Common sage Salvia officinalis is a widely grown culinary herb. Sage foliage varies greatly. The leaves may turn yellow for a variety of reasons, not all of which are cause for concern. However, yellow leaves should be treated as warning signs in some cases.
Plant common sage, also known as garden sage and kitchen sage, in a site with full sunlight and soil that drains well. Hundreds of different sage plants are grown in cultivation, and many of these naturally grow yellowish leaves. Salvia officinalis aurea "Golden Sage" is named for its colorful yellow foliage. When sage plants are properly cultivated, leaves will not turn yellow unnaturally.
Chlorosis is a condition caused by lack of chlorophyll in leaves. When the plant's roots have trouble accessing the moisture and nutrients it needs, reduced chlorophyll production results and leaves turn yellow. Chlorosis occurs in garden areas with poor drainage, which creates problems with all sage plants. If standing water remains for 30 or more minutes after rainfall, it's an indication that soil drainage should be improved. Raising the garden bed or amending the soil with compost lightens it up and allows water to flow more freely.
Multiple pests strike sage. Spider mites, aphids, thrips, spittlebugs and whiteflies all feed on sage leaves, sucking the nutrients right out of the foliage. As damage spreads, the leaves may turn yellow and wilt. Ladybugs, lacewings and many other insects are as natural predators to these pests, but pesticides may be needed to manage the problem.
Cotton root rot strikes some species of sage plants, most commonly those planted in clay soils. Cotton root rot is a disease occurring when the soil around the plant is too moist. As the name suggests, cotton root rot affects the roots, damaging and even killing them. Sage plants should only be planted in light, well-drained soils. When cotton root rot occurs, this is a signal that drainage must be improved in order for plants to survive.