Why are my leaves mottled yellow and green?

Mottling makes gardeners nervous because virus is the one plant problem with no answer, and the word gets applied to a great many leaves that are perfectly treatable. The good news is that the four causes produce genuinely different patterns, and the difference is visible without any equipment.

Step by step

1. Compare several leaves, not one.This is the whole test and it takes a minute. Virus mottling varies from leaf to leaf and from shoot to shoot on the same plant. Every other cause produces the same pattern on every affected leaf.
2. Ask whether the pattern respects the veins.Yellow tissue between a network of green veins is interveinal chlorosis, a nutrient or pH problem. On the oldest leaves it points to magnesium; on the newest, to iron or manganese unavailable because the soil is too alkaline for that plant. Both are addressable.
3. Ask whether the edges of the pale areas are sharp or blurred.Variegation and nutrient patterns have defined edges. Virus mosaic fades from light to dark green without a boundary, in irregular islands.
4. Check whether it was always like that.Variegation is present from the moment the leaf unfurls and is usually cream or white rather than yellow. A green plant that gradually became mottled is not variegated.
5. Look for distortion, which is the strongest virus signal.Puckered leaves, narrow strap-like growth, ring spots, streaked or broken flower colour, and stunting out of proportion to the conditions. Mottling plus distortion plus poor growth is a convincing virus picture; mottling alone is not.
6. Rule out weedkiller before concluding virus.Hormone weedkiller drift and contaminated manure produce twisting, cupping and mottling that mimics virus closely. The tell is that it affects several unrelated plants at once, appeared suddenly after spraying or mulching, and gradually grows out in later growth.
7. Rule out cold and light.Cold spring weather mottles and blotches new growth on many plants and then grows out completely. Pale areas confined to the side facing the sun or the glass are scorch, not mottle.
8. Act on what you found.Nutrient or pH: correct the feeding or the soil, or grow the plant in ericaceous compost with rainwater. Variegation: nothing to do, and if it is reverting to plain green, cut out the all-green shoots. Cold: wait. Weedkiller: wait, and identify the source. Virus: remove the plant, do not propagate from it, and control aphids.

Tips

• Take photographs of three or four leaves side by side. Variability across leaves is far easier to judge in a picture than by memory.
• Hosta with ink-blue bleeding along the veins usually has hosta virus X, which is spread by sap on tools when dividing.
• Tulips with flame-streaked petals are showing colour break, historically prized and caused by a virus that weakens the bulb.
• Do not confuse a reverting variegated shrub with disease. Plain green shoots on a variegated plant grow faster and take over unless cut out at the base.

Common mistakes

• Diagnosing virus from one leaf — The defining feature is variability between leaves, which one leaf cannot show. It is also the commonest reason a treatable nutrient problem gets a plant thrown away.
• Feeding a virused plant — Nothing changes. It also delays the decision to remove it while the aphids move the virus around the garden.
• Buying a mottled plant because it looks interestingly variegated — Virus-infected plants are attractive, which is exactly how they end up propagated and shared.

If it doesn't work

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