Oakleaf Hydrangea

Humusy, well-drained soil and no waterlogging. Improve drainage in heavy ground rather than trying to water around the problem.

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.

Large leaves and dense growth hold moisture, so leaf spot is common in a humid summer and still cosmetic. Water at the base rather than over the top.

Small brown, dark, reddish or purple-bordered spots and blotches on the leaves during prolonged humid, rainy spells, sometimes enlarging and merging, with premature yellowing or leaf drop on the worst-affected growth.

Remove badly affected leaves and improve airflow; that is normally the whole of it. The plant flushes clean new growth once the weather turns. Fungicide is seldom warranted for a cosmetic leaf spot on an established plant.

Give the plant airflow - space, thin or prune it rather than letting the growth mat together - avoid overhead watering, and water early in the day so the foliage dries. Rake up and remove fallen leaves, which is where these fungi overwinter, and keep the plant unstressed and properly watered.

A catch-all for the various fungal leaf-spot diseases that blotch foliage in warm, humid, wet weather. They spot and yellow the leaves and sometimes bring on early leaf drop, and they are worst on crowded, shaded or overhead-watered plants and on anything already stressed. On a healthy, well-sited plant they are a cosmetic nuisance that comes and goes with the weather rather than a real threat.