What Causes the Leaves of Kentucky Wonder to Yellow?
Kentucky Wonder is a cultivar of pole snap bean (Phaseolus vulgaris) prized for its clusters of flavorful, 9-inch pods and a climbing habit that can effectively save space in a small garden. Yellowing of Kentucky Wonder leaves could indicate a number of potential problems ranging from pest infestation or disease to a nutrient deficiency. Taking environmental and site factors into account and recognizing unique characteristics of the yellowing leaves helps to determine what it is most likely caused by.
The bean common mosaic, bean yellow mosaic and cucumber mosaic are all viruses that potentially impact Kentucky Wonder and other beans. These diseases, which are spread between plants by aphids and leafhoppers, appear on beans as a yellow and green mottling or mosaic, interveinal yellowing and sometimes leaf distortion or wrinkling. Mosaic viruses are prevented or addressed by controlling nearby broadleaf weeds that could act as a reservoir for the diseases and immediately removing any plants that show symptoms to prevent spread.
Fungal Leaf Diseases
Bean rust, caused by the fungal pathogen Uromyces phaseoli, appears on infected bean plants as dry orange, reddish or yellowish masses that are primarily located on the lower leaf surface. The fungus Erysiphe polygoni causes powdery mildew on beans. Older leaves on affected bean plants first develop yellow spots on the upper leaf surface before indicative white, powdery growth appears on both leaf surfaces. Leaves may completely yellow, die and drop. Fungal infections on aboveground portions of beans are best prevented by providing adequate space around plants to ensure excellent air circulation, avoiding overhead watering and, if necessary, applying a fungicide such as sulfur.
Fusarium Root Rot
Fusarium root rot caused by the fungus Fusarium solani incorporates the development of red lesions with undefined margins on the underground portion of the stem and roots. Infected plants wilt and turn yellow, and small red flecks are visible in tissue when the surface of the lesion is scraped. Fusarium solani can persist in the soil for years, so implementing a crop rotation system, practicing good sanitation and using soil solarization, a process that involves covering the soil with a thick, clear sheet of plastic for at least four weeks during hot weather, are required to achieve adequate control of Fusarium root rot.
Snap and other types of beans have many potential pests, but only a few could cause yellowing without other more indicative symptoms. Root-knot nematodes (Meloidogynes spp.) are microscopic roundworms that cause the formation of galls on bean roots. Affected plants demonstrate yellowing, stunted growth, dying leaf margins and wilt. Nematodes are most problematic in sandy, dry and warm soils. Crop rotation and fumigation or solarization offer nematode control. Spider mite feeding on bean leaves generally first appears as a light stippling; if feeding persists and the mite population grows, webbing often appears and leaves may develop a bronze, yellow or reddish color and drop. Aphid feeding on beans usually causes leaf distortion, but may also force leaf yellowing. These pests excrete a sticky, sweet substance known as honeydew that hosts sooty mold.
Nutrient Deficiencies and Toxicities
Nutrient deficiencies or toxicities can appear on a bean plant as yellow, tan or bronze foliage discoloration that generally begins at leaf margins while veins retain a green color. Plant growth is often stunted. Soil pH too high or low outside the ideal bean-growing range of about 6.0 to 6.5 or poor drainage greatly influence nutrient availability. The most common nutrient deficiencies in beans are magnesium and phosphorous. Possible nutrient toxicities include aluminum, boron and manganese; toxicities often accompany a low soil pH. Nutrient problems are best addressed by conducting a soil test and correcting pH and amending or fertilizing the soil appropriately.
- UC IPM Online: Beans
- UC IPM Online: Mosaic Viruses
- University of Wisconsin-Extension: Handbook of Bean Diseases
- UC IPM Online: Bean Rust—Uromyces phaseoli
- UC IPM Online: Fusarium Root Rot—Fusarium solani
- UC IPM Online: Powdery Mildew—Erysiphe polygoni
- University of Illinois Extension: Beans
- University of California Cooperative Extension: Snap Bean Production in California
- Jupiterimages/Comstock/Getty Images