Vein clearing is nearly always a sign of viral disease. It can also indicate herbicide poisoning and bacterial or fungal infection.
Normal, healthy leaf veins are green or white and opaque. In the case of disease or overspray, those veins may become pale yellow or so light they are nearly translucent. It 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. Cucumber green mottle, pea seed-borne and squash mosaics can exhibit vein clearing.
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 is also a symptom of chemical overspray. Overspray occurs when herbicides and other chemicals 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 something is poisonous to plants. We apply insecticides, oils, or other treatments with the best intention. But extreme sunlight, high temperatures, and other factors can turn these treatments into toxins, causing symptoms such as vein clearing, wilting, leaf loss, or flower drop.
Cherry vein clearing, for example, is believed to be a genetic mutation spread by grafting affected scions onto healthy wood. Some researchers believe this mutation is due to a boron deficiency in the soil, but no one is sure yet.
In some cases, vein clearing is a short-lived symptom as more disease-specific symptoms appear. 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 you can see bands of translucent and opaque green, yellow, or white.
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