Plant problems

Early blight is the single most common tomato disease in home gardens.

Septoria is early blight's more numerous cousin and often shows up on the same plant.

Late blight is the disease that caused the Irish potato famine, and it is the one garden disease genuinely worth acting on immediately.

Powdery mildew is the most recognisable plant disease there is, and one of the least dangerous.

Downy mildew is not related to powdery mildew despite the name, and it is far more destructive.

Black spot is the defining disease of roses in humid climates, and the reason so many rose bushes are bare-legged by August.

Rust is easy to identify because the spores genuinely look and behave like rust — an orange powder that transfers to whatever touches it.

Fusarium is a soil fungus that enters through the roots and blocks the plant's water-conducting tissue, so the plant wilts even in wet soil.

Verticillium is fusarium's cool-weather counterpart, and it attacks a much wider range of plants including maples and roses.

Anthracnose is mostly a fruit-rotting disease in the vegetable garden and a leaf disease on trees.

Botrytis is the mould that turns a forgotten punnet of strawberries grey, and it behaves the same way in the garden.

Root rot is the commonest killer of houseplants and container plants, and it is caused by soil staying wet rather than by watering too often.

Sooty mould does not infect the plant at all.

Aphids are the most common garden pest anywhere, and they concentrate on the softest new growth because that is what their mouthparts can penetrate.

Spider mites are the pest that kills houseplants that were doing fine last month.

Whiteflies are unmistakable because of how they behave — brush the plant and a small snowstorm lifts off it.

Thrips rasp the surface of leaves and petals and drink what leaks out, leaving a silvery scarred appearance quite unlike anything else.

Scale insects are missed for months because they do not look like insects — people take them for bark texture or a growth on the stem.

Mealybugs are the classic houseplant pest, and they hide in exactly the places that are hardest to spray — leaf joints, under the rim of a pot, and in the roots.

A hornworm can defoliate a tomato branch in a night and still be almost impossible to see, because its colour and stripes match the foliage precisely.

If you have white butterflies over your brassicas, you will shortly have caterpillars in them.

This is the single most common reason a healthy squash or zucchini plant dies suddenly in midsummer, and it is almost always misdiagnosed as a disease.

Squash bugs feed by injecting saliva that kills leaf tissue, so a heavy population yellows and then kills leaves progressively.

The chewing damage is minor.

Japanese beetles produce a very distinctive kind of damage — skeletonised leaves where only the vein network remains.

Slug damage is easy to confirm because of the slime, which dries to a silvery trail visible in low light.

Flea beetle damage is unmistakable: a leaf peppered with small round holes.

Fungus gnats are almost entirely a nuisance rather than a threat — the adults do no damage at all and the larvae eat fungus and decaying matter, only occasionally nibbling fine roots.

Bacterial spots are distinguished from fungal ones by looking water-soaked and by having angular rather than round edges, because bacteria spread through the leaf's plumbing and get stopped by veins.

Bacterial wilt is spread by cucumber beetles feeding on the plant, and there is no treatment once it is inside.

Mosaic viruses distort growth rather than spotting it, and the mottling has no defined edges — it is a blend rather than a lesion.

Blossom end rot is not a disease and cannot spread.

Nitrogen is mobile inside a plant, so when it runs short the plant strips it out of old leaves to feed new ones.

Iron chlorosis is usually not a shortage of iron in the soil but an inability to take it up, and the most common reason is soil that is too alkaline.

Overwatering kills more houseplants than every pest and disease combined, and the cruel part is that it looks exactly like drought.

The distinction between too much and too little water is the single most useful thing a plant owner can learn, because the above-ground symptoms are nearly identical and the treatments are opposites.

Sunscald is literally sunburn, and it happens when fruit or leaves that were shaded suddenly become exposed — usually because the plant lost foliage to disease or was pruned hard in hot weather.

Growth-regulator herbicides such as 2,4-D and dicamba cause distinctive twisting distortion at very low doses, and tomatoes are extraordinarily sensitive to them.