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Bean, Downy Mildew

Downy Mildew caused by Phytophthora phaseoli infects broad bean, pea, and lima bean (Phaseolus lunatus). Photo: R.L. Wick

Phytophthora phaesoli

Downy mildew caused by Phytophthora phaseoli infects broad bean, pea, and lima bean (Phaseolus lunatus). The disease can be a major problem in cool, moist regions of Europe and the United States. The pathogen is adaptable and continues to produce new races (strains) on previously resistant cultivars. There may be physiological differences which limit cross infection among the various races and hosts.


Local lesions begin as small, yellow blotches on the upper sides of leaves which are limited by leaf veins. The undersides of these lesions produce a gray to purple, fuzzy sporulation. Lesions become dry and brown and may be invaded by secondary organisms like Botrytis. Pods can also be attacked and exhibit large, yellow blotches on the pods and white mycelial growth within the pods.

Life Cycle:

Seedlings may be infected at emergence and die, although infected seeds usually fail to germinate. Systemically infected plants are light green with a silvery appearance. The pathogen is seedborne and soilborne. Disease development is most rapid under cool, wet conditions. The disease is disseminated by wind and rain.

Cultural Controls & Prevention:

  • Rotate with non-host crops to prevent the buildup of soilborne inoculum.
  • Plow under infected crop residue after harvest.
  • Plant resistant cultivars where available. Resistance may be overcome by new races of the pathogen.
  • Use seed treatments to protect emerging seedlings from infection.

Chemical Controls & Pesticides:

For Current information on disease recommendations ins specific crops including information on chemical control & pesticide management, please visit the New England Vegetable Management Guide website.

Crops that are affected by this disease:

Last Updated: 
January 2013

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