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Roses – Pests and diseases

Fortunately, if the roses have been planted correctly in good soil in the right position, root troubles are mercifully few.

Pests. Holes in the leaves, are also produced by cockchafers and leaf-cutter bees, which take bites out of the edges. As well as capsid bug, flea beetle, caterpillar, pea and bean weevil and earwig.

Damaged flowers can be caused by chafer beetles, tortrix moths, capsids, aphids or thrips.

Marked leaves and damaged young growths are a sign of leaf miner,  and also rose slugworm, which skeletonizes the leaves.

Rolled leaves are usually produced by tortrix moth caterpillars, leaf-rolling sawfly, or damage by hormone weedkillers.

White froth on stem (cuckoo spit) is caused by frog-hoppers.

Diseases. White patches and powder on leaves, which become distorted — powdery mildew.

Black spots with yellow edges, leaves turn yellow and drop off-black spot.

Orange swellings on undersides of leaves — rose rust.

Remedy. If you are spraying regularly with both a fungicide and insecticide at monthly intervals, your problems will be controlled to a large extent. Sometimes bugs and blights develop a certain amount of resistance to chemicals used regularly and so some variation should be introduced.

Other problems

Leaf discoloration. Healthy roses have nice green leaves. They can be dark green or lighter, depending on variety, but should be of one colour. Sickly leaves with pale green parts or reddish or purple tinges or spots are almost certainly an indication of a shortage of major and/or trace elements. You can avoid to a large extent the problem by regular feeding as described. Occasionally the condition can be caused by virus and although there is no remedy, regular feeding will improve the symptoms.

Sunken, brown patches on the stem. This is caused by the canker fungus, which enters through a small wound, insect damage, frost, etc. After a while the canker encircles the whole of the stem and the part above it dies. There is no cure for this and the affected stem should be cut back to a healthy bud.

Reddish, furry growth on leaves (robins pincushion). A gall caused by the gall wasp. It does not hurt the plant but if you do not like it you can cut it off.

Shoots or stems dying back. This condition is known appropriately enough as die-back. It has several causes: frost, canker, other diseases, water logging, plant food deficiencies, etc. A common cause is faulty pruning when the cut has been made too high above the bud. The stem between the cut and the bud often dies back right into the new growth coming from it. The treatment is to remedy the trouble that has caused the die-back where possible, and to cut the affected part back to a healthy bud below the dead piece.

Blindness. A term used to describe a new stem where the terminal bud does not develop into a flower. Some roses are more prone to this than others, but frost damage and poor feeding are a contributory factor. Sometimes if you leave it alone, the shoot recovers itself and puts on more growth which eventually ends in a flower bud, or sometimes buds in the leaf axils grow out into flowering lateral branches. If this does not happen, cut the shoot back to half its length to a well positioned bud which will then develop into a flowering stem.

A word about hygiene

Plant diseases are infectious, just like any others. When you cut out diseased parts, you will almost certainly contaminate your secateurs. If you then use contaminated tools on healthy tissue you will probably pass the infection on to that as well. It is advisable that whenever you use secateurs on infected parts you wipe them over, especially the blades, with neat Jeyes Fluid or methylated spirit. Do not handle infected tissue and then immediately touch healthy growth without washing your hands first. Bear in mind that dirty gardening gloves can also spread infection.

 

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