Leaves of deciduous plants and needles of evergreens occasionally lose their normal green color. The key reason that plants turn yellow is that they are not able to maintain adequate levels of the green pigment, chlorophyll. When this vital substance is absent, yellow pigments that are also present take over and the leaves or needles fade.

Yellowing of plant parts that are normally green is called chlorosis. Affected parts are said to be chlorotic. Diseases, insects and changes in the plant's environment cause leaves and needles to become chlorotic. Determining which is to blame is not always easy.

Fungal diseases such as apple scab or anthracnose in maples, oaks and sycamores cause leaves to yellow. Needlecast diseases on pine, spruce and fir cause a similar effect on needles. Viruses cause the loss of chlorophyll in a mottled pattern on leaves and stems.

Sucking insects such as aphids, mealybugs and scale remove nutrient laden sap from the plants restricting their ability to make more chlorophyll. Spider mite feeding produces a mottled pattern of yellowing similar to a virus.

Many environmental factors cause a plant to yellow. The most basic is a lack of adequate light. When plants become shaded by other plants or by branches higher up on the same plant, the chlorophyll fades away and the leaves yellow.

Low light levels in the fall trigger the normal development of fall color. Warm days and cool nights then encourage the production of the pigment anthacyanin which results in the bright red colors we all enjoy.

Nutrient deficiencies also cause chlorosis. Nitrogen is a vital component of chlorophyll so inadequate levels of this element make leaves turn yellow. Iron deficiencies cause chlorosis between the veins in leaves while the veins themselves maintain their green color.

Too much or too little water can cause yellowing. Under water stress, plants shut down internally and cannot transport nutrients to the leaves. Chlorophyll production can be affected causing loss of color.

Low temperatures can affect leaf color. Under cool conditions, chemical reactions are slowed and, again, chlorophyll production may be reduced.

Herbicides (weed killers) may also cause yellowing. Some of these chemicals, including the widely used Round-up, work by stopping photosynthesis in the plant. Often, they do this by blocking the production of chlorophyll.

 

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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