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Odd that it is blooming now, but hold off on pruning until blooms fade. Normally you prune these shrubs back folling their blooming in spring. Here is more information: https://www.gardeningknowhow.com/ornamental/shrubs/weigela/pruning-weigela.htm

This article will help with pruning: https://www.gardeningknowhow.com/ornamental/shrubs/weigela/pruning-weigela.htm

The following article should help answers about nonchemical fungicides: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
This article should help with pruning: https://www.gardeningknowhow.com/ornamental/shrubs/weigela/pruning-weigela.htm
And, since wigelias are decidious, the diseased lookingj leaves could just be ready to fall off. If you think or know it is a disease, spray with neem oil anyway. Gather (rake) the leaves, place them in a plastic bag and put in landfill. Do NOT compost or burn diseased leaves, composting will not kill it and burning will spread it.

The following article has all the information on trimming your weigela: https://www.gardeningknowhow.com/ornamental/shrubs/weigela/pruning-weigela.htm

Fletcher, it sounds as if one of the plants is "pitching a fit"! As you state, close proximity, adequate rain and drainage... All things considered, they should both be doing will. Maybe this article can help: https://www.gardeningknowhow.com/ornamental/shrubs/weigela/growing-weigela.htm

This article should answer your question: https://www.gardeningknowhow.com/ornamental/shrubs/weigela/pruning-weigela.htm