Ageratum: Lower leaf purplish black discolouration
In an effort to diagnose the cause of these symptoms, the substrate was tested for pH and EC. Tests indicated that the electrical conductivity (EC) was 3.81 mS/cm [saturate media extraction method (SME)]. Benary suggests growing ageratum by fertilizing with 100 to 150 ppm N weekly, while PanAmerican suggest higher levels of 150 to 200 ppm N weekly.
Click here to read the complete e-Gro Alert report written by Brian E. Whipker at egro.org (PDF)