ROSE DISEASES

Various types of ROSE DISEASES

Roses are susceptible to a number of disease and insect pests. Roses may survive without a basic pest control program, but they may not be very attractive. A pest control program starts with proper site selection, good soil preparation, good drainage, proper spacing, cultivar selection, and plant maintenance. These all foster healthy roses that are better able to withstand the pressure of disease and insects.

Blackspot

This fungal disease can cause almost complete defoliatiation of bushes by early fall, resulting in a weakened bush on which cane die-back and cankers become severe. Blackspot is identified as circular black spots that appear on the upper surface of the leaves, starting at the bottom of the plant and moving upward. Infected leaves turn yellow and fall off prematurely. The fringed margin and black color distinguish this leaf spot from others. Infections on canes are identified as reddish-purple spots. Splashing water spreads blackspot. Infection occurs after leaves are wet for several hours, making it more serious during rainy periods.

Downy Mildew (Peronosporasparsa) –

Downy mildew is a rapid and destructive fungal disease that appears on the leaves, stems and blooms of roses as dark purple, purplish-red, or brown irregular blotches. 

Rust (Phragmidiummucronatum )–

Rose rust appears as yellow patches on the surface of leaves, with orange pustules of sporesunderneath the leaf. The fungus is spread by wind. Affected leaves fall prior to healthy ones and plants may be defoliated in serious infections.

Canker (Leptosphaeriaconiothyrium and Cryptosporellaumbrina)–

 Cankers present as small yellowish or reddish spots on bark slowly increasing in size. Leptosphaeriaconiothyrium turns brown, increases in size, and may eventually girdle the stem. The tissue within the infection begins to dry out and shrink, presenting a shriveled appearance. If the disease infects only part of the stem, growth above the canker will continue. If it girdles the stem, however, growth will cease and the stem will die.

Botrytis Blight

Botrytis blight is a fungal disease that generally attacks dying tissue. It is frequently found on older flowers and other plant parts. Under certain conditions it may also attack healthy tissue. Botrytis favors moist, wet conditions, often causing the disease to attack entire flowers and produce a gray fuzzy mold. This disease is often called graymold. Good garden sanitation and removing spent flowers often result in good control of this disease. When this is insufficient in providing adequate control, a preventative spray program may be necessary.

Rose rosette

Rose rosette is becoming more common and can result in significant damage. This pathogen (not yet positively identified) is spread by an eriophid mite. Symptoms include rapid growth of shoots, development of "witches' broom," development of tufts of small, deformed reddish leaves and excessive thorniness. Plants decline over time. Because affected plants can't be cured, it is best to dig out the affected plant and destroy it. 

Rose Mosaic Virus –

Actually a virus and not a fungal attack, it causes reduced vigor, distorted leaves, and reduced flowering. Roses with rose mosaic virus are best discarded from the garden or rose bed, and the only sure way to tell if a rose bush has this is to have it tested.

Powdery Mildew (Podosphaerapannosa) 

– P. pannosa produces a very fine, powdery coating on the surface of buds and leaves. Significant cases have stems and particularly thorns, infected. Attacks on young leaves and buds will cause deformity with retardation of growth. Infected buds will fail to open. The disease is likely in hot, humid weather, with fungal spores overwintering on the stems and fallen leaves.

Prevention

Plant disease-resistant varieties- Some roses are very susceptible to these fungal diseases, others are virtually immune, and many fall somewhere between. 

Grow healthy plants- Plant roses in full sun in rich, well-drained soil. Soil pH is ideally between 5.8 and 6.2. If the pH is lower, add limestone. If it's higher, add sulfur. Space plants about 3 feet apart to allow good air circulation, and mulch roots in summer to help maintain even soil moisture and to prevent competition from weeds.

Water regularly- Generally, you want water to reach everywhere within the plant's root zone, which may be as deep as 18 inches and as wide as the crown of the plant. Get to know your soil and how water penetrates and travels within it. If the soil around your roses is dry to a depth of 4 inches, you should definitely water, but it's best not to let it get that dry.

Water roots, not leaves- Wet leaves encourage some fungi, so avoid wetting them unless you are washing off dust or aphid infestations with a hose. Drip irrigation is better for watering roses than sprinklers.

Fertilize correctly- Rose plants that are either under- or overfed will be more susceptible to problems. One simple method is to buy a fertilizer packaged for roses and to follow the label directions.

Use preventive maintenance- In the fall, remove all fallen leaves on the ground below the plants, and dispose of them. In late winter or early spring, prune plants and apply horticultural oil to smother disease organisms that may have overwintered on branches. Spread a fresh layer of mulch to cover any disease spores that might be on the surface of the old mulch.