Serviceberry

Uncommon on serviceberry but it is a rose-family host, and this is the one disease here that kills wood rather than just spotting leaves - so the shepherd's-crook shoot tip is worth recognizing. Never prune it in wet weather with unsterilised tools.

Sudden wilting and blackening of blossoms and shoot tips that look burned rather than diseased, shoots bent over into a hook, dark sunken cankers on limbs, and amber bacterial ooze on the bark in warm humid weather.

Cut strikes out at least 8 to 12 inches below the visible damage, sterilizing the saw or pruners between every single cut, and remove and destroy the prunings rather than leaving them lying. Cut cankers out in dry dormant weather. Copper or biological sprays at bloom help in high-pressure areas - use only a product labeled for the crop you are treating, follow the label exactly, and observe the pre-harvest interval before you pick.

Where a resistant option exists for your crop, variety choice matters more than any spray - check before you buy. Avoid heavy nitrogen and hard pruning, both of which force the soft succulent growth the bacteria enter through, and never prune or work the plant in wet weather. Keep susceptible hosts spaced apart rather than planted hard against each other, and do not prune two of them in the same wet spell.

A fast, destructive bacterial disease (Erwinia amylovora) of the rose family. It enters through open blossoms and soft shoot tips in warm, humid weather, then runs back down the shoot, blackening leaves and wood as though scorched and bending the tip into a telltale 'shepherd's crook'. From there it moves into limbs as dark sunken cankers, and it can kill a young tree in a single season. Apple, pear, quince, loquat, hawthorn, serviceberry and toyon are all hosts, and susceptibility varies enormously between species and between varieties.

Serviceberry also gets minor rose-family leaf spots in the same humid, crowded or shady conditions, and both are more common in the muggy South. Usually cosmetic on a healthy tree: site it in sun with airflow, rake fallen leaves, and choose resistant cultivars.

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.

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.

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.

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.