Answer: In a quick search I am not finding specific mention of Albizia julibrissin as a source of allelopathic problems, however it is possible that the tree is competing against the plants for sun and water and nutrients and thus weakening them and making them susceptible to disease. (These annual flowers do best in full sun, with a soil that is reasonably fertile, and evenly moist yet well drained. If you are not replenishing the soil each year by adding organic matter such as compost and using an organic mulch that will help feed the soil, this may be a contributing factor as well.) Some diseases inlcuding some vascular wilts are soil borne and can carry over from year to year, so that might also explain it. Another possibility is that there is a general soil problem under the tree; these trees are able to grow in many very inhospitable spots where other plants fail to thrive. YOur county extension or certified nurseryman might also have some suggestions to help your trouble shoot, or could assist with soil testing to explore that avenue. I'm sorry I can't be more specific for you.
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