Most of the problems affecting roses are seasonal and dependent on the climate. Certain varieties of roses also appear to be more inclined to certain diseases. Keeping the soil in good condition with ample organic material and applications of a good, well-balanced fertilizer (at least twice a year), as well as good drainage and irrigation are the best methods of avoiding problems from diseases.
For ease of use, we have grouped the various diseases and pests according to their most obvious symptoms: spots on leaves, insects, and other disorders.
Spots, patches or powder on leaves
Rounded black spots with fringed edges appearing from July onward are the most obvious symptom of this disease. The big black spots appear
first on the lower, older leaves and then spread upwards. As the leaves grow they may turn yellow and eventually fall off.
Spray with a systemic fungicide, such as Benomyl, Triforine, Burpirimate or Carbendazim.
Burn affected leaves.
Powdery Mildew (Odium)
White powdery-looking coating on leaves, buds and flower stems, spreading quickly. (see further under "Flower buds do not open").
Downy mildew (Peronospora sparsa)
Peronospora causes purple-red to dark-brown spots on the leaves with irregular, often angular, borders. Stems, petioles and flower stalks can split and spotted with purple marks. Buds, sepals, petals and calyces can be affected and will present purple spots. New growth affected will be deformed. The disease is spread by wind.
Rose rust is caused by a fungus and appears as yellow patches on the surface of leaves, with orange pustules of spores underneath the leaf, which later turn black. The fungus is spread by wind. Affected leaves fall prior to healthy ones and plants may be defoliated in serious infections.
Anthracnose Sphaceloma rosarum
Spots caused by this fungus originate from a point where leaves are watersoaked, usually unnoticeable at first, until they turn black with a very distinct defined edge. As the spots enlarge the center becomes gray and may fall out resulting in a shot-hole appearance. Defoliation may occur but is often not serious.
Adult females lay their eggs in leaf margins and the same time inject a toxin which causes the leaf to roll up lengthwise, protecting the grub when it hatches inside. The rolled leaves contain an egg or a pale green caterpillar-like sawfly larva that feeds within the rolled leaflets. This occurs during late April to early June. Only preventive spraying in late April with fenitrothion is effective. Damaged leaves can be found throughout the summer.
Rose Flower buds do not open or rot
On roses grey mould is primarily a disease of the flowers and buds, leaves are infrequently attacked. Infected buds rot on the stem and infection may progress down the stem. On petals botrytis cineria produces pink rings.
Powdery Mildew (Odium)
White powdery-looking coating on leaves, buds and flower stems, spreading quickly. 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. Spray with benomyl or triforine, so the same spray will deal with both black and mildew.
Insects and other animals
Irregular holes eaten in leaves. Remove by hand or spray with fenitrothion, gamma BHC (HCH) or permethrin.
Small green, occasionally light-brown insects which cluster on rose shoots and leaves, sucking the sap. They may cover (in a colony) the complete growing tip of the plant. Aphids are most active in spring and summer and multiply at a prodigious rate. In large quantities they may seriously retard the growth of the plant and ruin buds. They are particularly damaging to the new shoots with subsequent damage to the emerging leaves which become malformed.
Any insecticide will deal with these. Systemic insecticides, such as dimethoate and heptenophos (and fungicides) remain active longer than others, as they penetrate the plant tissue and so are not washed off by rain.
Japanese Beetle (Popillia japonica)
This invasive, non-native species in North America, also referred to as jitterbug, feed on a variety of trees, shrubs, as well as the flowers, fruit and leaves of grapes, rose, cherry, hibiscus, dahlia, zinnia, etc. Japanese beetles travel and feed in groups. A swarm of beetles have been known to strip a peach tree in 15 minutes, leaving behind only bare branches and the fruit pits. They are native to east Asia, but were accidentally introduced into the United States in 1916. In North America they occur from Georgia west to Missouri, north to Ontario and east to Nova Scotia, with some populations now in California. Insecticides kill adult beetles, but do not prevent reinfestation. Some of the beetle's natural predators such as wasps and flies have been imported from Japan to help control the population. Moles, shrews, skunks, and birds also significantly decrease the population by eating the larva form. Biological control is available using a bacterium, Bacillus popilliae, which causes milky disease in the larvae--thereby greatly reducing Japanese beetle populations.
Red spider mite
Minute animals which such sap from the undersides of the leaves, usually troublesome only in the open garden in hot dry weather. Very difficult to control, but they do not like water. Dimethoate (systemic) spray may sometimes help.
Blobs of froth where leaves join shoots are the signs. Froghoppers, palle yellowish-green, sap-sucking insects, hide in these. Pick off by hand or spray as for greenfly.
This pest usually occurs in hot, thundery weather, especially on some of the climbing Hybrid Teas and the more doubled Bourbons, Noisettes and Teas. Spraying with a systemic or contact insecticide just before the flower buds begin to open will prevent the little flies from disfiguring the petals of the flowers.
A disease that mostly attacks older plants, especially those that have been heavily pruned over the years. Canker spores enter through a wound left after cutting, forming brown, cracked areas. Cut away shoot below canker. Cuts should then be left clean and smooth and coated with a layer of grafting wax.
The most common nutritional ailment of roses is probably iron deficiency. A typical sign is the yellowing of the leaves, especially younger leaves, either along the margins, or along the veins, or both. The leaves eventually turn completely yellow and fall.
Rose sickness (replant disorder)
Rose sickness is a form of replant disorder that occurs when a new rose is planted on the same spot as an old one. Why exactly this problem occurs is unknown, but the rose plant will almost always fail to grow properly and will appear very sickly. The soil that has had roses grown in it for any length of time is called "rose sick". The problem does not arise with existing roses, which can thrive in the same ground indefinitely, but only when the same spot is replanted with new roses. Various theories have been advanced, such as poisons or microscopic organisms known as nematodes stemming from the roots of the previous roses, but the exact nature of the problem is not clear yet. If possible, plant the new rose about 2 ft away from the original spot or replace the soil completely to a depth of 1 to 2 ft (according to the size of the rose) before replanting a new rose. According to David Austin the problem of rose sickness tends to be less in humus-rich soild, so adding large quantities of humus may alleviate the problem.