Plant diseases and pests

Symptoms on leaves are rarely unambiguous, because the same appearance can come from completely different directions. That is why the first question is always: where on the plant is the symptom, and how has it spread over time?

Symptom to cause

• White, powdery coating on the leaves — powdery mildew, a fungus. Attacks readily in humid air with fluctuating temperatures. Remove affected leaves and improve the air circulation.
• Grey, fuzzy coating on stems and flower buds — grey mould. Often comes after overwatering in a dense, cool environment. Remove affected material, lower the humidity.
• Fine webbing and grey-speckled leaves — spider mites. An animal, not a disease, and it thrives in dry, warm air. Rinse the leaves and raise the humidity.
• Sticky leaves, small insects in the shoot tips — aphids. Often followed by honeydew and black mould on the sugar-rich excretions.
• White, cotton-like lumps in the leaf axils — mealybugs. Pick them away with a cotton bud and follow up, because the larvae stay in the soil.
• Brown, sticky spots on stems — scale insects. They sit tight and are scraped off mechanically.
• Silvery streaks and deformed leaves — thrips. Thrives in dry air and attacks new leaves first.
• Yellow leaves from the bottom upwards, or purple/yellow discolouration between the veins — calcium or magnesium deficiency and nitrogen deficiency respectively. Do not fertilise blindly: over-fertilising produces its own symptoms.
• Brown, dry leaf tips — usually low humidity, draught or dried-out soil, not disease.
• Root rot: soft, brown and foul-smelling root — often overwatering in dense soil without drainage. Hard to save; change the soil, remove dark roots and water less.

Overwatering or underwatering?

The most common misjudgement. Both give yellow, drooping leaves, but different signs:

• Overwatering: the soil is heavy and smells sour, the leaves turn yellow from the bottom, the edges become soft and brown, and small black flies thrive in the soil.
• Underwatering: the soil pulls away from the edge of the pot, the leaves become dry and brittle, hang clearly, and recover quickly after watering.

Photograph like this for the best diagnosis

• Take a close-up of the symptom in daylight, not against the light.
• Take an overall shot showing the plant, the pot and the soil surface.
• Take a third photo of the underside of an affected leaf — that is usually where the animal sits.
• Select Disease & pests in the tool, so the analysis is adapted to symptom images instead of species identification.

When the plant has to go

If the rot has reached the stem, the soil smells rotten far down, or the infestation is spread across the whole plant with no healthy plant tissue to cut back to — then salvage is rarely worth the effort, and a contagious disease can affect the neighbouring plants. Throw the plant away with its soil, clean the pot, and do not put the soil in the compost.