Chlormequat chloride

May 10, 2023

This osteospermum crop (Fig. 1) started to exhibit chlorosis on leaves. The distribution of the symptoms across the crop as well as individual plants, made it clear this wasn't any nutrient deficiency. It started showing up after an application of chlormequat chloride (Citadel, formerly Cycocel now Altercel) was made.

Chlormequat chloride is an active ingredient that is useful in controlling excessive osteospermum growth. Foliar sprays of chlormequat chloride are effective and recommended as a plant growth regulator (PGR). In addition to using it as a sole active ingredient, tank-mixing it along with daminozide is another good strategy.

Chlormequat chloride is an effective PGR for many crops but is well-known for causing chlorosis on foliage. Geraniums can be particularly susceptible to yellowing in response to chlormequat at moderate concentrations. Sometimes the "halo" of chlorosis is used as a confirmation that a sufficient active ingredient was applied. For other crops chlorosis may be more common at concentrations above 1,500 ppm. However, was the chlorosis here caused from an excessive concentration or application volume?

In looking at the pattern of how the chlorosis is distributed in the crop, there are pockets up front near the aisle where chlorosis is more concentrated and prevalent (Fig. 2). As you look back towards the back of the block, there is little to no chlorosis. The degree of this chlorosis on affected leaves, combined with the lack of chlorosis in areas around affected areas, is a good indicator the chlorosis was caused by an excessive volume of solution from a less- than-even application and not an excessive concentration.

There are several best management practices for foliar PGR sprays. Among them is to apply 2 quarts of solution per 100 square feet of growing area. Uniform applications require applicators to calibrate their spraying equipment and use of that equipment to get to a point where they are confident in their application volume.

Chlormequat chloride does have root activity — meaning it can be taken up from the substrate. In those spots where excessive solution was applied, it likely ran down the stem into the substrate, potentially causing excessive growth control. Watch for excessive growth control and consider using GA+BA (Fresco, Fascination) to counter any over-regulation.

For modest chlorosis, tissue can regreen by the end of the crop cycle (Fig. 3). However, in this instance, there was some necrosis on leaf margins at high concentrations — and this won't green up. Instead, you must promote growth and development of new, unfolding leaves. As they unfold and mature, the new leaves will cover necrotic tissue if there are enough new leaves that grow to an adequate size. This can become a bigger concern for crops with terminal flowers, like osteospermum, because once flowers have initiated, no more leaves will be formed.

Christopher is an associate professor of horticulture in the Department of Horticulture at Iowa State University. [email protected]

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