I got my answer over on the Florida Gardening forum. In case anybody is interestd, other people have found that yellow lantana does negatively affect other species of plants.

Lantana quite a pest in Hawaii, maybe it's allelopathic. I haven't paid that much attention, it's not winter hardy here. Otherwise, individual evergreen azaleas declining and dying or becoming chlorotic in the vicinity of others that look much better not a rare sight.

If you google "lantana allelopathy" you will find many references to this problem.

Another effect is that lantana is quite a robust plant and azaleas have very shallow roots. Hence, root competition may be all that is necessary for lantana to have a detrimental effect on azaleas.

Here is a link that might be useful: Some lantana is invasive in Florida.

I'm pretty sure that this comes up every year in several of the forums. Lantana camara, in particular, is pretty successful at inhibiting the growth AND germination of seeds of other plants. Just so happens that it's the L. camara hybrids and cultivars that are the most popular! ;-(