Snapdragon

The dense flower spike is the weak point - florets rot from the middle of the spike outward in damp weather. Cut faded spikes off rather than leaving them standing on the plant.

Browned, collapsed flowers and buds; soft, water-soaked brown patches on leaves, stems or fruit; and a dusty gray-brown fuzz over the dead tissue. Rot jumping between two pieces of fruit or two petals that touch is the signature, and it often shows up again in the fridge or the storage box days later.

Cut out and destroy infected tissue as soon as it appears, and bag it rather than composting it. Harvest ahead of a prolonged wet spell and store the crop dry and cold. Protective fungicides help through long damp weather, but airflow and sanitation do most of the work.

Space and prune for airflow, water at the base rather than over the top, and go easy on nitrogen, which produces the soft growth it prefers. Clear dead leaves, spent flowers and rotting fruit at every pass, since that debris is where it overwinters. Handle fruit, buds and bulbs gently, because it enters through bruises, and cure and store anything you keep dry and well ventilated.

Botrytis cinerea and its relatives - the fuzzy gray-brown mold of cool, wet, humid weather. It blights flowers and buds, rots ripening fruit, and moves into soft, aging, bruised or frost-damaged tissue through the smallest wound, coating whatever it kills in dusty gray spores. It overwinters on dead plant debris, spreads on air currents and by direct contact between touching tissue, and keeps working after harvest, which is why it also ruins fruit and bulbs in storage.

A common secondary snapdragon disease alongside rust. If you are already choosing rust-resistant varieties and watering at the base, you are handling both.

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.