Sunscald and Bronzing in Dry Edible Beans
Sunscald is caused by intense concentration of the sun's heat on plant tissue. Sunscald can occur on leaves, stems or pods and most often affects new succulent leaf tissue. Affected leaves can exhibit brown scorched leaf tissue or white discolouration of upper exposed leaves. Leaf tissue becomes necrotic and crumbles easily, resulting in a ragged leaf appearance. Injury often occurs when bright sunny days follow cloudy, warm and humid conditions. Sunscald is not considered to affect yields.
High temperatures can burn top leaves causing wilting or brown necrosis similar to frost injury.
Bronzing damage affects the upper leaves of dry bean plants and is caused by ozone.
Bronzing is caused by exposure to ozone (O3). Ozone is caused by air pollution and lightning produced during storms. Conditions of intense sunlight or high temperature favour ozone damage. Under dry conditions plants are more tolerant to ozone, therefore symptoms may be more severe under moist conditions. The amount of damage found on the plant, or in an area, often corresponds to air pollution alerts or heavy thunderstorms. Damage appears as reddish-brown flecking or "bronzing" on the upper surface of leaves. Pods may also be affected, but damage is usually superficial and seeds are typically not affected. Bean cultivars vary in their susceptibility to ozone. Black beans appear to be particularly susceptible.
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