Lilac

Lilacs want sharply drained ground and will not tolerate a low spot that puddles, a downspout outfall, or heavy clay that stays sodden through winter. The decline is slow - a few thin years, then dieback and death - so it is rarely recognised for what it is. Plant high, on a berm or raised bed in heavy soil, and never plant a replacement lilac in the same wet hole.

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.

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.

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.

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.

On a mature shrub it is a lost season, not a lost plant.

Young shoots that turn brown to black from the tip down, often collapsing at a soft, water-soaked spot partway along the stem. Flower buds and open trusses that brown and shrivel without opening properly, which is where the older name 'blossom blast' comes from. Angular brown spots on young leaves, sometimes with a yellow halo. Damage appearing suddenly after a cold, wet, windy spell in spring, and worst on the youngest, softest growth.

There is no cure for infected tissue, so cut it out. Prune affected shoots back well below the visible damage, into clean healthy wood, and do it in dry weather; disinfect the blades between every cut with rubbing alcohol or a household disinfectant, because the pruners themselves spread it. Destroy the prunings. Copper sprays applied as buds break can reduce infection where the disease recurs every year, but they protect healthy tissue only and will not rescue blackened shoots. Most established shrubs regrow and bloom normally the following spring.

Do not force soft growth - skip high-nitrogen feeding, which is the single biggest risk factor. Prune for air movement so foliage dries quickly, avoid overhead watering, and space plants properly. Make all pruning cuts in dry weather. Rake up and remove fallen infected leaves and prunings rather than composting them, since the bacterium overwinters in debris and in infected wood. In areas where cold wet springs are the norm, favour cultivars with a reputation for resistance and avoid siting a lilac in a frost pocket.

A bacterial disease (Pseudomonas syringae pv. syringae) that strikes in cool, wet springs, entering through frost cracks and pruning wounds and blackening young shoots and flower trusses from the tip down. Damage looks alarming and is rarely fatal on an established shrub, but a bad spring can take out most of a season's bloom. Soft, sappy growth forced by heavy nitrogen feeding is the most susceptible, and white-flowered cultivars are generally hit harder than purple ones. The same bacterium causes blight on many woody plants, so a nearby infected tree can be the source.

One whole cane wilting, yellowing, and dying back in summer while the rest of the shrub looks perfectly healthy - the classic sign. Small round exit holes about a quarter inch across on the lower two or three feet of older trunks. Coarse sawdust-like frass packed in bark crevices or piled at the base of a stem. Swollen, cracked, or roughened bark over a tunnel, and stems that snap off in wind at a weakened point. Papery brown pupal cases left sticking out of exit holes in spring.

Cut infested stems out at ground level and burn or bin them - do not compost or stack them. There is no way to reach a grub already inside the wood with a spray, and systemic treatments perform poorly against this insect. Where the borer recurs annually, a bark spray applied to the lower trunks in spring at the start of adult flight, timed by a pheromone trap and labelled for this use, protects the wood from egg-laying; that is a professional-grade approach and rarely necessary in a garden. For most gardeners the answer is simply to keep renewing the old wood.

Renewal pruning is the real control: cutting a third of the oldest stems to the ground each June keeps the shrub young and removes the thick old wood the borer needs. Keep mowers and string trimmers away from the base, since wounds are where eggs are laid, and keep mulch pulled back off the stems. Do not leave pruning stubs. Keep the shrub unstressed with adequate water in drought, and remove and destroy infested stems promptly rather than stacking them nearby, because the adults emerge from cut wood.

The larva of a clearwing moth (Podosesia syringae) that looks convincingly like a wasp in flight. Adults emerge in spring and lay eggs on the bark of older stems, particularly near wounds and mower scars; the grubs tunnel into the wood and feed under the bark for a year, girdling stems from the inside. Stems more than about an inch thick are the targets, so this is a disease of neglected, overgrown lilacs full of old trunks and of shrubs damaged at the base. The same insect attacks ash and privet, which are close relatives.

It is armoured rather than soft, so there is no honeydew and no sooty mold to warn you. Crawlers hatch in late May to June - that is the window when oil actually works - and a dormant oil in late winter plus renewal pruning of the crustiest old stems does most of the job.

Rows of small brown, tan, black, white, or waxy bumps along stems, twigs, leaf midribs, and leaf undersides that do not brush off but can be scraped loose with a fingernail. A sticky shine on the leaves - and on the floor, table, or path beneath the plant - that later turns black with sooty mold. Ants running steadily up and down the stems, since they farm and defend scale for that honeydew; often the first sign anything is wrong. Yellowing leaves, weak thin growth, and dieback of individual branches or canes that are heavily encrusted. On fruit crops, scale covers and sooty mold directly on the fruit. Check the leaf undersides and the shaded inner wood, which is where colonies build up unseen.

Horticultural oil is the main control: it works by smothering, so it has to coat the insects thoroughly - stems, twigs, and leaf undersides - and it usually needs repeating, because the waxy covering shields them and no single pass reaches everything. On deciduous woody plants a dormant-season oil smothers the overwintering stage while the leaves are out of the way, and timing a growing-season spray to the crawler stage, when the young are mobile and uncovered, is far more effective than spraying settled adults. On a small plant or a light infestation, scrub or wipe the scales off by hand, or dab them with a cotton swab of rubbing alcohol. Prune out and destroy badly encrusted wood rather than trying to save it. Wash off the sooty mold once the scale is gone - it lives on the honeydew, not on the plant, and stops once the supply does. Anything you spray must be labeled for the plant, and oils can burn delicate foliage, so test first.

Inspect. Look over leaf undersides and stems regularly - on a crop you pick, every harvest is a scouting chance - and check every new plant thoroughly before you buy it, plant it, or set it beside anything else, because scale almost always arrives on a plant rather than flying in. Quarantine new houseplants for a few weeks. Keep plants unstressed and not over-fertilized, since soft, forced growth is what populations build on. Prune for airflow and light to open up the shaded inner growth where scale hides, and prune out heavily infested wood as routine maintenance - on anything that gets a hard annual pruning, that alone removes most of any buildup. Leave the lady beetles, lacewings, and parasitic wasps alone, since they normally keep scale down to nothing, and keep ants off the plant where you can, because ants protect the colonies from those predators. Indoors, avoid hot, dry, still winter air.

Scale insects are sap-suckers that spend most of their lives immobile, fixed to a stem, twig, or leaf underside beneath a waxy or shell-like cover. They look far more like bumps, blisters, or crusty encrustations than like insects, which is why an infestation is usually well established before anyone notices it. Soft scales excrete large amounts of sticky honeydew that drips onto everything below and grows a black sooty mold - and those blackened leaves are often the first thing a gardener actually sees. Armored scales produce no honeydew but sit under a hard shield that protects them from most sprays. They spread as tiny mobile crawlers in the weeks after hatching, which is the only stage that moves and the only stage that is easy to kill. Populations build on stressed plants, on crowded shaded growth, and above all on plants kept indoors in dry, still air over winter.