All leaves turn yellow, then flowers and leafs drop. Is this caused by virus or bacteria? How should I treat the disease? Thank you.
The image is a bit blurry, so I am suggesting some options.
First, it is possible that your bean plants are infested with spider mites. There is a good image of the result of their presence here: https://extension.umn.edu/plant-diseases/bean-insects-and-diseases
Click on the link they provide (More information on spider mites) to see close-up images of the two-spotted spider mite, which is the usual culprit, how to identify it using a magnifying glass, and the damage it causes.
As that site suggests, you can blast the mites away with water from the hose or try an insecticidal soap treatment, although, if the infestation has been in place for a long period, the plants may struggle to recover from their impact.
The other possibility, as suggested at the top of the first web page, it that your beans are suffering from bean mosaic disease, which is quite common, and is usually spread by aphids. You may have noticed, and even rid the plants of aphids, but the disease virus may have already taken hold.
There are two types:
- Bean common mosaic, described and pictured here: https://www2.ipm.ucanr.edu/agriculture/dry-beans/Bean-Common-Mosaic/
- Bean yellow mosaic, described and pictured here: https://www2.ipm.ucanr.edu/agriculture/dry-beans/Bean-Yellow-Mosaic/
Unfortunately, there is no treatment for these diseases and the plants must be destroyed.
However, from the picture you provide, I think that spider mites are more likely.
Good luck with salvaging your bean crop.