Vein clearing is nearly always a sign of viral disease. It can also indicate herbicide poisoning, bacterial disease, or fungal disease.
Normal, healthy leaf veins are green or white, and opaque. In the case of disease or overspray, those veins may become lighter than normal to the point of becoming translucent, clear, or pale yellow. This is a form of chlorosis.
Viral diseases and vein clearing
Viral diseases, such as cucumber vein yellowing, papaya ringspot, and turnip vein clearing, often appear initially as lighter colored veins. Many mosaic viruses, such as cucumber green mottle, pea seed-borne mosaic, and squash mosaic start with similar symptoms.
Other diseases and vein clearing
Fungal diseases, such as Fusarium wilt, occasionally cause vein clearing. Bacterial wilts, such as Verticillium wilt, may also exhibit vein clearing during the initial stages of infection. In bacterial wilts, the xylem walls either dissolve or rupture, releasing fluids into nearby cells and leaving the veins looking translucent.
Vein clearing and overspray
Vein clearing can also be seen when overspray occurs. Overspray describes the way herbicides and other chemicals can drift on a breeze to unintended plants. After landing on a plant, the herbicide is absorbed and transported throughout the plant in the xylem. Older leaves generally exhibit damage to margins (edges) and interveinal (between vein) areas. Younger leaves respond differently, showing chlorosis of the veins, especially the midrib.
Vein clearing can also be a phytotoxic symptom. Phytotoxic means “poisonous to plants”. In some cases, we apply insecticides, oils, or other treatments with the best of intentions. Whether due to extreme sunlight or temperatures or something else entirely, these treatments can go awry, causing symptoms such as vein clearing, along with wilting, leaf loss, or flower drop. In other cases, these symptoms may appear for no obvious reason at all!
Cherry vein clearing, for example, is believed to be a genetic mutation that is spread by grafting affected scions onto unaffected wood. Some researchers believe this mutation is caused by a boron deficiency in the soil, but no one is sure just yet.
In some cases, vein clearing is a short-lived symptom. As the disease or toxicity progresses, vein clearing may resolve itself and show up as completely different symptoms, depending on the initial cause. These symptoms may include dwarfing, puckered leaves, variegated yellow and green on leaves, and vein banding. Vein banding is similar to vein clearing except that bands of translucent and opaque green, yellow, or white are seen.
If you see vein clearing, take a closer look, note any other symptoms and send me pictures!
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