Garden beans are vulnerable to a number of pests and diseases that keep them from performing their best. Among the most common diseases are bronzing and sunscald. As their names suggest, these conditions are the bean tissue equivalent of sunburn on human skin. Both are related to the environment and weather, and both affect bean plants across the United States. Because the occurrence of bronzing and sunscald is entirely unpredictable, no reliable prevention measures for them exist.
Bean plant bronzing results from high atmospheric levels of ozone, say Colorado State University Professor Howard F. Schwartz and colleagues. Ozone descends from the upper atmosphere during heavy thunderstorms. It also develops on warm, sunny days when air blooms--including sulfites and nitrites--are high. Sulfur- and copper-based leaf pesticides sprayed on hot days may also cause bronzing-like discoloration of leaf surfaces.
Sunscald results when bean plants have too much exposure to sunlight. It's most common when sunny, clear weather follows a period of warm, humid, cloudy conditions. The sunlight usually damages tender new leaf tissue or plants that are missing some foliage.
Bronzing causes upper leaf discoloration with reddish-brown specks on the upper leaf surfaces. Over time, some leaves become yellow and fall from the plants. The condition's symptoms resemble those of fungal rust, but the bean plants are free of telltale rust spores. Bronzing symptoms are also distinguishable from peroxyacetyl nitrate (PAN) discoloration, which occurs only on the leaves' undersides.
Sunscald can strike any of an older bean plant's aboveground tissues. However, symptoms are most common on the top, outside leaves where sunlight is strongest. Early stage sunscald browns the leaves between their veins. The discolored area expands, drying the leaves until they crumble when touched. Leaf loss occurs in severe cases. Sunscald may also produce red or brown spots on bean pods.
Affected Bean Species
Bronzing and sunscald strike all commercially grown beans–including navy, kidney, wax, snap, lima and pinto. Some bean cultivars, however, are more resistant to bronzing than others, says University of Wisconsin plant pathology professor D. J. Hagedorn and research associate D. A. Inglis. Resistant green bean varieties include Tenderwhite, Provider and Stringless Black Valentine.
Bronzing is most likely to affect being plants growing in moist soil. Soil moisture results in the plants' stomates--pores on the leaves' undersides--staying open to release water vapor and absorb carbon dioxide for photosynthesis. When ozone is high, the stomates absorb it as well. Bean plants in heavily fertilized soil may be more susceptible to sunscald as they develop new tissue.
No chemical or biological methods of bronzing control exist. Research on resistant bean cultivars is ongoing. Planting beans in well-drained soil may discourage stomate ozone absorption. Sunscald control is also a matter of planting beans in well-drained soils and avoiding excessive fertilizer.