Powdery Mildew

Excerpt from Publication 310, Integrated Pest Management for Apples,
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Table of Contents

1. Symptoms
2. Biology
3. Monitoring and management

Powdery mildew, caused by the fungus Podosphaera leucotricha, requires living host material to survive (obligate fungus). The fungus infects apples, pear and quince, and infects all plant parts above the ground including green shoots, leaves, flowers and fruit.

Symptoms

The first symptoms of powdery mildew appear in the spring on infected buds three to four days after they open. Infected blossoms often open several days later than normal, or may be killed if winter temperatures are cold. Under optimum conditions, the fungus spreads to cover leaves and blossoms. Symptoms on leaves of new shoots appear as white, felt-like patches of fungal growth containing mycelium and chains of conidia (asexual spores) that eventually colonize and cover the leaf and stem (Figure 4-127). The lower surface of leaves is usually initially colonized by the fungus. Colonies of powdery mildew are observed on the upper surface of leaves under optimum environmental conditions (Figure 4-128). Infected leaves become narrow, folded and brittle frequently with a pinkish-red discoloration or hue on the leaf margins. By mid summer, infected leaves often turn brown and senesce prematurely. Infected flowers appear covered with a powdery growth. On less susceptible cultivars, such as Red Delicious, infected leaves have poorly defined pale spots with reddish or lavender borders. Powdery mildew reduces vegetative shoot growth vigour, winter hardiness and productivity. Infection during flowering results in failure to se fruit, or russeting of fruit (Figure 4-129). Rusetted fruit are downgraded to juice.

Figure 4-127. Powdery mildew symptoms on the leaves appear as white, felt-like patches of fungal growth with a pinkish-red discoloration or hue on the leaf margin

Figure 4-128. The lower surface of leaves is usually colonized by the powdery mildew fungus

Figure 4-129. Early infections of flower buds with powdery mildew causes net russeting on fruit

Colonized shoots and buds - particularly on young trees - have reduced vigour and are less productive and more susceptible to cold injury. Temperatures below -24ºC kill most infected buds, but healthy buds survive -26 to -30ºC temperatures. Although cold winters reduce the survival of infected bud, they also reduce the inoculum potential of the disease.

Both lateral and terminal buds can be infected as early as one month after they form. The fungus can remain dormant in infected buds until bud break the following spring. Although lateral buds remain susceptible to infection for a longer period, terminal buds are more likely to harbour the fungus over the winter.

Biology

Powdery mildew overwinters as strands of the fungus (mycelium) in dormant fruit buds and flower buds that were colonized the previous summer (Figure 4-130). When the buds open in spring, the fungus grows onto emerging leaves, shoots and blossoms. The fungal mycelium grows on the surface of plant tissue producing chains of conidia giving the leaves a white powdery appearance.

Figure 4-130. Powdery mildew colonizing a terminal bud in the fall - note white, felt-like growth on the terminal bud

Conidia produced by overwintering mycelium serve as the primary inoculum in spring, and are easily dispersed by wind currents to susceptible tissue within the same tree or to other trees. Only new leaves are susceptible to infection and only for a few days after emergence - but fruit become infected between pink and bloom. Conidia landing on susceptible tissue germinate when temperatures are between 10-25ºC with optimum germination between 19-22ºC and high relative humidity (90%). Some conidia germinate when relative humidity is as low as 70%. Unlike other disease causing fungi, leaf wetting is not necessary for powdery mildew infections. In fact, conidia will not germinate in free-standing water and rain or dew frequently washes the powder mildew conidia off leaf surfaces. Under optimum conditions, the fungus produce abundant conidia on infected shoots and young leaves resulting in a rapid build up of inoculum. The pathogen continues to infect until young susceptible tissue is no longer present.

Monitoring and management

Look for infected terminals and shoots during regular orchard inspections.

Commercial cultivars of apple vary in susceptibility to powdery mildew. Do not interplant less susceptible varieties with mildew-susceptible varieties to facilitate spray applications where extra fungicides are required on more susceptible blocks. Table 1. Susceptibility of apple cultivars to powdery mildew lists some apple cultivars and their susceptibilities to powdery mildew.

Table 1. Susceptibility of apple cultivars to powdery mildew

Cultivar	Susceptibility
Rome Beauty	Very Susceptible
Cortland	Very Susceptible
Idared	Very Susceptible
Paulared	Very Susceptible
Gala	Moderately Susceptible
Mutsu	Moderately Susceptible
Golden Delicious	Moderately Susceptible
Jonagold	Moderately Susceptible
Jonamac	Moderately Susceptible
Wealthy	Moderately Susceptible
Macfree	Moderately Susceptible*
Red Declicious	Slightly Susceptible
McIntosh	Slightly Susceptible
Empire	Slightly Susceptible
Northern Spy	Slightly Susceptible
Freedom	Slightly Susceptible*
Jonafree	Slightly Susceptible
Liberty	Resistant*

*Scab-resistant varieties

Many of the fungicides applied to control apple scab help suppress powdery mildew infections. However, powdery mildew infections often occur during periods of hot, humid weather when trees are growing rapidly and apple scab infection periods are unlikely. Not all scab fungicides are highly effective on powdery mildew. Ensure fungicide programs include fungicides that are active against both scab and powdery mildew. Additional sprays may be necessary for control of powdery mildew in susceptible cultivars such as Idared, Cortland and Paulared.

Begin fungicide applications to control and prevent powdery mildew infections at tight cluster and continue until terminal growth stops. Apply fungicides to control powdery mildew prior to infections occurring (preventative sprays). Spraying early in the season - particularly from tight cluster until petal fall - is critical to managing powdery mildew successfully. For a list of fungicides and timings of sprays effective against powdery mildew see OMAFRA Publication 360, Fruit Production Recommendations.