The following is information on bract edge burn on poinsettias.
Bract necrosis/Bract Edge Burn
This disorder, which can be recognized by brown bract margins and eventual internal necrosis, and also referred to as bract burn occurs most frequently on the varieties 'Gutbier V-14 Glory' and 'Supjibi'. Generous fertilizer applications, particularly ammonium sources of nitrogen, continued into the late stages of the season, seem to increase the likelihood of bract burn. Research also has shown increased incidence of the problem when calcium was deficient. Excess soluble salts in the growing substrate causing root injury, reduced water absorption or stress from inadequate or excessive irrigation, damage from pesticides or pollutants, and high relative humidity are also associated with the disorder. Perhaps the greatest damage caused by bract necrosis in the increased chance for Botrytis infection of the damaged tissue, and then the need to control this persistent disease.
The following suggestions have been made:
1) Reduce fertilizer rates and frequencies as the crop matures.
2) Primarily use nitrate nitrogen, rather than fertilizer high in ammoniacal nitrogen.
3) Apply adequate amounts of calcium (calcium levels of 0.5% or less in the leaves are considered to be deficient). Consider supplemental calcium sprays (see: Calcium).
4) Do not use excessive amounts of slow-release forms of fertilizer, as nutrients cannot be withheld late in the season.
5) Avoid unnecessary irrigation which can result in soft bracts.
6) Try to avoid high relative humidity. This can be achieved by ventilating and heating late in the afternoon to remove moisture from the greenhouse atmosphere. A policy of not irrigating in the afternoon can also be helpful. Steps taken to reduce relative humidity will not only reduce the incidence of bract necrosis, but will also help control Botrytis if it does occur.
7)Use approved fungicides for Botrytis prevention or control.
Information from the Poinsettia Problem Diagnostic Key on Physiological Disorders from North Carolina State University http://www.ces.ncsu.edu/depts/hort/poinsettia/corrective/a11.html