Butterfly Bush Has Brown Leaf Spots: Fixes For Buddleia Leaves With Spots

Purple Butterfly Bush
(Image credit: Ken Griffiths)

The wild beauty and sweetly scented flowers of the butterfly bush (Buddleia davidii) makes it an irreplaceable member of the landscape. These tough bushes grow quickly; attract pollinators, like butterflies; and resist disease like champs. They're excellent choices for low maintenance landscapes, but even these low care plants may run into an occasional problem. Brown spots on butterfly bush leaves are a common symptom in Buddleia when issues do occur. Buddleia leaf spot isn't anything to worry about, though, as long as you figure out what's causing it and manage it promptly.

Butterfly Bush Has Brown Leaf Spots

Growers are often alarmed when leaf spots appear suddenly and spread across leaf surfaces. Buddleia leaves with spots can be caused by a few different problems, including fungal disease and sap-sucking pests. A butterfly bush with spotted leaves should be checked carefully from top to bottom before treatment, just to be sure you've fingered the right culprit. Fungal leaf spots and downy mildew cause many widespread spots, ranging in color from yellow to tan and even black or brown. These spots may be circular or irregular, but they usually develop fruiting bodies shortly after they make their appearance. Fungal diseases need humid conditions to take hold. Pests, like the four-lined plant bug and spider mites may cause brown spots where they've been feeding on the undersides of leaves. Four-lined plant bugs are elusive, but can be observed feeding on leaf tissues if you look carefully. These black bugs bear yellow-green stripes running from their fronts to their backs as adults, or appear red with small black markings as juveniles. Spider mites are so tiny that you may only notice small moving dots and fine silk where damage has occurred. They typically cause a damage pattern known as stippling, where many small tan to brown dots appear on plant leaf surfaces. These spots will grow together as the colony expands.

Treating Buddleia Leaf Spot

If the leaf spots in question are few and aren't spreading aggressively, treatment is not recommended, since many beneficial insects use butterfly bush as a food source. Simply pick off the damaged leaves and discard them away from the plant. The aggressive growth of Buddleia will quickly replace those missing leaves. Fungal diseases like leaf spots and downy mildew are encouraged by high humidity, so opening up the canopy by thinning the inside and pruning the bush away from structures may help destroy the fungus. If it's spreading rapidly, or pruning doesn't seem to be helping, spraying both upper and lower leaf surfaces with neem oil every seven to 10 days will destroy fungal leaf diseases in no time. Plant bugs can be hand-picked off of the plant and crushed or dropped into a bucket of soapy water if their numbers are large. Usually, this isn't necessary, since these bugs appear for only a short period and rarely in large number. Spider mites, on the other hand, should be treated with neem oil or insecticidal soap weekly until new damage stops; they're difficult to see, so you'll have to rely on your plant's health to know when these pests are gone for good.

Kristi Waterworth

Kristi Waterworth was a regular contributor to Gardening Know How for many years, answering countless queries on plant pests and diseases.