Powdery Mildew In Beans: How To Control Powdery Mildew On Beans

Powdery Mildew In Beans: How To Control Powdery Mildew On Beans

By: Bonnie L. Grant, Certified Urban Agriculturist

If you grow roses, you are likely familiar with the powdery white fungus that attacks plant leaves, blooms, and stems. This powdery mildew attacks many types of plants, including beans. Not only is the stuff unsightly, but it can ruin your crop, diminishing your harvest. However, powdery mildew on bean plants can be controlled and you can even prevent it. If you see even the first sign of bean plants with powdery mildew, it is time to jump into action and take steps to keep the fungal disease from spreading.

Recognizing Bean Plants with Powdery Mildew

Powdery mildew stems from the funguses Erysiphe or Sphaerotheca. It actually doesn’t matter which fungus is causing the damage when your crop is at stake. Treating powdery mildew on beans from either fungus requires the same method. Early control is essential, as the fungus spreads rapidly in warm, moist conditions and can quite literally decimate your bean crop, so recognizing powdery mildew on beans can protect your crop and prevent the spread of this fungal issue throughout your other vegetables.

Powdery mildew in beans is such a common occurrence it should have its own trademarked name. This fungus produces fruiting bodies in warm, moist conditions which spread across all parts of the plant and appear as ashy white powder.

Most powdery mildew occurs on legumes and cucurbits in the crop category, although they affect citrus and other crops as well. Once the spores are present and the right conditions occur, the fungus spreads rapidly in epidemic proportions. Preventing powdery mildew in beans is a crucial step to keeping an abundant harvest.

How to Control Powdery Mildew on Beans

A few cultural steps can help prevent bean plants with powdery mildew.

  • Avoid overhead watering where possible.
  • Water early enough in the day so the sun will dry off leaves and stems.
  • Provide support for climbing beans and give them plenty of air circulation. Crowded plants are more susceptible to acquiring the fungus.
  • The spores will overwinter in most areas, so cleaning up affected plant material at the end of the season is important.
  • Make sure the plants are well fed and watered so they can withstand a late season bout of the disease if it occurs.
  • If you have nearby roses or other ornamental plants that have the disease, spray those with a copper fungicide.

It is tricky treating powdery mildew on beans and other edible crops. This is because many of the products labeled for such control are not suitable for edible plants. Diluted compost tea (by 4 parts water) may offer some control without any toxicity.

If you have plants that develop powdery mildew habitually, apply a preventive fungicide early in the plant’s development. That means prior to flowers and fruit. Avoid eradicant fungicides, which will kill existing diseases but can contaminate fruit. Apply sulfur early in the season to protect plants from infection.

For existing infection, use a horticultural oil that is natural such as neem oil or jojoba. Finally, there are a couple biological controls in the form of beneficial microorganisms that combat powdery mildew. Look for products with Bacillus subtilus, the unique, non-toxic organism that prevents powdery mildew.

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How to Get Rid of Powdery Mildew in Your Garden

Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.

A watermelon vine that is infected with powdery mildew

Powdery mildew is a scourge of gardens, both vegetable and ornamental. There are some simple steps you can take to prevent powdery mildew or get rid of it once it infects your plants.


Powdery mildew can affect all above-ground parts of bean plants. Initial symptoms appear as small and white talcum-like spots (Figure 1), which most commonly are seen on the upper surface of leaves. These spots increase in size and run together to form a whitish, powdery growth, gradually spread over a large area of the leaves (Figures 2, 3, and 4), and can spread even farther to the stems. As the symptoms develop, infected leaves may gradually curl downward, color changed from pale yellow to brown (Figure 5), die, and fall off. Under severe conditions, the entire leaves and plants could be covered by white cottony mycelial growth of the fungus (Figure 6). Symptoms on infected leaves may vary with bean varieties, but powdery mildew may cause the leaves to be twisted, buckled, or distorted. The powdery mildew fungus usually does not grow on bean pods except pea pods (Davis et al. n.d.). However, powdery mildew spots can develop on snap bean pods (Pernezny and Stall 2005). The development of powdery mildew symptoms is not often observed on pole bean in Miami-Dade County, but it is apparent on both Italian bean and long bean (dark green type) (Figures 7 and 8) grown under the same conditions. Severely infested plants may have reduced yields, shortened production periods, and even completely die (Figure 8). Severe symptoms of powdery mildew infection can also be seen in snap beans when humidity is high (Figure 9).

Early stage of powdery mildew development on Italian bean.

What is Neem Oil and Why is it so Awesome?

Neem Oil is 100% organic, so it is a definite plus for all types of gardeners. The oil comes from the seeds of the Indian Neem Tree, which, as the name applies, is native to India and other parts of Southeast Asia. Not only is Neem Oil great at preventing bugs from munching on your leaves and acting as a fungicide, but it is also a bactericide and a miticide. When applied regularly, it does wonders for your garden. Bless the soul of whoever discovered that oil from a seed could be so magical.

We buy our Neem Oil in concentrated form. For small-medium sized gardens, one bottle should last you a whole season. Simply mix the required amount with a gallon of water and spray on the plants in your garden. We recommend using a hand sprayer to save your hand muscles and your sanity.

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