I planted this small Cottonwood Tree last year in a spot of our yard that has problematic, slow draining soils.  We are in the Grand Junction area, zone 7b.  The idea was that if any kind of tree could cope with the wet, impacted clay, then surely a Cottonwood could.   We had our soil tested last year and found no critical issues - just a general lack of NPK and other micronutrients.   We have not sprayed this area with any herbicides since we bought this property two years ago.  I amended the planting area with compost and sprinkled some slow release organic granular fertilizer on top.  Other than that, I haven't done anything but monitor the soil for moisture - rarely needing to add any water.  This Cottonwood came back to life this spring with vigor and was flush with leaves for a few weeks.  Then within 24 hours it went from seemingly healthy to suddenly wilting and it's only getting worse everyday.  The leaves are soft and brown, not dry and brittle.  What do you suppose is happening and is there anything I can do?