Petunia Problems: Budworm, Aphids, Gray Mold and Yellow Leaves

Budworm: buds that never open

The classic petunia disaster is a plant in full leaf that stops flowering. Look closely at the buds and you will find small round holes chewed through the side, and the buds either drop or open shredded. The culprit is the tobacco budworm caterpillar, which hatches from eggs laid on the shoot tips by a small pale green moth flying at dusk, and which feeds inside the bud where nothing can reach it.

The caterpillars are half an inch to an inch and a half (12 to 40 mm) long and take their color from what they are eating, so they come green, pink, purple or brown and are genuinely hard to see against a bud. The reliable evidence is not the caterpillar but the frass: dark green or black grains, about the size of coarse pepper, caught on the leaves and in the crown directly below the damaged buds. Where you see frass, there is a caterpillar within a few inches.

Because they feed inside the bud, contact sprays largely miss them. Hand picking works if you go out at dusk with a light and look at the shoot tips, which is when they move to a fresh bud. A biological spray based on Bacillus thuringiensis applied to the shoot tips is eaten as the caterpillar chews in and is the practical option on a large planting, but it has to be repeated every seven to ten days through the flight period because it does not persist. Once damage is heavy, cutting the plant back by a third removes most of the infested buds and the regrowth flowers clean.

Aphids and thrips on the shoot tips

Aphids cluster on the newest growth and the flower stalks, and on petunias they are usually green or black. The plant shows it before you see them: shoot tips curled and puckered, new flowers opening distorted, and a sticky honeydew film on the leaves below that later turns black with sooty mold. On a basket the whole underside of the planting can be tacky before the colony is obvious.

A hard jet of water into the shoot tips, repeated for three days running, knocks down a light infestation without anything else. For a heavier one, insecticidal soap sprayed until it runs off both leaf surfaces works, but it only kills what it wets, so the undersides matter more than the tops. Do not spray in full sun on a hot afternoon; petunia foliage scorches.

Thrips do a different kind of damage: silvery streaking on the petals, flowers that open with brown flecked edges, and growth that is distorted without the puckering that aphids give. Thrips also carry virus, and a petunia showing pale ring patterns or mottled deformed leaves alongside thrips damage is worth pulling out and binning rather than nursing, because there is no treatment and it acts as a reservoir for the rest of the planting.

1. Check the newest growth weekly Turn a shoot tip over and look at the underside. Aphids build up on the youngest tissue first, and a colony found in week one is a five minute job rather than a season long one.
2. Wash before you spray A firm jet of water clears most of a young colony. Reserve insecticidal soap for what survives three consecutive washes.
3. Pull anything with mottled distorted leaves Ring spots and mosaic mottling that spraying does not change is virus. Remove the plant, wash your hands, and do not take cuttings from it.

Gray mold and rot in wet weather

After a wet spell, flowers go brown and slimy, and a fuzzy gray growth appears on the dead petals and on any stem it is touching. This is gray mold, and on petunias it almost always starts in spent flowers that were left lying in the crown of the plant rather than on healthy tissue. From that beachhead it moves into the stem, which then browns and collapses above the infection point.

The fix is airflow and hygiene rather than a spray. Strip out every rotting flower and every dead leaf from inside the plant, thin out the densest congested stems so that air moves through the middle, and water at the base rather than over the top. A basket packed so tightly that the interior never dries out will get gray mold in every wet week of the summer, regardless of what you spray.

Separately, a plant that wilts and does not recover after watering, with a brown soft patch right at soil level, has collar or root rot from standing water. That one is not recoverable. Remove it, tip out the compost rather than reusing it, and check that the container drains freely before replanting.

Record the date gray mold first appears each year and what the weather was doing. It arrives at a predictable point in your own conditions, and that is when the next season's thinning out should be done in advance rather than in response.

Yellow leaves: three different causes

Yellowing is the most commonly asked petunia question and it has at least three separate answers, told apart by which leaves are affected. Yellowing on the oldest lowest leaves, working upward, with the plant otherwise growing, is plain hunger in exhausted compost and is fixed by feeding. Yellowing between the veins on the newest leaves at the tips, with the veins themselves staying green, is iron shortage and is fixed by a chelated iron feed, not by more nitrogen.

Yellowing everywhere at once, with limp leaves that do not perk up overnight, is a root problem: either the compost has been permanently wet, or the root ball has gone so dry that water is running down the outside without wetting it. Lift the plant or push a finger deep into the compost to tell which. These two look identical from above and want opposite treatments, which is why guessing costs so many baskets.

There is also a plain seasonal yellowing of the lowest leaves on a leggy midsummer plant, where the interior is shaded by its own growth. That one is not a deficiency and feeding will not stop it. It is the signal to cut the plant back.