Powdery Mildew

Table of Contents

1. Introduction
2. Description
3. Biology
4. Damage
5. Management

Introduction

Powdery mildew, Podosphaera leucotricha, infects apples, pears, and quince. The fungus affects green shoots, leaves, flowers and fruit.

| Top of Page |

Description

Powdery mildew overwinters as mycelial strands in buds infected during the previous summer. The first symptoms are on infected buds three to four days after they open in the spring. The fungus spreads to cover leaves and blossoms. The first symptoms on the leaves of new shoots are white, felt-like patches of mycelium and spores that spread and cover the leaf and stem (Figures 1 & 2). Infected leaves become narrow, folded, discoloured, and brittle (Figure 3). By mid-summer, these leaves often turn brown and die. Flowers do not develop normally and are likely greenish white. Symptoms on fruit are a gold net-like russeting at harvest (Figure 4). On less susceptible cultivars such as Red Delicious, infected leaves have poorly defined pale spots with reddish or lavender borders.

Figure 1. Infection of young terminals - (note felt-like patches of white on leaves).

Figure 2. Powdery mildew on young terminal shooty.

Figure 3. Severely infected terminal growth - note dried up leaves.

Figure 4. Powdery mildew on mature fruit appears as golden net-like russeting.

| Top of Page |

Biology

When the buds open in the spring, the fungus grows on emerging leaves, shoots and blossoms. The fungal mycelium grown on the surface of plant tissue, but root-like feeding organs called "haustoria" acquire nutrients from plant cells of the leaf.

The mycelia produce spores called "conidia". Wind and splashing rain carry conidia to other parts of the same tree and to other trees. Spores germinate at temperatures between 18° and 27°C when relative humidity is greater than 90%. Unlike other diseases caused by fungi, leaf wetting is not necessary for powdery mildew infections. Spores will not germinate in freestanding water.

Once established, the fungus continues to grow on shoots, regardless of moisture or humidity. If conditions are ideal with no rain, high humidity and temperatures within 18° - 27°C, the fungus sporulates well and the disease will spread rapidly (Figure 5).

Figure 5. Apple tree heavily infested with powdery mildew.

New leaves and shoots are more susceptible to infections than older leaves, especially when conditions are good and shoot growth continues. Blossom infection is less common than leaf and shoot infection, but it is important because diseased blossoms do not set fruit. Fruit infection starts when fruit are small, but russeting is only visible when fruit size increases and fruit begins to colour.

Powdery mildew infects new buds when they begin to form until they are mature. Once the terminal buds have set, they are no longer susceptible.

| Top of Page |

Damage

Powdery mildew infections can kill vegetative shoots and prevent fruit formation when blossoms are infected. Russeted fruit is downgraded to juice. On young trees, on in heavily infested trees, vigour and productivity are reduced.

Infected buds are more sensitive to cold temperature injury than healthy buds. Most infected buds freeze and die at temperatures below -28°C. Powdery mildew can only survive on live tissue, so that infected winter-killed buds actually reduce mildew inoculum.

| Top of Page |

Management

Most commercial cultivars of apple vary in their susceptibility to powdery mildew, but all will become infected if powdery mildew inoculum is high and conditions are favourable for the disease. Less susceptible varieties should not be inter planted with mildew-susceptible varieties to facilitate spray applications where extra fungicides are required on more susceptible blocks. The following is a list of apple cultivars and their susceptibility to powdery mildew:

Very susceptible: Rome Beauty, Cortland, Idared, Paulared

Moderately susceptible: Gala, Mutsu, Golden Delicious, Jonagold, Jonamac, Wealthy, Macfree*

Slightly susceptible: Red Delicious, McIntosh, Empire, Northern Spy, Freedom*, Jonafree*

Resistant: Liberty*

*Scab-resistant varieties

Fungicides applied to control apple scab help suppress powdery mildew infections. However, powdery mildew infects during periods of hot, humid weather when trees are growing rapidly. Sufficient fungicide residues from a scab control program may not be present or may not have redistributed to new growth under such growing conditions. Also, not all scab fungicides are highly effective on powdery mildew. Therefore, special sprays may be necessary for control of powdery mildew in susceptible cultivars such as Idared, Cortland and Paulared.

Apply fungicides to control powdery mildew prior to anticipated infection (preventative sprays). Sterol-inhibiting fungicides (i.e., Nova) also offer some post-symptomatic activity. A five to ten day spray interval may be necessary if weather conditions remain favourable to the disease.

| Top of Page |