It sounds like it is probably Bean common mosaic virus (BCMV), which is transmitted via seeds in Phaseolus species (like runner beans and bush beans). It is also transmitted among seedlings and mature plants by aphids in a non-persistent manner. This means that the aphid just has to taste the infected plant in order to acquire the virus, and will spread it to the next susceptible plant it tastes. Because of this, insecticides are not very effective at preventing aphids from spreading non-persistent viruses like BCMV. I expect that the reason you are seeing it in your plot is that some of the seeds planted were infected with the virus already and that the resulting infected plants then served as a source of virus for spread by aphids to other parts of the garden. Since the virus spreads just by aphids tasting, you many not even see large colonies of aphids associated with the infection, since they do not have to colonize the plants to spread the virus. Unfortunately there is no way to cure plants of virus, and BCMV does cause some effects on pods/seeds, which may make them less appetizing. If you want to stop the spread of the virus you can remove the plants that are showing symptoms and bag them up so that the tissue dies before it is disposed of (if it sits around on a weed waste pile aphids may probe on it while it is still viable, or disperse from it if they are already on the infected plants). You can also try to just take out all the beans and start a new crop, preferably of a different variety from a different seed provider (check for resistance to BCMV, some varieties may be bred for this). However, these removal tactics will only work if everyone in the garden agrees to remove the infected plants. Also make sure there is not a source of virus in the general area - a farmer growing other Phaseolus species for instance. If this is the case, without resistant cultivars your new planting will again become infected.

Here is a good fact sheet about BCMV
http://www.dpvweb.net/dpv/showdpv.php...