Rose powdery mildew
Podosphaera pannosa; previously it was known as Sphaerotheca pannosa.
Wide. Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from Australia, New Caledonia, New Zealand, and Tonga.
Powdery mildews obtain nutrients from living hosts: they are 'obligate parasites'. Infections caused distortions to leaves, stems and flowers. A white to pale-grey powder-like growth occurs over young leaves, stems and buds. The powdery appearance is due to long chains of spores ('conidia') that stand erect from strands of the fungus (the 'mycelium') that covers the plant surface. The conidia are asexual spores, formed without mating.
As the fungus grows over the plant, slender projections penetrate the plant to absorb nutrients.
Warm, dry days followed by cool, humid nights encourage outbreaks of the disease. Spread occurs by spores carried in wind. Unlike other fungi, they can germinate on dry surfaces, they do not need a film of water on the leaf; in fact, water on the plant surface may prevent germination.
In temperate climates, the fungus overwinters as sexually formed 'cleistothecia', which discharge their spores in the spring. These are not seen commonly in the tropics. Here, the powdery mildew spreads by conidia throughout the year. However, wherever it grows it can survive in buds, emerging with the appearance of the flowers.
Infections by powdery mildew distort leaves and stunt bud growth, producing disfigured flowers. A powdery mildew-infected flower has much lower economic value than a healthy one.
Look for the white powdery growth on leaves, stems and buds.
There are a number of measures that can be taken to limit the impact of the disease:
There are many rose species and hundreds of varieties, so obtain advice from nurseries, garden centres, or from growers' catalogues about tolerance or resistance to powdery mildew before making a purchase.
Synthetic (commercial) fungicides:
AUTHOR Grahame Jackson & Mani Mua
Information from Podosphaera pannosa. Wikipedia. (https://en.wikipedia.org/wiki/Podosphaera_pannosa); and Podosphaera pannosa (powdery mildew of rose) (2018) Crop Protection Compendium (www.cabi.org/cpc); and from Pegg K & Manners A (2015) Powdery mildew: A myriad of nursery pathogens. Agri-science Queensland, Department of Agriculture, Fisheries, Ecosciences Precinct, Brisbane QLD. Photo 1 Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org.
Produced with support from the Australian Centre for International Agricultural Research under project HORT/2016/18: Responding to emerging pest and disease threats to horticulture in the Pacific islands, implemented by the University of Queensland and the Pacific Community.
This fact sheet is a part of the app Pacific Pests and Pathogens
The mobile application is available from the Google Play Store and Apple iTunes.