Quebec Farmers Advocate – Ask the Green Thumb – Neem oil also plays a very important role in controlling black spot in roses

Powdery mildew

Rose types and cultivars vary in susceptibility to powdery mildew. Some climbers are especially

susceptible. Awhite, powdery fungus growth covering the leaves and young shoots is

often the first sign of the disease. 

Unlike black spot and rust, it survives the winter in live leaf buds, not on dead material.

Spores are blown from plant to plant. Also unlike black spot and rust, powdery mildew does

not require water in order to thrive and is most active during the summer.

The younger leaves tend to curl, exposing the lower surface.  If the infection is severe,

the growing tip may be killed; infected buds cannot open properly and leaves drop prematurely.

The disease can easily be identified by the felt-like white fungus covering the diseased


Young succulent plant tissue is most susceptible to infection.  As the rose tissue ages, it is

becomes more resistant.  Temperature, relative humidity and the presence of free water

greatly influence the growth of the fungus. The optimum temperature for growth is 20 to 25°C

with high relative humidity of 97 to 99 per cent. The fungus is less able to infect the plant if its

leaves are wet. Frequent misting of the foliage prevents infection by powdery mildew but, unfortunately,

this provides ideal conditions for the growth of black spot.


Powdery mildew can be controlled by cultivar selection and sanitation. Since the fungus

over winters on infected canes, in buds or on old leaves, it is important to prune infected

plant parts and remove leaves before new growth starts in the spring. If you plant susceptible

varieties, ensure the location is sunny with good air circulation.

Pruning rose canes to ensure good air circulation may also help.

Fungicides may be required for susceptible varieties. Fungicides active against powdery

mildew include folpet, lime sulphur, sulphur, triforine, thiophanate-methyl and benomyl.

These chemicals are contained in a number of home garden pesticide products.

Biological control of black spot and powdery mildew For biological control

Potassium bicarbonate is a chemical relative of baking soda and is used in antacid over-thecounter

medications. It controls powdery mildew and black spot on roses. It is sold under several

names including: Bi-Carb Old Fashioned Fungicide, Kaligreen,  and Bonide Remedy.

Bacillus subtilis is a microbial pesticide. It is a bacterium that is commonly found in soil, air,

and water. According to the product label, it is effective in controlling all important rose


Neem oil also plays a very important role in controlling black spot in roses.