Oidium spp., Podosphaera spp., and Sphaerotheca spp.

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Image above: Powdery Mildew on older leaves of zucchini
(Image courtesy of Elise Dando)

What is Powdery Mildew

Powdery mildew refers to a group of fungal pathogens that attack a wide range of edible and ornamental plants. The pathogen can be host specific (e.g., Podosphaera leucotricha attacks apples and pears), while others can attack a range of different plants (Oidium affects cucurbits, grapes and strawberries). Depending on the pathogen, the disease can be prevalent in warm and humid, or warm and dry conditions.

Powdery mildew is one of the most easily recognisable plant diseases as the leaves appear to be covered in a white powder or ash-grey film. Fungal spores are easily dispersed by wind and rain which land on leaves and cause infection. It generally only affects leaf surfaces, but severe infection can weaken shoots and cause flower buds to fail.

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Image above: Powdery Mildew on zucchini leaves
(Image courtesy of Elise Dando)

Here is how to recognise powdery mildew and how to control the disease on edibles and ornamental plants.

Symptoms

Small white powdery spots appear on the leaves and gradually increase in size to cover the whole surface in white powder or ash-grey film. Lower, mature leaves usually succumb first to attack, but the pathogens can also affect buds, stems, flowers, and fruit. Young leaves may yellow and become slightly puckered along the edges, while flower buds may fail to open, or the petals are distorted. Older leaves eventually blacken, and overall plant growth is reduced.

On strawberries, the ash-grey film may not always be present. Instead the leaves curl upward, and purple blotches appear on the upper leaf surface. Flowers may prematurely die, or immature fruit remain small and hard. Mature fruit maintain a dull appearance.

Plants impacted:

  • Cucurbits (cucumbers, pumpkin, zucchini, squash)
  • Strawberries
  • Roses
  • Zinnias
  • Dahlias
  • Sweet peas
  • Grapes
  • Crepe myrtles
  • Hydrangeas
  • Mangoes
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Image above: Powdery Mildew on Strawberry leaf
(Image courtesy of Elise Dando)

How to Get Rid of Powdery Mildew

Control

To help control the powdery mildew on vegetables (except cucumbers and rockmelon), spray at first sign of the disease with Yates Mancozeb Plus Garden Fungicide and Miticide and repeat as needed, especially if conditions are favourable. To help control powdery mildew on cucumbers, rockmelon and other vegetables, spray with Yates Lime Sulfur. For roses and ornamentals, powdery mildew can be treated with Yates Rose Gun Black Spot & Insect Killer or Yates Rose Shield Concentrate.

Prevent

To help prevent powdery mildew on certain plants, spraying at the right time is key. If treating apples, you will need to apply Yates Lime Sulfur during dormancy to late bud swell. For grapes, apply Yates Liquid Copper when shoots are 10 cm long and repeat as needed.

The disease favours warm and humid or warm and dry weather. To reduce the risk of infection, avoid overhead watering, use disease-free seeds or planting material, and ensure there is sufficient spacing between plants. To treat infected plants, remove the worst affected plant parts and bin them. Ensure you sterilise secateurs or pruning tools between cuts with a diluted bleach solution to prevent the disease from spreading.

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