult. Powdery mildew develops in the absence of rain, and at temperatures above 20°C. Rain is harmful to mycelium and suppresses production of spores. The time between germination of spores on leaf surfaces and infection and production of fresh spores is short, so there is a rapid recovery after any reduction in the number of spores. Plenty of spores are usually available for infection.

The emergence of new leaves on rapidly-growing laterals makes complete protection with fungicides difficult. Also, the buds in leaf axils are protected by leaves, and are difficult to reach with fungicides. Control measures must be applied thoroughly to be effective.

Removal of infected buds

The terminal bud is most commonly infected with powdery mildew, and therefore can be an abundant source of spores for infecting new leaves and buds. Remove infected terminal buds during pruning where practicable.

Modification of the environment

The humidity at the surfaces of leaves is an important factor influencing germination of spores. Maximum germination occurs at high relative humidity, but not in free water. High relative humidity occurs at leaf surfaces when the air is calm, but it is reduced by air turbulence. Trees with open canopies have more air turbulence and therefore less powdery mildew than in trees that have dense foliage. Similarly, wind breaks should be managed in such a way that turbulence is reduced enough to prevent fruit damage, but not so much as to result in completely calm conditions that favour mildew development.

Monitoring

Monitoring for mildew during winter and the growing season will assist with decisions about mildew control. The incidence of mildewed terminal buds in winter can be used to help plan mildew control strategies for the coming spring. Shoots with infected terminal buds can be assessed immediately after flowering using a minimum of 10 trees per block. For assessment of secondary mildew infection, label 15 trees per ha and assess 10 extension shoots per tree. Inspect the top 5 unfolded leaves per shoot for mildew, and record the incidence of mildewed shoots. Assess the same trees at intervals during the growing season. If mildew levels increase over time then control measures may need to be improved.

Control by spraying

There is no adequate substitute for a series of spring and summer sprays to prevent infection of fruit and leaf buds. Detailed work on powdery mildew at East Malling in the U.K. has shown that infection of fruit buds occurs before petal fall, and that infection of terminal buds can occur until lateral extension is complete. Thus, frequent spraying from pink bud until the end of lateral extension is needed to protect fruit, leaf and terminal buds. Frequency of spraying is more important than the dosage of fungicide, and two weeks should be the maximum period between sprays if control in orchards with a history of mildew is to be effective. High volume sprays are generally more reliable for mildew control, although recent work has shown that low-volume sprays can be equally effective.
A range of fungicides is registered for the control of powdery mildew, and your choice will depend on costs and any experience with phytotoxicity.

Contacts/services available from DEPI

For effective pest and disease control, correct diagnosis is essential. Phone Crop Health Services on (03) 9032 7515 or fax (03) 9032 7604.

Acknowledgements

This Information Note was published in December 1999.
It was reviewed by W.S. Washington, Plant Standards in August 2010.

ISSN 1329-8062

Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
Melbourne, Victoria

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