Is Robinia pseudo-acacia allelopathic?

First, shade is not very much to their liking......that may be what's causing the issue. It is some sort of leaf spot/leaf blight of which there are several that flowering quince are prone. Treating early in the season with an appropriate fungicide would help. But good air circulation and avoidance of overhead watering together with plenty of sunlight is best.

Verticillium wilt does not display symptoms like that......and Chaenomeles is considered to be resistant as well.

Thanks so much for the follow ups.
Plantknitter: It is not allelopathic, as far as I know. Just a total water hog.

Gardengal48: Really glad to read it's not verticillium. I have nothing but shade to give them, so I guess I will leave them where they are and water with soaker hoses. And fungicide next year maybe. Thanks again!

The bottom leaf has dots on the brown part, maybe these are fungal fruiting bodies.

Can't fungus come after the fact?
Not necessarily being the primary causal agent , but an opportune invader after the fact?

Depends on the specific fungal pathogen. Some colonize already necrotic plant tissue (usually bark or woody portions of the plant) but most fungal problems associated with leaf spots or leaf blights are themselves the cause of any necrosis or dieback.

If you are asking if stressed or compromised plants are more prone to disease issues, then yes, in many situations that is the case. Just like humans with compromised immune systems :-) But I have never encountered an instance where an allelopathic reaction generated secondary disease problems. Typically it just interfers with robust growth or outright kills the plant. And Robinia's allelopathic properties seem to be rather limited anyway.