Gadenin日本語Get the appCanna leaf roller: open the roll and look inside
This is the pest that defines canna growing. A moth or skipper lays eggs on the newest growth, and the caterpillar that hatches uses silk to stitch the still furled leaf shut around itself. It then feeds inside that closed tube, protected from weather, birds and any spray applied to the outside of the plant. The leaf keeps growing while being eaten, so when it finally opens it is a ragged fan of matching holes and slashes in mirrored rows.
The damage you can see is always old news. By the time a shredded leaf has unfurled, the caterpillar has moved on to the next furled leaf above it. So the inspection has to be of the tight new spear at the top of each cane, not of the damaged leaves below. A spear that will not unfurl on schedule, feels slightly stiff, or shows fine white silk threads or dark crumbs of frass at its edge has something living in it.
The treatment is direct. Unroll the leaf with your fingers, find the caterpillar, and crush it or drop it in soapy water. One pass through the clump every few days in warm weather keeps numbers down without any spray at all. Where a clump is badly infested, cut the worst canes out at the base and remove them from the garden rather than composting them.
Never judge leaf roller by the shredded leaves. Those are the record of a caterpillar that has already moved up the cane. Always open the tight new spear at the top, because that is where the live larvae are.
Keeping leaf roller numbers down between generations
There are usually several generations in a season, so pressure builds through summer and is worst in late summer. Hand checking works well early, when there are few larvae. When numbers climb, a biological caterpillar control based on Bacillus thuringiensis, or a spinosad product, poured or sprayed directly down into the funnel of the new growth rather than over the leaf surface, reaches the larvae where they feed. Repeat every seven to ten days while damage continues.
The other half of the job is autumn sanitation. Larvae and pupae overwinter inside old dead canes and in the litter at the crown. Cutting the clump down after frost and removing every stem and dead leaf from the bed, rather than leaving them lying, removes most of next season's starting population. This one habit does more than any spray.
Cut canes should go out with the household waste or be burned where that is allowed, not onto a cool compost pile, which is a comfortable place for pupae to spend the winter.
| Problem | What you see | Action |
|---|---|---|
| Canna leaf roller | New leaves stitched shut with silk, opening ragged with rows of matching holes | Unroll and crush larvae, treat the growing funnel, remove all canes in autumn |
| Canna rust | Small raised orange to brown pustules on leaf undersides, yellowing above | Remove affected leaves, improve air flow, avoid overhead watering |
| Bud or bacterial rot | Emerging buds and new leaves blacken and turn slimy in warm wet spells | Cut affected canes to the base, clear debris from the crown, do not water into the funnel |
| Virus (mottle or streak) | Fine yellow streaks or mottling along the veins, distorted narrow leaves, poor flowers | No cure. Dig up and destroy the whole clump including rhizomes |
| Spider mites | Fine pale stippling on the leaves, dusty look, webbing in dry heat | Wash foliage with a strong jet of water, keep the plant well watered |
| Slugs and snails | Ragged holes at the base of young spears, slime trails | Clear mulch back from the crown, trap or collect at night |
Rust and bud rot: both are about air and water
Canna rust shows as small raised pustules on the undersides of the leaves, orange at first and darkening to brown, with matching yellow blotches on the upper surface. Badly affected leaves dry from the tip and hang dead on the cane. It spreads in warm humid weather and is far worse in crowded plantings and where the foliage is wetted every evening by a sprinkler.
Manage it by removing affected leaves as soon as you see the pustules, watering the soil rather than the leaves, watering in the morning so any splash dries, and thinning a congested clump so air moves through it. There is no need to strip a plant bare; taking off the worst leaves and improving the conditions is usually enough to hold it until the end of the season.
Bud rot is the more alarming one because it hits the growing point. In prolonged warm wet weather the emerging bud and youngest leaves blacken, go soft and smell sour. Cut those canes out at ground level immediately, take them off the site, and clear old leaf litter away from the crown so the base of the clump dries between waterings. Avoid pouring water down into the leaf funnels in humid spells.
- Inspect the newest spear on every cane Once or twice a week in warm weather, run your fingers up each cane to the tight new leaf. Look for silk, frass crumbs, or a spear that has failed to open on time. This finds leaf roller while there is still one caterpillar to deal with.
- Unroll anything suspicious Gently open the furled leaf. If a caterpillar is inside, crush it or drop it into soapy water. Accept the damage to that leaf; it was already eaten. Do not leave the roll closed hoping it will unfurl cleanly.
- Turn leaves over for rust Check the undersides, especially on the lower older leaves and in the crowded center of the clump. Remove any leaf carrying pustules and take it out of the garden rather than dropping it at the base.
- Act decisively on streaked leaves Fine yellow streaking along the veins with narrowed distorted leaves is virus, and there is no treatment. Dig the clump out with its rhizomes and destroy it. Do not save pieces for storage or pass them on, and clean tools before working on healthy plants.
- Strip the bed after frost Once frost has blackened the foliage, cut every cane down and clear all stems, leaves and litter away from the crown. This removes overwintering leaf roller and most rust spores in a single operation.
Virus is the one to be ruthless about
Canna is affected by several viruses that produce fine yellow streaks running with the veins, mottled patches, narrowed or distorted leaves and poor twisted flowers. It is easy to mistake early streaking for an attractive variegation, but variegation is regular and repeats leaf after leaf, while virus streaking is irregular, gets worse through the season and is accompanied by distortion and weak flowering.
There is no cure and no treatment. An infected clump does not recover and remains a source of infection for everything around it, spread by sap on tools and by insects. The only sound response is to dig up the entire clump, rhizomes included, and destroy it. Do not store the rhizomes, do not divide them, and do not give pieces away.
The practical defense is care with tools. Wipe blades between clumps when cutting canes, particularly if any plant looks suspect, and inspect new rhizomes before planting them beside an established collection. Since these viruses travel in the rhizome, a clean start is worth more than any spray.
What to record so problems shrink each year
The most useful record for canna is the date you first found leaf roller damage each season. Over two or three years the dates cluster, and knowing that pressure starts around a particular point in your summer lets you begin weekly inspections just before it, when hand removal alone is still enough.
Also note which clumps and which positions in the garden were worst hit. Rust and leaf roller both concentrate in still crowded corners, and repeated notes on the same spot are an argument for wider spacing or a different position rather than for more spraying.
Finally, record any clump you destroyed for virus, its exact position and the date. It saves you wondering later why there is a gap, and it flags the spot to check carefully in the following seasons.