Warm-weather vegetables, green beans (Phaseolus vulgaris) are a common plant for the home gardener. They thrive in U.S. Department of Agriculture plant hardiness zones 3 through 10, and enjoy full sun exposure and a nutrient-rich, loamy soil. They can be either pole or bush varieties. While generally easy to care for, fungal diseases can damage these tender plants, leaving them with white leaves.
1. White Mold
Also known as Sclerotinia trifoliorum and Sclerotinia sclerotiorum, white mold occurs when the foliage of green beans is too dense, so excess moisture is trapped on and around the foliage. This is more common with bush bean varieties as they are frequently grown close together to minimize the need for staking. When persistent moisture exists with little air flow, a white fungus can appear on the surface of the infected foliage. In most cases, however, the infected areas dry out quickly and turn a light brown, tan or whiteish color. White mold can infect only a portion of a plant or the entire plant, and can spread through contact to other plants.
2. Prevention of White Mold
It's best to prevent white mold from developing in the first place. Soil-level irrigation will help reduce the likelihood of white mold fungi taking hold, as the spores need at least 48 to 72 hours of standing, still water on the foliage before the fungus can take hold. Space the bean plants so that the fully grown foliage of the plants will not lead to cramping in too close quarters. Similarly, prepare your soil to ensure it is well-draining to reduce the likelihood of standing water exposure.
3. Powdery Mildew
Powdery mildew is a common fungal infection for beans and other delicate plants. Caused most commonly by the Sphaerotheca fuliginea or Erysiphe polygoni fungi, powdery mildew is more likely in damp, cool conditions as the spores are transmitted through water. When the infection first takes hold, tiny yellow spots may appear on the foliage, but they rapidly become powdery white blotches that will cover entire leaves and stems. As the infection proceeds, the leaves will wither, die and fall off, leaving the beans susceptible to both disease and sunscald.
4. Prevention of Powdery Mildew
To prevent powdery mildew, space your plants sufficiently apart so that they do not become cramped to the extent that the fully grown foliage allows for little or no air circulation, creating the damp, humid conditions favored by these fungal spores. If it is too late to properly space your plants, prune off some of the leave from the plants. While this may reduce your overall harvest, you will also likely prevent a full decimation of your bean crop as powdery mildew can spread rapidly between plants. If a sustained period of cool, damp weather occurs, spray your bean leaves with neem oil as it can act as a deterrent for the spores as well as potentially contain the infection.