Identification Guide: Diseases

Downy Mildew (Peronospora sparsa)




Downy Mildew (Peronospora sparsa) prefers humid, cool conditions. The leaves develop purplish red to dark brown irregular spots. The leaflets turn yellow and unlike Powdery Mildew the grayish spores are only produced on the undersides of the foliage. Advanced disease conditions cause severe defoliation. Remove and destroy infected plant parts and provide good air circulation by proper spacing and plant location.

Root Rot (Phytophthora megasperma)




Root rot occurs in wet or poorly drained soils. The symptoms are similar to plants that are stressed for lack of water. New shoots wilt and die, older leaves tend to yellow. Sometimes the stems near the soil surface sometimes become water soaked and if the hairlike feeder roots are exposed the bark like root covering easily slips off leaving "wire roots" near the tips. These roots are usually brittle and not mushy. Root Rot prefers wet poorly drained soils. Provide good drainage and dry out the soil. Aliette can be used as a foliar spray or drench to control Phytopthora spp.

Black Spot (Diplocarpon rosae)




Black spots with fringed margins on the upper surface of the leaves and young stems. Yellow areas develop around the spots and the leaves may drop. Black spot is spread by splashing water, so avoid overhead watering. Prune out diseased tissue, and clean up fallen leaves.

Powdery Mildew (Sphaerotheca pannosa)

White to gray growth, often powdery, on the leaves, shoots and buds. Leaves become distorted and may drop. Powdery mildew spreads as wind blown spores. It prefers shade, and moderate temperatures. Disease can be reduced by planting in sunny locations with good air circulation. Pruning out infected tissue, and midday overhead sprinkling may reduce the spread of this disease.

Rust (Phragmidium mucronatum)

Small orange pustules, primarily on leaf undersides. Upper leaf surface may discolor, and drop. The airborne spores are favored by cool temperatures and moist conditions. It is spread by splashing water so avoid overhead watering.

Botrytis Blight (Botrytis cinerea)

The flower petals are spotted, and the buds rot. The twigs dieback and cane cankers are often present. It produces woolly, gray fungal spores on decaying tissue. Botrytis is favored by high humidity. Prune out infected parts and clean up fallen leaves. Avoid overhead sprinkling, and prune out to improve air circulation.

Cercospora Spot of Rose (Cercospora puderi and C. rosicola)




Circular spots 1 to 4 mm in diameter but coalescing to make irregular large spots. Usually purplish, or reddish brown with pale brown, tan, or gray centers. The spores are spread by wind, rain and overhead watering. Keep plants well spaced, and fallen leaves cleaned up.

Crown Canker of Rose

The canes are attacked at or just below the graft union, the bark darkening into a black, water soaked area. The cankers girdle but do not kill the canes and the plants produce fewer and inferior blooms.. This fungus lives in the soil and enters through wounds under moist conditions.

Rose Brown Canker aka Stem Canker (Crytosporella umbrina)


Begins with small purplish spots, then the area turns white with a purplish margin. During the winter cankers are formed, often several inches long, with tan centers and purplish margins. The leaves on the affected canes are sometimes spotted, yellow, or wilted. Affected canes usually die back. Prune out infected canes in the early spring and treat the entire plant with Thiolux. Repeat applications every 10 to 14 days as long as the weather is wet or humid.

Verticillium Wilt (Verticillium albo-atrum V. dahliae)




Verticillium Wilt is a soil born disease that infects the plant and inhibits the plants ability to move water from the soil to the foliage. The symptoms begin with wilting of the new canes and yellowing of the lower leaves. Advanced cases result in defoliation from the base up and eventual death of the cane or plant. These symptoms appear during periods of stress. Verticillum is very difficult to control. Infected plants are candidates for removal and replaced by varieties with resistant rootstocks.

Crown Gall (Agrobacterium tumefaciens)

Crown Gall (Agrobacterium tumefaciens) Is a bacterial disease that usually infects a wounded plant at or just below the soil surface and sometimes on the aerial portion of the plant. Once infected the plant grows a tumor like growth called galls. These galls are rough surfaced, start out soft and eventually become dark and woody. Infected plants usually produce inferior plants that are stunted, have poorer quality foliage and fewer and smaller blooms. To prevent Crown Gall avoid injury to the roots or crown during planting and cultivation. Remove and discard infected plants, and surrounding soil when first noticed.

Rose Mosaic

Rose Mosaic are characterized by yellow or whitish chlorotic lines, rings, mottles or net like mosaic patterns on the foliage. The virus does not affect the flowers however infected plants tend to be less viforous than non infected plants.
There are 3 different patterns of Rose Mosaic virus:

1. Rose Ring causes foliar distortion and ring patterns. It also causes color breaks in the petals.

2. Rose Streak causes veinbanding or brownish green rings and premature defoliatio.

3. Rose Wilt causes symptoms similar to Verticillium Wilt. The infected plants develop general decline, dieback and premature defoliation.
In cases where Rose viruses are suspected no chemical control controls are available. Remove and destroy the infected plants.

Alternaria Leaf Spot

Alternaria Leaf Spot causes leaf spots during rainy conditions. After initial spotting, leaves become brittle and change from yellow-brown to dark brown. Spots enlarge to show concentric rings on the ridges.

Spot Anthracnase