Lily diseases: telling leaf blotch, basal rot and virus apart

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Three quite different problems account for nearly every diseased lily, and telling them apart matters because two are survivable and one is not. The clue is where the damage starts: on the leaves, at the base of the bulb, or in the pattern of the foliage itself.

None of them is treated the same way, and the commonest mistake is to spray a plant whose problem is under the soil.

Lily disease: brown blotches on the leaves

The commonest of the three, and the least serious. It starts on the lower leaves after a wet spell and works upwards.

• What you see: oval brown or tan spots, often with a paler halo, spreading until whole leaves collapse. Stems can be girdled and buds may rot without opening
• What causes it: prolonged leaf wetness. Rain, overhead watering, crowded planting, still air
• What it costs: this season’s flowers and some of the bulb’s reserves. The bulb itself survives

What actually works:

• Space plants so air moves between them, and water the soil rather than the foliage
• Remove affected leaves as they appear, and never compost them
• Cut and remove the stems at the end of the season rather than leaving them standing — the fungus overwinters there
• Where the problem recurs every year, treat preventively as the shoots emerge, not once the spots are visible

Basal rot: the one that kills

This is a soil-borne fungus attacking the base plate, the disc from which the roots grow. Everything above ground is a consequence.

• What you see: yellowing that begins at the bottom of the stem and climbs, stunted growth, a plant that wilts although the soil is wet. Lift the bulb and the base is brown and crumbling while the upper scales may still look sound
• What causes it: wet, poorly drained soil and warm conditions. Damage to the bulb at planting gives the fungus its way in
• What it costs: the bulb. There is no treatment

The response is preventive:

• Plant into free-draining ground or a gritty compost; add a third of grit where the soil is heavy
• Discard any bulb that is soft at the base when you buy or lift it — one infected bulb contaminates the ground around it
• Do not replant lilies in the same spot for several years after a loss
• In containers the risk is entirely about drainage — see growing lilies in pots

Lily mosaic virus: the incurable one

• What you see: pale streaks, mottling or mosaic patterning on the leaves, twisted growth, flowers streaked or misshapen. Vigour declines year on year
• What causes it: aphids, which carry it from plant to plant. It is not soil-borne and does not spread on its own
• What to do: dig up and destroy the plant. There is no cure, and leaving it in place infects the rest of the collection

Controlling aphids is the only genuine prevention. Buying from a reputable supplier matters too, since virus travels in the bulb.

Telling the three apart in one minute

• Damage starts on the leaves, brown spots with haloes → lily disease. Cultural, survivable
• Yellowing climbs from the base, plant wilts in wet soil → basal rot. Lift and check the bulb
• Streaked or mottled leaves, distorted flowers, gradual decline → virus. Remove the plant
• Ragged holes and chewed edges → not a disease at all, but the lily beetle

For the growing method itself, see the lily care guide. And if a cat shares the house, remember separately that the plant is toxic to cats.