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Ministry of Agriculture, Food and Rural Affairs

Powdery mildew

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 margins The lower surface of leaves is usually colonized by the powdery mildew fungus Early infections of flower buds with powdery mildew causes net russeting on fruit Powdery mildew colonizing a terminal bud in the fall – note white, felt-like growth on the terminal bud
Click to enlarge.

Beginner

Scientific Name
Podosphaera leucotricha

Identification

  • First symptoms appear on infected buds three to four days after they open.
  • Symptoms on leaves of new shoots appear as white, felt-like patches of fungal growth containing mycelium and chains of conidia that eventually colonize and cover the leaf and stem.
  • 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. 
  • Infected blossoms often open several days later than normal, or may be killed.
  • 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 on the underside of the leaf with reddish or lavender borders.
  • Infection during flowering results in failure to set fruit, or russetting of fruit.

Often Confused With

  • Russetting on fruit due to weather, or spray burn- Powdery mildew on fruit appears a fine russetted net over the surface of the fruit. Spray burn (from copper) is usually more concentrated russetted area on the fruit, and frost often results in russet rings on the fruit. Powdery mildew infections appear as wild felt like patches on the undersides of the leaves as they emerge from the terminal shoot. Young new growth of the shoots is white and hairy as the leaf emerges from the bud. This is on both sides of the leaf and the leaf grows quickly into green tissue. Severely distorted leaves with powdery mildew can be similar in appearance to Round up damage.

Period of Activity
The first symptoms of powdery mildew appear in the spring on infected buds three to four days after they open. Only new leaves are susceptible to infection and only for a few days after emergence – but fruit become infected between pink and bloom. The pathogen continues to infect until young susceptible tissue is no longer present. Both lateral and terminal buds can be infected as early as one month after they form.

Conidia landing on susceptible tissue germinate when temperatures are between 19-22ºC and high relative humidity (90%); some conidia germinate when relative humidity is as low as 70%.

Scouting Notes
Look for powdery mildew from green tip to half inch green through to terminal growth set. Monitor leaves and terminals for powdery mildew during scouting. Monitor susceptible varieties (Cortland, Ida Red, Paulared and Gala) and particularly young plantings of these varieties closely.

Thresholds
None established.

Advanced

Powdery mildew 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.

Scientific Name
Podosphaera leucotricha

Identification
First symptoms appear on infected buds three to four days after they open. Symptoms on leaves of new shoots appear as white, felt-like patches of fungal growth containing mycelium and chains of conidia that eventually colonize and cover the leaf and stem. 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. Infected blossoms often open several days later than normal, or may be killed if winter temperatures are cold. 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 on the underside of the leaf with reddish or lavender borders. Infection during flowering results in failure to set fruit, or russetting of fruit. Powdery mildew reduces vegetative shoot growth vigour, winter hardiness and productivity.

Often Confused With

  • Russetting on fruit due to weather, or spray burn- Powdery mildew on fruit appears a fine russetted net over the surface of the fruit. Spray burn (from copper) is usually more concentrated russetted area on the fruit, and frost often results in russet rings on the fruit. Powdery mildew infections appear as wild felt like patches on the undersides of the leaves as they emerge from the terminal shoot. Young new growth of the shoots is white and hairy as the leaf emerges from the bud. This is on both sides of the leaf and the leaf grows quickly into green tissue. Severely distorted leaves with powdery mildew can be similar in appearance to Round up damage.

Biology
Powdery mildew overwinters as strands of the fungus (mycelium) in dormant fruit buds and flower buds that were colonized the previous summer. 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.

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. 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 powdery 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.

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.

Period of Activity
The first symptoms of powdery mildew appear in the spring on infected buds three to four days after they open. Only new leaves are susceptible to infection and only for a few days after emergence – but fruit become infected between pink and bloom. The pathogen continues to infect until young susceptible tissue is no longer present.

Conidia landing on susceptible tissue germinate when temperatures are between 19-22ºC and high relative humidity (90%); some conidia germinate when relative humidity is as low as 70%.

Scouting Notes
Look for powdery mildew from green tip to half inch green through to terminal growth set. Monitor leaves and terminals for powdery mildew during scouting. Monitor susceptible varieties (Cortland, Ida Red Paulared and Gala) closely.

Thresholds
None established.

Management Notes.

  • Commercial cultivars of apple vary in susceptibility to powdery mildew.
Cultivar Susceptibily
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 Delicious
Slightly susceptible
McIntosh
Slightly susceptible
Empire
Slightly susceptible
Northern Spy
Slightly susceptible
Freedom
Slightly susceptible*
Jonafree
Slightly susceptible*
Liberty
Resistant*
*Scab resistant cultivars

 

  • 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 Gala, Idared, Cortland and Paulared. 
  • Begin fungicide applications, especially in blocks with a history of mildew problems, 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, Guide to Fruit Production - Chapter 3 Apples (PDF) or Apple Calendar only (PDF):