I've seen this on two liriodendron and found cankers at the base of both. You might want to have a look to see if there are any lesions just about at soil level. It's touted as being an easy tree and not terribly pest prone but I've found that not to be the case in this area, especially with nursery acquired stock.

Lombardy Poplar has a reputation for being a problem tree. One site simply says too many diseases to list. Fast growth is usually poor quality growth. The good news is now you have a chance to replace them with something of quality. See link for Univ of Florida site. Here is a link that might be useful: Univ of Florida factsheet

This message has kind of gotten hijacked from the original post to an argument over the risks/benefits of genetic engineering. Certainly genetically engineered crops are being developed to produce a marketable result (GM seeds) and side effects may not be of the utmost concern. However, an underlying theme of developing these crops is to reduce the potential side effects of other pesticides by reducing their use and substituting pesticides that are safer, non-toxic or less toxic to non-target species, and have a reduced potential to cause damage to non-target species or areas. There is the potential for Bt crops to kill Monarch caterpillers, however it is still probably less toxic than the alternatives. Bt crops do not produce a bug free field. Many species of insects are immune to Bt. Caterpillars are generally not immune to it. Insects that are not eating the corn are not a target of the Bt corn, and Monarchs do not eat the corn. The risk lies in the movement of corn pollen carrying the Bt to the Monarch's food source. Still, the bloom period for corn will run a couple weeks in the summer. The rest of the growing season, it is not shedding any Bt material. Thus, there is risk to non-target Monarch larvae for a couple weeks. Adults would not be harmed, just larvae during this period. Without Bt corn, the control of insect pests in the crop would generally require sprays or soil treatments that would kill a much broader spectrum of insects, including all adult Monarchs that might be in the field. Thus while the use of Bt crops might not be totally safe for non-target species, it is still probably much safer than the alternatives, and it is more effective on the target pests. Seems like a winning solution here--and with time it can still be improved. But if the early steps aren't profitable for the developers, they will never move on to develop even safer products down the road. With herbicide resistant crops, the plants are producing and carrying no pesticides. What the genes do is allow farmers to spray the crop with broad spectrum, low toxicity herbicides and the plant has the ability to deactivate the herbicide quickly so it suffers no damage. There are valid concerns with these technologies, however we need to explore them as they offer the potential to reduce the use of toxic pesticides and non-target damage dramatically compared to conventional technologies. When it comes to the potential for the bunny cottonwoods to spread by seed or cross into other poplars growing wild, Monsanto had techniques to render their genetically modified crop seeds sterile so they could not transfer into other plants, however that was killed with the help of those claiming that potential outcrossing was a terrible risk. Sometimes I think the anti-GM people are less interested in controlling the risks of GM technology than they are in finding issues to generate memberships and donations to feather their nests and perpetuate the discord.

what a lot of theory and work ... i would cut them off the tree.. and stick them in the ground.. year around.. with or without water ... had more than i ever wanted ... they are almost all gone.. because the are fast growing, fast dying, disease infected crumby trees ... and they sucker in the extreme when they die ... or are cut down ... try it.. enjoy it... but really decide if you want all the problems associated with them in the long run ... if you are trying to cover some land... they are a good TEMPORARY solution.. but i suggest you plant other hardwood trees.. so that when all the poplars die.. you are left with other things... been there... done that.. and regret it... good luck ken