Salal

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.

Wants acidic, humus-rich, well-drained ground and will not take a wet low spot. Raised or bermed beds help on heavy soil, and it is hard to reverse once established.

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.

Salal also picks up minor dark fungal leaf spots on older foliage in the same dense, stagnant conditions. Both are more disfiguring than damaging: renew a leggy patch with a hard late-winter cut-back and skip spraying on a landscape planting.

Root weevils - black vine weevil and its relatives - do their obvious damage as adults and their serious damage underground. The adults are flightless, dull black or grey, about a quarter to a third of an inch long with a short snout; they feed only at night and hide in mulch and leaf litter by day, so most gardeners see notched leaves for weeks before ever seeing the insect. The larvae are small white legless grubs with tan heads that live in the soil, eat fine roots, and then strip the bark from the root crown itself, which is what actually kills plants. Males barely matter - the females reproduce without them, so one insect can start an infestation. Container plants and newly set shrubs suffer most, because a small root system has nothing to spare.

Watch for: Neat crescent-shaped notches chewed into leaf margins, appearing overnight and building up through summer - unmistakable once you have seen it. Damage with no insect in sight during daylight; go out after dark with a flashlight and you will find the adults feeding. Wilting, dull foliage, or an unexplained slow decline in a plant whose leaves are also notched, which is the combination that points at larvae on the roots. Container plants that loosen in their mix or lift out with almost no fine roots left. Chewed, girdled bark at the root crown just below soil level on a young shrub that has collapsed.

Prevent: Check new plants before you buy, since weevils travel in nursery containers and that is how most gardens acquire them. Keep deep loose mulch and leaf litter pulled back from the base of vulnerable shrubs, because that is where the adults shelter through the day. Keep plants vigorous and steadily watered so root feeding is easier to shrug off. A sticky barrier band around the base of a container, or a smooth-sided pot, stops the flightless adults climbing back up. Encourage ground beetles, birds, and shrews by leaving some rough ground, and avoid broad-spectrum insecticides, which do nothing about the larvae underground and remove the predators that eat the adults.

Treat: The notching itself is cosmetic and needs no treatment on an established plant, and spraying for the adults is largely wasted effort since they feed at night and hide by day. The larvae are the target. Drench beneficial nematodes into moist soil in late summer or early autumn while the grubs are small and the soil is still warm - that is the single most effective home control and it works especially well in containers. Repeat the following year if damage continues. Hand-collect adults after dark from small collections of plants. Repot badly affected container plants into fresh mix, washing the roots and removing every grub you find, and cut your losses on a shrub whose root crown has already been girdled.