P&PDL Picture of the Week for
March 29, 2010

Chlormequat chloride (Cycocel or Citadel) Phytotoxicity Symptoms

Roberto G. Lopez , Ph.D., Assistant Professor & Floriculture Extension Specialist, Purdue University and Chris J. Currey, PhD student, Purdue University

The plant growth regulator (PGR), chlormequat chloride (Cycocel, OHP, Inc. or Citadel, Fine Americas, Inc.) is commonly used on geraniums, poinsettias, and other herbaceous plants to reduce stem elongation. However, foliar sprays of chlormequat chloride can cause yellow spotting, “haloing”, or discoloration of newly expanding leaves (Figures 1 to 3).  The yellow spotting is a result of damage to the chloroplast and often visible within 3 to 5 days after the spray application.  In general, problems occur when the concentration of chlormequat chloride sprays exceed 1,500 ppm, though some cultivars and species are sensitive at lower rates.  Leaves will usually “green up” within a few weeks and as new leaves expand, they will generally cover any yellow or discolored leaves. Remember that excessive chlormequat chloride rates may cause brown necrotic areas on leaf margins, which will not recover.  It is always recommended that trials be conducted with the use of any PGR and especially before application rates of 1,500 ppm or greater of chlormequat chloride are used.

Click image to enlarge

Geranium

Figure 1. Geranium crop exhibiting phytotoxicity from 1,500 ppm spray of chlormequat chloride

Geranium closeup

Figure 2. Closeup of a geranium leaf exhibiting phytotoxicity from 1,500 ppm spray of chlormequat chloride

Poinsettia

Figure 3. Poinsettia leaf with yellow spotting and discoloration after an excessive spray application of chlormequat chloride

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service