Why Are My Lilac Leaves Turning Yellow?

What happens when your usually dependable lilac shrub (Syringa spp.) starts developing yellow leaves? Is it time to panic? The bad news is that a number of factors might cause yellowing of leaves, calling for some detective work. The good news is that many diseases, pests, or nutrient deficiencies provide specific visual clues, along with the general presence of yellow. Start by looking for what signs might be accompanying the yellowing leaves. Is the problem all on one section of the lilac shrub? Alternatively, are the affected leaves more on the inner leaves, or the ones on the outer edges? Can you spot any pests on the branches or leaves? Do the yellow leaves have brown margins, or perhaps yellow tissue but green veins? Is flowering affected?

Verticillium wilt is a common plant disease in which a whole section of the lilac may suddenly turn yellow and die back. Peeling back an affected branch will typically show streaking. If parts of your lilac are looking "twiggy," with leaves that are both small and yellow, witch's broom disease could be the culprit. Sometimes branches develop tufts, or "brooms" at their ends. The yellow leaves will be widespread, often with brown edges. Unfortunately, diseases such as witch's broom (also known as "ash yellows") and verticillium wilt are often fatal to the shrubs and trees they affect. In both cases, however, taking special care of your lilac's watering and feeding needs can extend its life for several seasons before it succumbs.

Certainly, before removing a lilac, it's better to call in a professional who can confirm the diagnosis and may be able to help with specialized pruning and other treatment. If you need to replace your lilac, look for types that are resistant to these diseases. Common lilac, for example, is more resistant to witch's broom than more exotic lilac species and hybrids.

Along with plant diseases, other problems can result in yellow leaves on lilacs. An infestation of the pest known as oystershell scale, for example, pairs yellowing and dropped leaves with the presence of the "oyster shaped" scales on branches. If oyster scale is suspected, watching and waiting for beneficial insects to devour the damaging scale pests may be the least intrusive course. Otherwise, use an insecticidal soap or horticultural oil.

Nutrient deficiencies in the soil surrounding the lilac can also show themselves as yellowing leaves. If the leaves turn yellow, but the veins are still green, iron chlorosis may be an issue. Sometimes iron deficiency will also produce brown spots or edges on leaves, and perhaps the lilac isn't blooming like it used to. It's also possible that other soil deficiencies may be causing the chlorosis, such as nitrogen, manganese, or zinc. These three deficiencies tend to show as yellow at the inner, older leaves first, while lack of iron shows as yellowing at the outer, newer leaves initially, before moving inward.

For suspected nutrient deficiencies, a detailed soil test is a crucial first step. Not only can it pinpoint which nutrients are low in the soil near your lilac bush, but also whether the pH level is "locking up" these nutrients. In general, lilacs flourish best with a pH in the neutral to alkaline range, or about 7.0 to 7.8. If the soil isn't in that range, add the recommended amendments at the indicated rates. The soil test will also tell you which nutrient is low in the soil. As an immediate treatment, consider foliar-feeding the lilac the missing nutrients. Try not to panic and over-fertilize, which is a common mistake people make when growing lilacs.