Marin Rose Society

FAST FACTS - POWDERY MILDEW
by Nanette Londeree, Master Rosarian

SYMPTOMS

White, gray or silver talcum-powder-like growth on the tops of leaves, primarily new growth

May appear on stems, buds or sepals

First observed on new growth and distorts or curls the edges of leaves; leaves may be permanently twisted

Growing tips and buds may look badly deformed or be killed

When the disease is severe, plants become stunted and leaves curl and drop

Mature leaves are generally not infected

Powdery Mildew CAUSE

Infection by the fungus Sphaerotheca pannos var rosae

Airborne spores infect tender new growth

OPTIMAL CONDITIONS

Most prevalent in spring and fall

Temperatures 60 – 80°F, with 40 – 70% relative humidity during the day, and up to 95% at night

Does not need free water to reproduce

Fungus can overwinter as dormant mycelium or resting spores on infected stems and leaves

TREATMENT

Prevention:

Buy and plant disease-free plants

Choose resistant varieties; glossy foliaged varieties generally have better resistance to most fungal diseases

Plant roses in areas with good soil drainage and ventilation; avoid shady spots and dense plantings

Maintain good garden sanitation; remove and destroy infected leaves and canes during the season

Water leaves daily (overhead) in the morning

Spray with baking soda and horticultural oil, anti-transpirants, botanicals such as Neem oil or systemics such as Funginex

Dormant spray with horticultural oil and copper or lime sulfate after pruning

Elimination:

Spray with baking soda and horticultural oil, Banner Maxx, Bayleton/Strike, Erase, Fungi-gard, Immunox, Rally or Rubigan

GOOD GUY / BAD GUY?

Definitely a Bad Guy

One of the most common diseases of roses and other plants

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