Rose Problems: Black Spot, Powdery Mildew and Aphids

Black spot: the one that defoliates a rose

Black spot shows as dark, roughly circular blotches with feathery, ragged edges, usually on the lower leaves first. The tissue around each spot yellows and the leaf drops. A badly affected bush can be bare by midsummer, and a bare rose cannot fund the next flush, which is why this is the disease that costs you flowers rather than just looks.

The fungus needs about seven hours of continuous leaf wetness to infect, and it overwinters on fallen leaves and on the canes. That gives you three practical controls, none of which is a spray: keep the leaves dry, remove infected leaves the day you see them, and clear every fallen leaf out from under the bush in autumn so there is nothing to start from in spring.

1. Pick off spotted leaves immediately Remove each affected leaf and put it in the bin, not the compost pile. Doing this the day the first spots appear can hold the disease at the bottom of the plant for weeks.
2. Never water overhead Water at the base, in the morning. An evening sprinkler over the foliage gives the fungus the whole night of leaf wetness it needs.
3. Open the plant up Prune to an open center so air moves through. Two roses crammed together stay wet after dew for hours longer than one with space around it.
4. Strip and clear in late autumn Rake up every fallen leaf under and around the bush and remove it from the garden. This single job cuts next year's starting inoculum more than anything else you can do.
5. Mulch over the ground in spring A fresh layer of mulch buries any spores left on the soil surface so rain cannot splash them back onto the lower leaves.

Some roses are genuinely resistant and some are not, and no amount of care makes a susceptible rose in a damp, still corner behave. If one bush defoliates every year while its neighbors do not, the honest fix is to replace it.

Powdery mildew: dry roots, not wet leaves

Powdery mildew coats new shoots, buds and young leaves in a white or gray felt, and the affected leaves curl and distort. Unlike black spot it does not need wet foliage. It thrives on warm days and cool, humid nights, and it is far worse on plants whose roots are dry, which is why roses against a warm wall or in containers get it first.

That makes the treatment counterintuitive: the answer to mildew is usually more water at the root, not less. Keep the root zone evenly moist, mulch to stop it drying, and cut out the worst affected shoot tips. Overfeeding with nitrogen makes it worse by producing exactly the soft young growth mildew prefers.

Powdery mildew and black spot want opposite conditions. A hot dry summer gives you mildew, a warm wet one gives you black spot, and a season that alternates can give you both.

Aphids: catch them in the first week

Aphids appear on the softest growth in spring, clustered on the tips of new shoots and on the flower buds. They are usually green but can be pink, brown or black. They suck sap, so shoots grow distorted and buds fail to open properly, and they coat everything below in sticky honeydew that goes on to grow black sooty mold.

Numbers double very quickly, so timing beats method. A colony caught in its first week comes off with a thumb and finger or a firm jet of water and does not come back for a month. The same colony left for three weeks has spread to every shoot on the plant and brought sooty mold with it.

1. Check the shoot tips twice a week in spring From the moment the buds break, look at the top two inches (5 cm) of every new shoot and at the flower buds. That is the only place they start.
2. Squash or wash them off Run finger and thumb up the shoot, or knock them off with a jet of water from a hose in the morning. Aphids knocked to the ground largely do not climb back.
3. Leave the predators alone Ladybird larvae, hoverfly larvae and lacewings clear aphids for you. Broad spectrum sprays kill them too, which is why a sprayed rose often has a worse aphid problem a fortnight later.
4. Wash off sooty mold afterwards The black film is growing on honeydew, not on the plant. Once the aphids are gone, wash the leaves with plain water so they can photosynthesise again.
5. Ease off the nitrogen Heavily fed roses put out lush, soft shoots that aphids prefer. If one plant is always the worst, look at how much feed it is getting.

The other things you will see

Beyond the big three, a handful of problems turn up often enough to be worth recognizing on sight. Most need nothing more than removing the affected part; only rose rust and stem canker warrant cutting into the plant, and only rose replant problems require you to change the soil.

Anything you cut out because of disease goes in the bin, never on the compost pile, and never left lying under the plant. Clean the blades with a wipe between plants when you have been cutting canker.

Building a routine instead of reacting

Every one of these problems is easy in its first week and hard in its fourth. The practical answer is a two minute walk round the roses twice a week from bud break to late summer, looking at three places: the tips of the new shoots, the undersides of the lower leaves, and the flower buds. Almost everything shows up in one of those three.

Write down the date you first see black spot each year and the date you first see aphids. After two seasons those dates tell you when to start looking rather than when to start worrying, and you will find you are acting a fortnight earlier than you used to.