Taro Pests and Problems: Leaf Blight, Rot, Aphids and Skin Irritation

What actually goes wrong with taro

For a crop that spends six or seven months in the ground, taro is remarkably untroubled. Its leaves are unappetising to most mammals, the sap deters a good deal of chewing insect damage, and the dense canopy shades out competition. The problems that do occur cluster in two places: fungal leaf blight in prolonged warm wet weather, and soft rot of the corm, which arrives through wounds or through planting material that was already infected.

That means most of your effort should go into planting sound corms, avoiding damage at hilling and harvest, and keeping air moving through the row. Spraying is rarely the answer, and there is nothing to be done for a corm that is already rotting. The table below is the quick diagnosis; the sections after it deal with each problem in turn.

Taro leaf blight, the one disease that matters

Leaf blight begins as small water-soaked spots that enlarge into brown or purplish patches with yellow margins, often with a fine pale bloom on the underside in humid weather. Spots run together, the leaf turns to a papery brown and collapses, and in a warm wet spell an untreated plant can lose most of its canopy in two or three weeks. Because the corm is filled by those leaves, a bad attack in midsummer directly and permanently reduces the crop.

It spreads by spores in water, which is why it explodes during long warm rainy periods and why splashing overhead irrigation onto the leaves makes it worse. Control is cultural and it is mostly about air and water discipline: full spacing so leaves dry, watering at the base rather than over the canopy, and prompt removal of infected leaves. Copper-based fungicides give some protection if applied before an outbreak takes hold, but in a home garden good spacing plus removing infected material usually keeps the plant productive to harvest.

1. Cut off and remove infected leaves Take the whole stalk down at the base as soon as spots run together. Carry the leaves off the plot rather than dropping them on the row.
2. Never water over the canopy Deliver water at the base of the plant. Overhead watering in warm weather spreads spores from leaf to leaf within hours.
3. Keep the full two feet of spacing Crowded plants stay wet all morning. The gap between plants is what lets leaves dry, and it is the cheapest control there is.
4. Do not compost blighted foliage Bag the leaves and remove them from the garden. A cool home heap does not reliably kill the spores.
5. Clear everything at the end of the season Take all foliage and every leftover corm off the bed after harvest. Overwintering material is where next year's infection comes from.

A plant that has lost half its leaves to blight will still make a crop, just a smaller one. Do not pull it out; remove the infected leaves, keep it watered, and let the remaining canopy finish the corm.

Corm rots, and how they get in

Soft rot turns the base of the plant into a wet, foul-smelling mush and the corm with it. It is a bacterial and fungal problem of wounds, and it arrives in three ways: on a seed corm that was already soft at planting, through spade or hoe damage during hilling, and through soil piled up the leaf stalks so that the crown stays permanently wet against rotting material. Dry rot, which shows as firm brown patches inside a corm that looked sound, more often traces back to the planting material.

There is no cure once a corm is rotting. The response is removal: lift the affected plant with the soil immediately around it, take it off the plot, and do not plant taro in that spot next season. Everything else is prevention, and it works well. Start with firm sound seed corms, hill with loose soil kept off the stalks, cure the crop properly after digging, and sort ruthlessly before anything goes into store.

Aphids, mites and the animals

Aphids cluster on the undersides of young leaves and on the leaf stalks, leaving sticky honeydew and attracting ants. The direct feeding damage is minor on a plant this vigorous, but aphids carry the viruses that cause mosaic and feathery yellow patterning, and a virused plant is a permanent loss whose corms should never be saved for seed. Knock aphid colonies off with a jet of water at the base of the leaf stalks and encourage the ladybirds and hoverflies that follow them.

Spider mites are the opposite problem: they appear when the plant is hot and dry, which for taro means the watering has already fallen short. Fine pale speckling and bronze dry patches are the sign. Correct the water first and hose the undersides of the leaves; the mites usually disappear as the humidity around the plant rises. Voles and rats occasionally tunnel into corms in mulched beds. If you find fresh tunneling in autumn, pull the mulch back, clear grass around the bed, and dig the crop rather than waiting for size.

1. Check leaf undersides fortnightly Turn two or three leaves per row. Aphid colonies and early mite speckling both show there long before the top of the leaf changes.
2. Wash aphids off rather than spraying A firm jet of water into the leaf axils clears most colonies and leaves the predators alive to deal with the rest.
3. Treat mites as a watering fault Mites on taro mean the plant has been dry. Fix the water supply, hose the foliage, and they rarely persist.
4. Remove virused plants entirely Mosaic or feathery yellow patterning does not recover. Take the plant out, and never keep its corms as seed for next season.
5. Pull mulch back if you find rodent runs Mulch is cover as well as moisture. Where voles are tunneling, remove it near the affected plants and dig on the early side.

The hazard to the gardener, and what to record

The injury most people actually suffer from taro is not to the crop. Raw corm, sap, leaves and stalks all carry needle-shaped calcium oxalate crystals that cause hours of burning and itching on skin and painful swelling in the mouth if anything raw is tasted. Wear waterproof gloves and long sleeves whenever you cut leaves, hill, dig or peel, wash with soap and cool water rather than scratching if you get sap on you, and cook every part of the plant until it is completely soft before eating it. Children should not handle raw peeled corm at all.

As for the crop, keep a simple problem log alongside your planting and harvest dates: when blight first appeared and what the weather was doing, which positions in the row were affected, which plants you removed for rot or virus, and any rodent damage. Blight and rot both build in the same ground year after year, so knowing which end of which bed produced them decides where taro goes next season. Mark those spots on a plan; six months later you will not remember.

1. Gloves and sleeves for every job on the plant Cutting leaves, hilling, digging and peeling all bring you into contact with the sap. Waterproof gloves, not fabric.
2. Cook thoroughly, never taste raw Corm, leaves and stalks all need cooking until completely soft. Raw or undercooked taro burns the mouth and throat.
3. Note the date blight first appeared Record the date and the weather that preceded it. After two seasons you will know which spell of weather to prepare for.
4. Mark the positions of removed plants Write down where rot or virus took a plant out. Those spots should not carry taro next year and you will not recall them otherwise.