Powdery Mildew
| Author: |
Margaret Appleby
- IPM Systems Specialist/OMAFRA
|
| Creation Date: |
01 April
1999
|
| Last Reviewed: |
20 April
2005
|
Table of Contents
- Introduction
- Description
- Biology
- Damage
- Management
Introduction
Powdery mildew, Podosphaera leucotricha, infects apples, pears,
and quince. The fungus affects green shoots, leaves, flowers and fruit.
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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.

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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.
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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.
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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.
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For more information:
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Local: (519) 826-4047
E-mail: ag.info.omafra@ontario.ca
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