Petunia

The classic petunia pest: caterpillars of the tobacco budworm moth (Chloridea/Heliothis virescens) that bore into unopened flower buds and eat the developing petals, leaving hollowed, holed buds that never open properly. Also attacks geraniums and nicotiana.

Watch for: Round holes chewed in buds and petals, buds that fail to open or open shredded, dark pinhead frass (droppings) around the buds, small green-to-brown striped caterpillars feeding at dusk.

Prevent: Inspect buds by flashlight after dark when the larvae feed, remove and destroy affected buds, and clean up debris where pupae overwinter; the caterpillars often match the flower color.

Treat: Act at the FIRST sign of small caterpillars or fresh chewing on bud tips, not once buds are hollowed. Btk (Bacillus thuringiensis var. kurstaki) is a stomach poison that only works on small larvae eating treated surfaces - a budworm already inside a bud will never touch it, and Btk is weak on this species even when well timed. Spinosad is the more reliable choice here: apply it only at dusk and let it dry overnight, because it is highly toxic to bees while wet and petunia flowers are actively foraged by bees and hummingbirds. Whatever you spray, pick off and destroy every already-holed bud - no spray reaches the larva inside.

Tobacco budworm (a.k.a. geranium budworm) is the single most damaging and characteristic petunia pest - a plant that keeps forming buds that never open is almost always budworm.

Aphids are small soft-bodied sap-suckers - green, black, gray, pink, or waxy white, and mostly wingless - that gather in dense colonies on the tenderest new growth: shoot tips, buds, and the undersides of young leaves. They breed extremely fast in mild weather, so a few can become hundreds inside a week. Their feeding curls and distorts soft growth, and the sticky honeydew they excrete coats everything below and turns black with sooty mold. Ants often farm and defend the colonies for that honeydew. A few aphids are normal on almost any plant and a healthy plant shrugs them off; only dense, persistent colonies do real damage, and some aphids also spread plant viruses as they feed.

Watch for: Dense clusters of tiny insects packed onto shoot tips, flower buds, and the undersides of young leaves. Curled, cupped, or distorted new growth that never opens properly. A sticky shine on the leaves - and on whatever sits under the plant - that later goes black with sooty mold. Tiny white cast skins left behind where a colony has been feeding. Ants running steadily up and down the stems, which is often the first thing you notice. Mottled, streaked, or misshapen leaves that do not grow out can mean a virus the aphids brought with them rather than the feeding itself.

Prevent: Go easy on nitrogen: the soft, lush growth that heavy feeding produces is exactly what aphids breed on. Check new growth once a week through spring and early summer so you catch colonies while they are still small, and rub the first few out with your fingers. Leave flowering plants nearby and skip broad-spectrum sprays - ladybugs, lacewings, hoverflies, and parasitic wasps do most of this work for free, and they only stay if there is something for them to eat. Keep ants off the plant where you can, since they protect aphids from those predators. Water steadily so plants are not stressed, and check any new plant over before you bring it home or move it indoors, since aphids usually arrive on something else.

Treat: Knock the colonies off with a hard jet of water and repeat every few days - on most plants that alone ends it. Pinch out or prune off the worst-infested tips rather than treating the whole plant. If colonies keep rebuilding, spray insecticidal soap or horticultural oil directly onto the insects, covering leaf undersides, and repeat weekly until they are gone; treat early or late in the day, and never spray a wilted or heat-stressed plant. Avoid broad-spectrum insecticides - they kill the ladybugs, lacewings, and parasitic wasps that were about to solve the problem for you, and aphid numbers usually come back worse afterward. Anything you spray must be labeled for the plant you are treating; follow that label and observe its pre-harvest interval. On anything you eat, wash leaves, heads, and buds well before use, since aphids hide down in crowns, folds, and tight growth.

Petunias are worth watching mainly because aphids transmit viruses between bedding plants; the feeding itself is rarely serious.

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.

Watch for: 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.

Prevent: 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.

Treat: 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.

Petunia blooms, especially the large grandiflora types, collapse into gray mush after rain. Deadhead the battered flowers, and shear a rain-wrecked plant back to refresh it.

Tiny white sap-sucking insects that cluster on leaf undersides and lift off in a cloud when the foliage is brushed. They weaken growth, and the honeydew they excrete coats the leaves below in a sticky film that blackens with sooty mold. Some species also transmit plant viruses. They breed fast in warm, still, sheltered air, which is why they are far more trouble under cover and on plants overwintered indoors than on the same plant outside.

Watch for: Clouds of tiny white insects rising when you shake the plant, flat scale-like nymphs stuck to the leaf undersides, a sticky shine on the leaves below them, black sooty mold over that, and yellowing, weakened growth.

Prevent: Inspect the undersides of any new plant before it joins the others, and quarantine anything you are bringing under cover or indoors for winter. Avoid crowding and heavy nitrogen feeding, which produce the soft growth they prefer. Hang yellow sticky traps early to catch the build-up while it is still small, and encourage lacewings, ladybugs and parasitic wasps outdoors.

Treat: Hose off light infestations, then spray insecticidal soap or horticultural oil onto the leaf undersides where they actually sit. Sprays do not kill the eggs, so repeat every few days for two or three weeks or the population simply rebuilds. Yellow sticky traps knock back the adults, and outdoors natural enemies usually finish the job.

Common on petunias in warm weather and under glass. The dense growth of a well-fed hanging basket suits them perfectly, so check the leaf undersides while you deadhead.