powdery mildew (ornamentals)

Symptoms

A white-to-grey powdery coating spreads over the upper leaf surface, shoots and flower buds; under it the tissue yellows, and young leaves may pucker, curl or be stunted; severe attacks brown and shrivel the leaves. Unlike downy mildew the growth is on the UPPER surface and rubs off; worst in warm days with cool, humid nights and crowded, shaded plants.

Plants affected

• dahlia
• florist's chrysanthemum
• lupin
• peppermint
• African violet
• snapdragon
• delphinium (larkspur)
• foxglove
• common oregano
• zinnia
• common thyme

Easily confused with

• chrysanthemum white rust

  How to tell them apart: Puccinia horiana (chrysanthemum white rust) first shows as small pale-green to yellowish dimpled spots on the UPPER leaf surface; turn the leaf over and you find raised pink-to-buff/white blister-like pustules (telia) directly beneath each spot — these are firm, waxy and erupt through the epidermis. It thrives in cool, humid, wet-leaf conditions (15-25C, free moisture). Golovinomyces sp. (powdery mildew) instead forms a superficial, dry, dusty-white powdery coating that wipes off with a finger, sitting ON TOP of the upper leaf, stems and buds, with no pustules underneath and a felty rather than blistered texture; it favours warm days with dry leaves but humid nights. The decisive test: pustules erupting from the lower leaf surface = white rust (an EU-quarantine pest); a wipeable surface dust with nothing underneath = powdery mildew. White rust is notifiable — report any suspected case to your national plant protection organisation (NPPO) before treating.

• grey mould (Botrytis)

  How to tell them apart: Botrytis cinerea (grey mould) shows up as a fuzzy, greyish-brown mat of mould, usually on damaged, ageing or rain-wetted tissue — fading flower petals, buds, and soft stems that turn brown, water-soaked and collapse. When you brush it, a cloud of grey spores puffs off and the tissue underneath is rotting and mushy. It thrives in cool, damp, still air. Golovinomyces sp. (powdery mildew) instead coats living, otherwise healthy green leaves with a dry, flat white-to-greyish powder you can rub off with a finger, typically starting as discrete round patches on the upper leaf surface before spreading; the leaf stays firm (no rot), though it may yellow and curl underneath, and it favours warm days with dry leaves but humid nights.

• sclerotinia stem rot

  How to tell them apart: Golovinomyces sp. (powdery mildew) shows a dry, dusty, white-to-greyish powdery coating that sits on top of upper leaf surfaces, stems and flower buds and wipes off with a finger, leaving the green tissue intact beneath; it spreads in warm, dry, crowded conditions and rarely kills the plant. Sclerotinia sclerotiorum (white mould) instead produces a dense, cottony, wet-looking white mycelium concentrated at the stem base and on tissue that is already softening, accompanied by water-soaked tan-brown lesions, sudden wilting and stem collapse. The decisive sign is inside or on the rotting stem: Sclerotinia forms hard, irregular black sclerotia the size of rat droppings, which powdery mildew never produces. Cool, humid, wet conditions and a slimy soft rot point to Sclerotinia, not powdery mildew.

• two-spotted spider mite

  How to tell them apart: Powdery mildew forms a white-to-greyish powdery film or felty patches on the UPPER leaf surface, stems and buds, which you can WIPE OFF with a finger to reveal green tissue beneath. It begins as discrete circular floury spots that merge into a continuous mealy coating; there is no webbing and no moving specks underneath. It thrives in mild, humid air with poor air circulation, often in shade. Two-spotted spider mite causes fine, pale stippling that you see ON THE UPPER LEAF SURFACE as countless tiny yellow-white dots; as it worsens the whole leaf turns dull bronze or grey-green. Look UNDERNEATH the leaf for fine silk webbing and minute moving specks (the mites and pale shed skins). The discolouration is a pattern of dots, never a removable powder, and it is worst in hot, dry, dusty conditions.

Common questions

How do I recognise powdery mildew (ornamentals) on dahlia?

A white-to-grey powdery coating spreads over the upper leaf surface, shoots and flower buds; under it the tissue yellows, and young leaves may pucker, curl or be stunted; severe attacks brown and shrivel the leaves. Unlike downy mildew the growth is on the UPPER surface and rubs off; worst in warm days with cool, humid nights and crowded, shaded plants.

Do I have to report powdery mildew (ornamentals)?

No. powdery mildew (ornamentals) is not a notifiable organism, so there is nothing to report to an authority. Treat it as a normal garden problem.

How is powdery mildew (ornamentals) treated?

Mainly with biological and cultural measures — removing affected material, changing the conditions that favour it, and treating with agents approved in your country. The exact steps and their timing depend on the plant, the season and where you live: Dr. Phyto works them out in a free diagnosis.