Gadenin日本語Get the appRoot rot and stem rot are the main killers
Far more stevia plants are lost to waterlogging than to any insect. The roots are shallow and fine, and they suffocate quickly in compost that stays saturated. The pattern is recognizable: a plant that wilts even though the soil is wet, leaves that go dull and limp from the bottom up, and a stem that is soft and brown where it meets the soil. Pull it up and the roots are dark and slimy rather than white and firm.
By the time the stem has gone soft the plant is finished, so the answer is prevention. Plant into free-draining soil with compost worked in, use a container with real drainage and never leave it standing in a saucer of water, and water thoroughly but let the surface dry slightly between waterings. Raised beds and pots suit stevia far better than heavy clay.
If one plant collapses at the base, do not replant stevia in exactly that spot the same season. Move the replacement a foot (30 cm) away or use fresh compost in a container.
Aphids and whitefly on the soft tips
Regular pinching keeps stevia in soft, actively growing tip growth all season, which is exactly what aphids want. They cluster on the newest shoots and the undersides of the top leaves, and they leave sticky honeydew that goes on to grow black sooty mold. Whitefly do the same under cover and in a greenhouse, rising in a cloud when you brush the plant.
Because you eat the leaf, and because the leaf is often eaten raw or dried whole, do not reach for anything you would not want to consume. A firm jet of water on the undersides of the leaves knocks most aphid colonies off, and repeating it every few days for a week usually ends the problem. For a heavier attack, a soft soap spray applied in the evening and rinsed off the next morning works well.
| Problem | What you see | First response |
|---|---|---|
| Aphids | Green or black clusters on shoot tips, sticky leaves | Water jet on leaf undersides, repeated every two or three days |
| Whitefly | Tiny white flies rising when disturbed, scale-like nymphs beneath leaves | Yellow sticky traps plus soft soap; improve airflow under cover |
| Spider mite | Fine pale stippling, faint webbing, worst indoors in dry heat | Raise humidity, wash the foliage, isolate the plant |
| Slugs and snails | Ragged holes on low leaves, silvery trails, seedlings gone overnight | Clear hiding places, barrier around young plants, check after dark |
| Leaf spot | Dark spots with pale centers, spreading upward in wet weather | Remove affected leaves, thin the plant for airflow, water at the base |
| Root or stem rot | Wilting in wet soil, soft brown stem base | Usually fatal; improve drainage and take cuttings from clean top growth |
Leaf spot and the humidity problem
In a wet season, or on a plant grown too densely, you may see dark leaf spots with lighter centers appearing first on the lower leaves and working upward. It spreads by splash, so overhead watering and crowded growth make it much worse. Strip off affected leaves as you see them, thin the interior of the plant so air moves through it, and water at the soil surface rather than over the foliage.
The same conditions cause trouble at the drying stage. Leaf harvested in humid weather and then dried slowly can go moldy on the rack, and a moldy batch is not salvageable. If your harvest coincides with a damp spell, use a dehydrator or the lowest oven setting rather than air drying.
Cold damage is not a disease
Stevia is tender, and a lot of what looks like disease in autumn is simply cold. Below about 50F (10C) growth stops; a light frost blackens the leaves and a hard frost kills the top growth outright. Leaves that go dark and papery overnight after a cold snap have not been attacked by anything, and there is nothing to spray for it.
The practical response is to time the main harvest before the first frost rather than trying to protect the plant through it. If you want to carry a plant over, lift it or take cuttings well before the temperature drops, rather than waiting to see how it copes.
- Check the top growth first Aphids, whitefly and mite damage all show on the newest, softest growth. If the top of the plant is clean and the problem is at the bottom, look at water, drainage and airflow instead of pests.
- Feel the stem base Squeeze the stem just above soil level between finger and thumb. Firm means the roots are probably fine. Soft or spongy means rot, and the plant will not recover.
- Turn a few leaves over Most of what attacks stevia lives on the underside. A thirty second check of leaf undersides once a week catches nearly everything early enough to deal with by hand.
Keep a note of first sightings
Write down the date you first see aphids each year, the date leaf spot first appears, and which plants were affected. Pests arrive at close to the same time each season in a given garden, and knowing that aphids turn up shortly after the weather warms lets you start checking a week before rather than a fortnight after.
Note where the affected plants were standing as well. Recurring rot in one corner of a bed is a drainage problem you can fix over winter, and recurring whitefly in one greenhouse position is usually a ventilation problem. A note that says which plant, where and when turns a nuisance into something you can design out next season.