J. AMER. Soc. HORT . SCI . 115(1):73-78. 1990. Leaf Edge Burn and Axillary Shoot Growth of Vegetative Poinsettia Plants: Influence of Calcium, Nitrogen Form, and Molybdenum Peter M. Bierman 1 , Carl J. Rosen 2 , and Harold F. Wilkins 3 University of Minnesota, St. Paul, MN 55108 Additional index words. Euphorbia pulcherrima, ammonium, nitrate, gypsum, foliar Ca sprays, cutting production Abstract. Poinsettia (Euphorbia pulcherrima Wind. ex. Klotzsch CV. Gutbier V-14 Glory) plants were grown under conditions simulating commercial stock plant production to investigate the effects of NH 4 -N : NO 3 -N fertilizer ratios, foliar Ca sprays, medium-applied Ca, and medium-applied Mo on leaf edge burn (LEB) and cutting production. Leaf edge burn expression was nearly 100% greater with NH 4 -N : NO 3 -N fertilizer ratios of 1:2 or 2:1 than with NO 3 -N only. However, cutting production was 28% lower with NO 3 -N as the sole N source. There was little difference in either LEB or cutting production between the two NH 4 -N levels. Weekly Ca sprays at 500 mg·liter -1 were effective in reducing LEB, while medium-applied Ca as gypsum was ineffective. Foliar Ca sprays reduced both the number of LEB leaves (90%) and symptom severity of individual leaves. Spraying plants with tap water (Ca at 25 to 30 mg ·liter -1 ) plus wetting agent had an intermediate effect. Medium-applied Mo was ineffective in reducing LEB, despite greatly increasing leaf Mo levels. The Ca concentration in chlorotic, marginal leaf tissue was significantly lower than the Ca concentration in green leaf margins. There was also a strong, negative correlation between the Ca concentration in young leaves at the susceptible growth stage and the incidence of LEB in various treatment groups. Supplemental applications of Ca and Mo did not consistently affect cutting production. Leaf edge burn appears to be a localized Ca deficiency due to inadequate Ca uptake and/or translocation to the numerous axillary shoots simultaneously developing on poinsettia stock plants. Poinsettia leaf edge burn (LEB) is a marginal deterioration and hyponasty observed on young, expanding leaves from rap- idly developing axillary shoots. As marginal tissue ceases to grow, continued expansion of the leaf blade causes a puckering of the leaf surface and a downward curling of the leaf tip and margins. Symptoms typically appear after several flushes of cuttings have been removed and when numerous axillary shoots are developing simultaneously. Expression of LEB is often tran- sitory on a given shoot and limited to the first three or four unfolding leaves. Although LEB can occur on other poinsettia cultivars, ‘Gutbier V-14 Glory’ is especially sensitive. Leaves affected by LEB are highly susceptible to secondary Botrytis infection. This maybe a limited problem on stock plants, where prevailing environmental conditions are normally not conducive to its development and spread. However, Botrytis can be a severe problem during rooting of cuttings under mist and during cutting shipment. Symptoms of LEB are consistent with Ca deficiency symp- toms described in nutrient studies on poinsettia (Stuart and Rocke, 1951; Widmer, 1953; Struckmeyer, 1960; Tsutsui and Aoki, 1981). Leaf edge burn also resembles Ca disorders on other crops commonly labeled leaf tipburn (Shear, 1976). Preliminary research on vegetative ‘Gutbier V-14 Glory’ in- dicated that limited Ca translocation, N fertilizer form, and veg- Received for publication 27 Mar. 1989. Paper no. 16,952 of the Journal Series, Minnesota Agricultural Experiment Station. This research was supported in part by Paul Ecke Poinsettias, Inc., Encinitas, Calif. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. l Former Graduate Research Assistant, Dept. of Horticultural Science and Land- scape Architecture, Univ. of Minnesota. Currently Graduate Research Assist- ant, Dept. of Soil Science, Univ. of Minnesota, St. Paul, MN 55108. 2 Assistant Professor, Dept. of Soil Science, Univ. of Minnesota, St. Paul, MN 55108. 3 Professor, Dept. of Horticultural Science and Landscape Architecture, Univ. of Minnesota. Current address: Nurseryman’s Exchange, 2651 Cabrillo High- way North, Half Moon Bay, CA 94019. J. Amer. Soc. Hort. Sci. 115(1):73-78. 1990. etative growth rate were associated with LEB, but that direct ammonium toxicity was not a causal factor (Zakkour et al., 1986). Research on marginal bract necrosis of flowering ‘Gut- bier V-14 Glory’ found substantially more necrosis with 100% NH 4 -N fertilizer than with 100% NO 3 -N or various NH 4 -N : NO 3 -N ratios (Nell and Barrett, 1985). Maintenance of high fertility and moisture levels also increased bract necrosis (Nell and Barrett, 1986). Woltz and Harbaugh (1986) found foliar Ca sprays effective in decreasing necrosis. This effect was paral- leled by increased Ca concentrations in bract margins of sprayed compared to unsprayed plants. Conditions employed to limit Ca uptake resulted in a similar pattern of lower tissue Ca and in- creased bract necrosis (Woltz and Harbaugh, 1986). Numerous studies have shown that N-form has a profound effect on the cation-anion uptake balance of other nutrients (Haynes, 1986; Barker and Mills, 1980). Uptake of cations (including Ca) is depressed with NH 4 -N, while NO 3 -N stimu- lates cation uptake. There is also evidence that NH 4 -N inhibits xylem flow and, consequently, may inhibit Ca translocation (Haynes, 1986). Studies investigating variable NH 4 -N : NO 3 -N fertilizer ratios and their effects on poinsettia growth have found adverse re- sponses to NH 4 -N nutrition. Toxicity symptoms and poor growth are especially apparent when >50% of the N is supplied as NH 4 -N. Symptoms of NH 4 -N toxicity, including interveinal chlorosis and marginal leaf necrosis (Boodley, 1971; Cox and Seeley, 1984; Gaffney et al., 1982); short, brown, unbranched roots (Boodley, 1971; Cox and Seeley, 1984; Gaffney et al., 1982); leaf abscission (Boodley, 1971; Gaffney et al., 1982); and reductions in shoot growth (Boodley, 1971; Cox and See- ley, 1984; Gaffney et al., 1982; Tsutsui and Aoki, 1981) have been described. Leaf malformation, chlorosis, and marginal necrosis of poin- settia due to Mo deficiency also resemble LEB symptoms (Jungk et al., 1970; Winsor and Adams, 1987). Symptoms may be most characteristic on intermediate-aged leaves (Jungk et al., 1970) or may initially appear on younger leaves (see Royle, 73