Causes of Yellow Leaves
One of the first signs of concern with tree health is when leaves start to turn any other color besides green. Some causes are temporary and some are symptomatic of a more serious problem.
Some trees are selected genetically for their gold, lime, or yellow coloring. Those trees are healthy and are not being considered in this article.
Nutrition
Lack of sufficient nutrients in the soil is a common reason for yellowing in tree foliage and often shows up as chlorosis, a common symptom of mineral deficiencies visible by the mottling of yellow and green in the leaf. This can be caused by poor soil (insufficient nutrients), wrong soil pH, wet soils, or pest damage.
When trees are young, they should be fertilized with a liquid root stimulator and can be graduated to granulated fertilizers or tree spikes. But once trees reach a certain size, root systems are so large that fertilizing seems to help lawns and landscape plantings more than the tree itself. Mulching in a wide space around the tree is often times more beneficial, as the mulch breaks down to produce a natural organic fertilizer. Other slow-release organic fertilizers can also be applied in the root zone as the need arises.
Soil pH is responsible for most minor nutrient deficiencies. Iron and manganese are available in the soil only when the pH is acidic. Much of central Missouri has lime-based soils, meaning that the pH of our soils are alkaline (over 7.0), resulting in symptoms of iron and manganese deficiencies.
Moisture Issues
Excessively dry condiditons can cause older leaves to turn yellow and drop off. This is a survival technique. Leaves dropped off mid-summer don’t affect the trees health, as they have enough strength to either re-leaf within the same year or wait until next spring to resume photosynthesis, assuming the dry conditions are remedied through irrigation or natural rainfall.
When leaves turn yellow due to excessively wet conditions, this represents a much more serious problem, often times irreversible. See Tree Health Part 2 – Root Systems.
Pests
Insects – When leaves are affected by insects, through sap-sucking insects like aphids and mites or leaf chewing insects like caterpillars and beetles, the nutrient-producing area of the leaf may be depleted enough that the leaf abscises and drops off.
Boring insects will damage the vascular system, and sections of the tree will have yellow leaves before part or all of the branch dies.
Moles and Gophers – Ground-buroughing critters may disturb and expose root systems to the level that the tree becomes stressed. Leaves may turn yellow and drop off, or chlorosis may become evident.
Fungal Diseases – Like damage caused by insects, foliar diseases can block sunlight (powdery mildew) or reduce the chlorophyll-producing area sufficiently so that the leaf is no longer productive. It will then turn yellow and drop off.
Bacterial Diseases – Many bacterial diseases can affect foliage as well, but vascular-clogging diseases like verticillium wilt block the flow of food and water to the leaves, resulting in yellowing, stunting, and wilting, and possibly death to the tree.
Cankers developing on the trunk and branches show up as yellowing first on leaves and then eventually death to the branch.
Herbicide drift – When leaves get damaged, whether from herbicide drift or pest damage, trees will get rid of these unproductive leaves. First they will turn yellow, and then drop off.
Evergreen Trees
Yellow or Bronzed Needles – When the ground is frozen, evergreens can suffer wind damage resulting in a discoloration of the needles. The drying winds take moisture away from the needles, and because the ground is frozen, moisture can’t easily be replaced.
Sometimes, the tips get burnt, as well. Once warm weather and spring rains arrive by April, the healthy green coloring comes back.
Seasonal Drop of Older Needles – Starting in late summer, the older needles of all evergreens are shed in an annual ritual. Depending upon the amount of stress that plant has received will dictate if the needles from just one year are shed, or from multiple years. Three or four years of growth are maintained on a tree. When extreme soil moisture conditions or environmental stresses exist, all needles can be lost except those from the current year and still not compromise the long-term health of the tree.
Yews and arborvitae drop older needles during the growing season (April-June), and all other evergreens lose their needles starting in August and sometimes being highly visible as late as October.
