In this last Q&A session, I am focusing on leaf color change during the spring and summer growing season, not autumn. When you see leaves changing color out of season, it is time to do some detective work. Among the reasons why plant leaves turn color are environmental conditions, pests, and diseases. This change in coloration may also be a normal process, rather than a problem.
Q: “Why are my plants leaves turning brown?”
A: There are many factors that can cause leaves to turn brown. One reason is water stress indicating the plant is not receiving enough water, especially during the summer heat. Typically, these leaves show the browning at the tips and edges first; if the drought continues, the leaves begin to dry and die. This is sometimes referred to as environmental leaf scorch. If diagnosed early, drought stress can be remedied by proper watering. It should be noted that less frequent deep waterings are preferred to more frequent light watering, which causes the plant to develop shallow roots. Alternatively, too much watering or poorly drained soils can cause a root rot and this will result in the entire plant turning brown, losing leaves, and eventually dying.
Brown blotches, spots, or streaks on the leaves may be a sign that the plant has a fungal or viral problem. A few common examples are ‘black tar spot’ found on some maple trees and the brown spots on apple tree leaves. Both are caused by fungal pathogens that cause unsightly aesthetics and premature leaf drop. The good news is that in both cases these diseases do not kill healthy trees. A good control is to remove diseased leaves as they fall and dispose of them off site. Some insects can also cause browning of leaves, so careful inspection to determine the cause may be needed. Lastly, brown leaves can be caused by fertilizer burn, which would show up first towards the leaf tip and margins.
Q: ”Why are the leaves on my plants turning yellow?”
A: Some older leaves normally turn yellow, but yellow leaves may also be a sign that something isn’t right. Most commonly, the culprit is from too much water — or not enough. If plants do not receive enough water, they will drop leaves to help conserve resources. Too much water, especially in poorly drained soil, will create stress on the plants. Plants that are moved outdoors in spring may turn yellow due to too much light intensity. If you notice this, place them in a shady area to recover before exposing them to light again. The good news is most of these plants will recover, although they might experience a setback.
A nitrogen deficiency is also sometimes a cause for yellowing leaves during the active growing season.
As the end of summer nears, you may see some leaves turning yellow, which is normal as the plants prepare for winter dormancy. Yellowing is also seen on the interior needles of conifers. This is a natural process as new needles emerge. However, if your evergreen or conifer is losing the interior needles and they are not growing back, causing a sparse canopy, the culprit would most likely be a needle cast disease.
Q: “Why are my plant leaves turning black?”
A: If your plant leaves have a black substance coating them, it is most likely a fungus called sooty mold. This mold is typically found in conjunction with plant pests such as aphids, which secrete a sweet substance called honeydew that the sooty mold likes to grow on. The sooty mold is not toxic to your plants, but is unsightly and may get on structures such as cars and houses. To reduce sooty mold, take steps to control the aphids on your plants. Aphids are relatively easy to control with environmentally friendly insecticidal soaps and oils.
Q: “Why are my plant leaves turning white?”
A: White leaves are most likely caused by the colonization of fungi, which creates a fungal disease called powdery mildew. High humidity provides ideal conditions for powdery mildew. Increasing the sunlight or air circulation can control it. To increase light, thin out plants by pruning; adequate spacing between plants will help with air circulation. Do not overfertilize. Treat infected plants with an organic fungicide containing sulfur. In addition, many gardeners use home remedies such as a baking soda solution (1 Tbl baking soda and ½ tsp of castile soap (non-detergent) mixed into 1 gallon of water), which is then sprayed on the plant as a preventative and/or treatment.
Q: ”Why are my plant leaves turning purple?”
A: Generally speaking, a phosphorous deficiency may be the culprit. To investigate, I advise checking the pH level of the soil. Most plants prefer a slightly acidic soil ranging from 6.2−6.8. This is important because the nutrients may be in the soil, but with the wrong pH, the plant is unable to uptake and utilize them. If the pH seems in the acceptable range for your plants’ requirements, then applying a complete fertilizer that has adequate phosphorous should solve your problem.
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