Rosemary

Root rot is not one organism but a group of soil fungi and water molds - Pythium, Phytophthora, Fusarium, Rhizoctonia and others - that sit harmlessly in most soils and only turn destructive when the ground stays saturated. Waterlogged soil drives the air out, the roots suffocate, and the weakened tissue is invaded and goes soft, brown, and stringy. Because the damage is underground you usually see it first as something that looks like drought: wilting, yellowing, stunting, a plant that does not respond to being watered. Overwatering, heavy compacted ground, a low spot that collects runoff, a pot with no drainage or a saucer left standing full, and cold wet soil in winter are the usual causes. Once roots have actually rotted there is no cure - this is a problem you prevent rather than treat.

Watch for: Wilting in the middle of the day on a plant whose soil is damp - the classic tell, and the reason people water a rotting plant and finish it off. Leaves yellowing from the bottom up, dull off-color foliage, and growth that stalls for no visible reason. Whole shoots, or one side of the plant, collapsing while the rest still looks fine. Soft, dark, or mushy tissue at the base of the stem or at the crown, sometimes with a sour smell. Lift a failing plant and look at the roots: healthy ones are firm and pale, rotted ones are brown or black and slimy, and the outer layer slips off between your fingers leaving a thread of core behind. Seedlings that topple over at soil level are the same problem, called damping-off.

Prevent: Drainage is the whole defense. Match the plant to the site first - something that wants sharp drainage will not survive heavy clay or a low, water-collecting spot however well you tend it. Where the ground is heavy, plant on a raised bed, berm, or slope, or grow in a container. Set plants at the depth they were already growing, since burying the crown or the root flare is a common cause of rot, and keep mulch pulled back a few inches from the stem base instead of piled against it. Water the soil rather than the calendar: check that the top inch or two is dry before watering again, cut back hard in cool weather and winter dormancy, and never leave a pot standing in a full saucer. Do not plant or dig in soggy ground, and go easy on rich amendments for plants that want it lean. Rotate annual crops, and start with clean healthy plants and fresh mix.

Treat: There is no cure for tissue that has already rotted, so act on the conditions rather than looking for a spray. Stop watering at once and fix the drainage - open up compacted soil, or move the plant to a raised, sharply drained position or a container. Lift a badly affected plant, cut away every soft dark root and rotted section with a clean blade back to firm pale tissue, and replant only the healthy parts into fresh free-draining soil or mix. Often the better move is to give up on the plant and save its offspring instead: cuttings, offsets, or divisions taken from firm healthy growth root readily and give you a clean start. Bin the rotted material rather than composting it, and correct the drainage before you replant, because the rot comes straight back in the same wet spot.

Wet feet kill far more rosemary than cold does. Gritty lean soil, a raised bed or a well-draining pot, and water only to establish and then only in real drought. Take cuttings from healthy growth before you lose the plant.

Powdery mildew is a fungal disease that coats leaves - and often stems, buds, and fruit - in a white-to-gray powder that looks like spilled talcum. It is not one fungus but a large family of them, each fairly specialised to its own group of plants, which is why mildew on one crop rarely jumps to an unrelated one beside it. Unlike most leaf diseases it does not need wet leaves: it thrives on warm dry days followed by humid nights, and it takes hold fastest where plants stand crowded in still air or in shade. The coating blocks light, so heavily covered leaves yellow, brown, and drop early. On many plants that is mostly disfiguring and arrives late enough not to matter; where it strips away the leaves a crop still needed to finish, it costs you size, sweetness, and quality.

Watch for: Round white powdery spots on the upper leaf surfaces that spread and merge until whole leaves look dusted. It usually starts on the oldest, lowest, most shaded leaves and works upward, so check there first. Look at the undersides too, and at stems, buds, and flower stalks, which get coated as it advances. Leaves that yellow, brown, curl, or go crisp and drop early under the coating. New growth that opens small, crumpled, or twisted where the fungus has reached the shoot tips. On plants where it gets onto the fruit, a dull gray film, russeted or scarred skin, and cracking. Late in the season, tiny dark specks scattered through the white felt - that is the fungus setting up to overwinter on the debris.

Prevent: Give plants room. Crowding and still air are the biggest single driver, so space at the full recommended distance, thin seedlings properly, and divide or prune out congested growth until air moves through the plant. Full sun and an open position help; deep shade and a still corner against a wall are where it always starts. Water at the base rather than overhead, and water early in the day so nothing stays damp overnight. Keep plants evenly watered - drought stress at the roots leaves them far more susceptible, even though the fungus itself likes dry air. Go easy on nitrogen, since soft lush growth is the most vulnerable. Where resistant varieties exist for what you are growing, choosing one is the single most effective thing you can do on a plant that mildews every year. Clear infected leaves and stems away at the end of the season rather than composting them.

Treat: Start early: sprays protect healthy tissue, they do not clean up leaves that are already coated. Pick off and bin the worst leaves as soon as you see them, and open the plant up by thinning or pruning so air moves through. On anything that regrows quickly, shearing the whole plant back forces a clean flush and beats spraying. If it keeps spreading, potassium bicarbonate, horticultural oil, or neem applied weekly will hold it - cover both leaf surfaces. Sulfur also works, but it burns the foliage of some plants, should never go on in heat, and must not be mixed with or applied close behind an oil spray. Whatever you use must be labeled for the plant you are treating: follow that label and observe its pre-harvest interval on anything you eat. Do not compost badly infected material. Late-season mildew on a plant that is finishing anyway is usually cosmetic; on a crop still filling out it is worth acting on.

Common on indoor and greenhouse rosemary, where still air and a humid room are the trigger. Do not over-humidify potted plants, and give them some air movement.

Spider mites are not insects but tiny sap-feeding arachnids, smaller than a grain of salt, and you almost always notice the damage before you notice them. They live on the undersides of leaves and drain the cells one at a time, which is why the leaf goes finely stippled with pale dots before it bronzes and dries. They breed explosively in hot, dry, dusty, still air - a generation can pass in a week at high temperature - so a barely visible population becomes a webbed, bronzed plant within a fortnight. Drought-stressed plants and plants wintered indoors in dry heated air are the classic hosts. Broad-spectrum insecticides make matters worse rather than better, because they kill the predatory mites and lady beetles that normally hold mites in check.

Watch for: Fine pale stippling or speckling across the upper leaf surface, as though the color had been sandblasted out of it, and later a dull bronze or rusty cast over the whole leaf. Delicate webbing across leaf joints, shoot tips, and undersides - that comes with a heavy infestation and means you caught it late. Leaves that dry, curl, and drop early while the plant looks thirsty no matter how much you water it. Hold a leaf over a sheet of white paper and tap it: the specks that fall and then start walking are mites. With a hand lens, look on the undersides for the mites themselves, pale or reddish, and their round translucent eggs. Damage that shows up first on the hottest, dustiest, most sheltered side of a plant, or on a pot sitting near a radiator indoors.

Prevent: Do not let plants get drought-stressed - that is the single strongest trigger, and a plant watered steadily through heat is far less likely to have a mite problem at all. Hose the foliage down, undersides included, during hot dry spells: it knocks mites off, washes away the dust they thrive in, and raises the humidity around the plant for a while. Indoors and under cover, keep the air from going hot, dry, and stagnant, wipe or rinse the leaves occasionally, and keep pots away from radiators and heating vents. Inspect any plant carefully before you bring it indoors for winter or set it among others, since mites almost always arrive on something else. Avoid broad-spectrum insecticides: they wipe out the predatory mites, lacewings, and lady beetles that do most of this work for you, and reliably leave you with a worse outbreak than you started with.

Treat: Start with water. A strong jet aimed at the leaf undersides, repeated every few days for a couple of weeks, knocks populations down and breaks the cycle - on many plants that alone ends it. On anything that regrows quickly, shearing back badly infested growth resets the plant to clean foliage. If it keeps building, spray insecticidal soap or horticultural oil directly onto the undersides where the mites actually live, and repeat every five to seven days for three rounds: contact sprays do not kill the eggs, so a single application always fails. Anything you spray must be labeled for the plant you are treating - follow the label and observe its pre-harvest interval on anything you eat. Predatory mites are an effective release in a greenhouse or on an indoor plant. Above all, correct the heat, dust, and drought stress that caused it, or the mites simply come back.

A common reason indoor rosemary declines over winter. Keep an indoor plant cool and humid enough rather than warm and dry, and rinse the foliage periodically.

Aphids are small soft-bodied sap-suckers - green, black, gray, pink, or waxy white, and mostly wingless - that gather in dense colonies on the tenderest new growth: shoot tips, buds, and the undersides of young leaves. They breed extremely fast in mild weather, so a few can become hundreds inside a week. Their feeding curls and distorts soft growth, and the sticky honeydew they excrete coats everything below and turns black with sooty mold. Ants often farm and defend the colonies for that honeydew. A few aphids are normal on almost any plant and a healthy plant shrugs them off; only dense, persistent colonies do real damage, and some aphids also spread plant viruses as they feed.

Watch for: Dense clusters of tiny insects packed onto shoot tips, flower buds, and the undersides of young leaves. Curled, cupped, or distorted new growth that never opens properly. A sticky shine on the leaves - and on whatever sits under the plant - that later goes black with sooty mold. Tiny white cast skins left behind where a colony has been feeding. Ants running steadily up and down the stems, which is often the first thing you notice. Mottled, streaked, or misshapen leaves that do not grow out can mean a virus the aphids brought with them rather than the feeding itself.

Prevent: Go easy on nitrogen: the soft, lush growth that heavy feeding produces is exactly what aphids breed on. Check new growth once a week through spring and early summer so you catch colonies while they are still small, and rub the first few out with your fingers. Leave flowering plants nearby and skip broad-spectrum sprays - ladybugs, lacewings, hoverflies, and parasitic wasps do most of this work for free, and they only stay if there is something for them to eat. Keep ants off the plant where you can, since they protect aphids from those predators. Water steadily so plants are not stressed, and check any new plant over before you bring it home or move it indoors, since aphids usually arrive on something else.

Treat: Knock the colonies off with a hard jet of water and repeat every few days - on most plants that alone ends it. Pinch out or prune off the worst-infested tips rather than treating the whole plant. If colonies keep rebuilding, spray insecticidal soap or horticultural oil directly onto the insects, covering leaf undersides, and repeat weekly until they are gone; treat early or late in the day, and never spray a wilted or heat-stressed plant. Avoid broad-spectrum insecticides - they kill the ladybugs, lacewings, and parasitic wasps that were about to solve the problem for you, and aphid numbers usually come back worse afterward. Anything you spray must be labeled for the plant you are treating; follow that label and observe its pre-harvest interval. On anything you eat, wash leaves, heads, and buds well before use, since aphids hide down in crowns, folds, and tight growth.

Tiny white sap-sucking insects that cluster on leaf undersides and lift off in a cloud when the foliage is brushed. They weaken growth, and the honeydew they excrete coats the leaves below in a sticky film that blackens with sooty mold. Some species also transmit plant viruses. They breed fast in warm, still, sheltered air, which is why they are far more trouble under cover and on plants overwintered indoors than on the same plant outside.

Watch for: Clouds of tiny white insects rising when you shake the plant, flat scale-like nymphs stuck to the leaf undersides, a sticky shine on the leaves below them, black sooty mold over that, and yellowing, weakened growth.

Prevent: Inspect the undersides of any new plant before it joins the others, and quarantine anything you are bringing under cover or indoors for winter. Avoid crowding and heavy nitrogen feeding, which produce the soft growth they prefer. Hang yellow sticky traps early to catch the build-up while it is still small, and encourage lacewings, ladybugs and parasitic wasps outdoors.

Treat: Hose off light infestations, then spray insecticidal soap or horticultural oil onto the leaf undersides where they actually sit. Sprays do not kill the eggs, so repeat every few days for two or three weeks or the population simply rebuilds. Yellow sticky traps knock back the adults, and outdoors natural enemies usually finish the job.

Mostly a greenhouse and indoor problem on rosemary - it is the plants brought in for winter that suffer, not the ones left outside.