Hi.  Pierce County Master Gardening Office here.  It would be helpful if you would send pictures of your beans, with at least one close up of the problem on the leaves, stems etc. 

Your description of oily feel, looking like dirt, seemed like it would be a good identifier.  I have checked the various bean diseases.  None list an oily feel as an identifiable factor.  There are several bean diseases, fungus, blight etc. but they are described as wet spots that decay.  brown or black.  Not like dirt on the bean plant.  Sometimes beans, depending on how they are watered, can get dirt splashed up on them, or the leaves touching the ground pick up dirt.  I still cannot explain the oily feel. 

Are the beans being affected also, or just the leaves, stems?

If you will send pictures, we will perhaps be able to identify it. 

Thanks,

Victoria

Hello again,

After more investigation there are a couple of possibilities for the black spots on your beans. The most often seen is black aphids that love the plant and produce copious amounts of honey due that promotes a dark growth of  sooty mold ( black greasy stuff). The information below may help.

Good luck with your gardening.

Kathleen Z.

• Encourage natural predators including ladybird beetles, lacewings, syrphid (hover) fly larvae, and parasitic wasps. Avoid use of broad-spectrum insecticides which kill beneficial insects.
• Hand-wipe or prune to control small, localized infestations (when practical).
• Wash aphids from plants with a strong stream of water.
• Control honeydew-feeding ants, which may protect aphid colonies from predators.
• Provide proper nutrition. High levels of nitrogen encourage aphid reproduction. Switch to a slow-release or low-nitrogen fertilizer if necessary.

 

Caption: Bean aphid

Photo by: K. Grey