If your salvia or sage plant is dying, root rot could be the cause. Although most salvia varieties are highly resistant to insects and diseases, root rot, which is caused by a type of fungus, is a common problem among the hundreds of types of salvia that grow throughout the world today. To limit the chances of root rot in your salvia plants, make sure standing water does not sit around the plant, especially on warm days.
One of the first noted signs of root rot in a salvia plant is typically wilting leaves. When a salvia plant is the victim of root rot, the leaves droop, curl and often begin to lose their colour. Leaves may also turn yellow or a lighter green as they wilt. Wilted leaves in salvia plants with root rot are caused by a nitrogen deficiency. As the roots become infected with root rot and begin to die, the rest of the plant is unable to get nitrogen from the earth, causing the leaves to wilt.
Like with most fungi and diseases, young plants are often highly susceptible to root rot and may show signs of symptoms, such as stunted growth, first. Since root rot kills off the roots which supply the plant's food, it limits the plant's access to the nutrients it needs to grow. If a salvia plant is not getting enough of the nutrients it needs, its growth will become stunted. Small plants which have little new growth or few flowers in a season when such things are typically abundant should be checked for root rot.
Yellowing leaves or stems are signs of a number of plant problems, including natural nutrient deficiency in the soil, but nutrient deficiency can also be caused by diseases or fungi, like root rot. If the leaves of your salvia plant are yellowing and wilting, it could mean your plant has root rot. To be able to tell whether your plant has root rot for sure, dig up a section of the earth around the plant and look at the roots. Roots infected with root rot turn a deep brown colour as they die.