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Volume 44, Number 2
Spring 1990

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Nutrient Deficiency Symptoms in Rhododendron
Von D. Jolley & Tim D. Davis
Brigham Young University
Provo, Utah

        Rhododendron 'Catawbiense Album' plants were grown in 12 different nutrient solutions, each devoid of an essential element. Descriptions specific for deficiency symptoms of nitrogen, iron, boron, magnesium, and sulfur are provided. Less specific deficiency symptoms for copper, manganese, zinc, and calcium were observed. Deficiency symptoms did not develop for molybdenum, phosphorous, and potassium during the 306-day study period.

Rhododendrons (Rhododendron sp.) are among the most popular woody ornamentals cultivated in the U.S. This diverse genus is grown over a wide variety of conditions and is, therefore, subjected to a myriad of environmental stresses. As with most plant species, environmental conditions such as extreme temperatures, drought, flooding, or improper light lead to stresses which in turn predispose plants to damage from diseases, insects or nutrient deficiencies (Antonelli etal., 1984). Many of these stresses have been identified and described in detail and, in some cases such as cold hardiness, root weevil resistance, and shade tolerance, have been rated and categorized.
        Nutrient deficiencies are potential environmental stresses which have received limited attention in rhododendron literature. Identification and description of the symptoms are critical because deficiencies not only detract from the aesthetic value and enjoyment of rhododendrons, but also predispose plants to infestation by insects and diseases. This may eventually cause death of the plants. Our objective was to impose 12 essential nutrient stresses on rhododendron plants in hydroponic solutions and to describe the ensuing deficiency symptoms.

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Materials and methods
The nutrient deficiencies imposed were boron (B), calcium (Ca), copper (Cu), iron (Fe), magnesium (Mg), manganese (Mn), molybdenum (Mo), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), and zinc (Zn). Plants were also grown in complete nutrient solutions for comparison with the stressed plants.
        Liners of Rhododendron 'Catawbiense Album' received from the nursery had been grown in coarse peat/soil mix in 5-cm (2-in.) pots. Roots were cleaned and rinsed vigorously to remove all peat/soil mix and placed into dilute, but complete, modified Hoagland's (Hoagland and Arnon, 1950) nutrient solutions to allow adjustment to solution culture (any damaged tissues, either leaf or root, were discarded). Plants were grown in this pretreatment growth solution from March 21 through May 28, 1987 (69 days) and during this period there was one flush of growth.
        Individual plants were randomly selected to be placed in containers with 3.5 L solution per plant. Immediately before transfer into treatments, sufficient EDDHA [Ethylene-diamine di (O-hydroxyphenylacetic acid)] was added to the pretreatment growth solution for one hour to sequester any micronutrients from the roots of the plant, thus preventing their accidental transfer into the treatment solutions. Plant roots were thoroughly rinsed in double deionized water just before transfer into treatments. The complete nutrient solution control contained [in millimoles/L (mM)]: Ca, 2.38; Mg, 0.51; N, 7.12 (6.59 as nitrate and 0.53 as ammonium); P, 0.16; K, 1.82; S, 0.10; Cl, 0.44; Zn, 1.53 x 10-3; Mn 5.90 x 10-3, Mo, 0.26 x 10-3 Na, 0.53 x 10-3; Fe, 17.9 x 10-3. All treatments received 6.3 mM HEDTA (hydroxyethylethylene diaminetriacetic acid) chelate.
        The 12 treatment solutions were comparable except that the desired mineral stress element was excluded. At least four replications of each treatment were maintained for the entire time. Nutrient solution pH values were 6.2-7.0 initially and were adjusted periodically to 4.5 by the addition of dilute HCl. Treatments were imposed from May 28, 1987 to March 28, 1988 (306 days) and fresh solutions were provided on September 24, 1987 (120 days). Solutions were mixed in double deionized water and were maintained at a constant level by daily additions of the same water. Plants were also monitored daily for nutrient deficiency symptoms. Visual observations were recorded and photographs were taken periodically. Plants were grown under greenhouse conditions and during summer months shade was provided by glass-lime and shade cloth. Temperatures varied from 17 to 28° C.

Results and Discussion
Well-defined nutrient deficiency symptoms were observed for B, Fe, Mg, N and S. Less well-defined symptoms or no symptoms were observed for Ca, Cu, Mn, Mo, P, K and Zn. The development of the nutrient deficiency symptoms are presented herein. These symptoms developed under hydroponic conditions and may not be identical under field conditions. Furthermore, the study was conducted with one cultivar and it is unknown how other cultivars might respond to these same treatments.

Within 65 days after transfer into stress treatments, N deficiency symptoms were visible. Uniformly pale yellow leaves developed with severe stunting. Once these symptoms appeared there was no further bud development in the axils of old leaves and no extension of new leaves, but there was prolific root growth. Control plants (full nutrients) experienced three growth flushes in the same period during which N deficient plants made no growth. At later stages (250 days and thereafter) internodal tissues as well as leaves were chlorotic. The chlorosis observed was much more severe than that reported by Antonelli etal. (1984). Also, the developmental pattern of N deficiency symptoms was atypical compared with other species, wherein symptoms appear first in the oldest leaves.

Deficiency symptoms of Fe began to appear about 65 days after imposing treatments. Typical deficiency symptom development was for the uppermost leaves to develop yellow veins with green interveinal areas. New leaf buds appearing after this initial symptom were bright yellow before opening, and upon opening developed a mottled yellow-green appearance. Despite widespread leaf symptoms, new buds continued to develop. The second set of buds opening after these initial symptoms were also yellow, but as new leaves unfolded, they turned brown and became necrotic. As this occurred, the initially chlorotic leaves re-greened somewhat, but maintained a splotchy yellow-green appearance. Unlike with N, Fe-deficient plants continued to develop new buds throughout the experimental period, and except for the second set of buds, they developed new leaves and even flowered after about 300 days. Leaf symptoms observed were similar to those shown in Antonelli et al. (1984), but these bud symptoms have not been reported. Root growth was prolific despite the lack of Fe in the solution.

Development of B deficiency symptoms occurred about 95 days into treatments. There was a lightening of the terminal bud and expanding leaves became spindly and pointed. Two or three of the young leaves died within 5 weeks of the initial symptoms. No new buds developed after this date. By 175 days, these leaf buds and young leaves had dropped from the plant. There was considerable twisting or curling at the point of attachment of older leaves immediately following the initial symptoms. Little or no root growth occurred after initial symptoms appeared.

It was 110 days after initiation of treatments before leaf Mg deficiency symptoms appeared. As leaves expanded an uneven mottled appearance developed. With time mottling became more uniform with the midribs yellow and small veins green. Within 60 days mottled leaves took on a greyish tint. There was no root growth on the Mg-deficient plants during the 306 days of treatment. Scaife and Turner (1984) presented almost identical leaf magnesium deficiency symptoms for broad bean.

Although some yellowing in and around midribs was apparent after 260 days, it was not until near the end of the growth period (306 days) that distinct symptoms could be identified. The leaf canopy was uniformly chlorotic and stems took on a yellow cast as well. Prolific root growth was observed in S-deficient plants. The reason for this response is unclear.

Although symptoms were observed for some of the other tested nutrients, they could not be considered definitive or specific to one nutrient. In some cases, similar symptoms were observed for more than one nutrient e.g., Cu, Mn, and Zn. This could be explained by widely observed interactions between the various micronutrients in numerous other species (Olsen,1972). In the case of Ca, neither top nor root growth occurred in the hydroponic system and roots eventually turned black. There was no development of deficiency symptoms with Mo, P, and K. Hence, it seems unlikely that these latter nutrients would ever be deficient under most conditions.

The authors wish to thank the Research Foundation of the American Rhododendron Society for funding the project and the Briggs Nursery, Olympia, Washington, for providing rhododendron plants used in the study.

Antonelli, A.L., R.S. Byther, R.R. Maleike, S.J. Collman and A.D. Davison. 1984. How to identify rhododendron and azalea problems. E.B. 1229. Cooperative Extension, Washington State University, Pullman, Washington.
Hoagland, D.R. and D.I. Arnon. 1950. The water culture method for growing plants without soil. Circ. 347. Calif. Agric. Expt. Sta., Berkeley, California.
Olsen, S.R. 1972. Micronutrient interactions, p. 243-261. In. J.J. Mortvedt, P.M. Giordano and W.L. Lindsay (ed). Micronutrients in Agriculture. Soil Science Society of America, Madison, Wisconsin.
Scaife, A. and M. Turner. 1984. Diagnosis of mineral disorders in plants. Volume 2. Vegetables. Chemical Publishing, New York, New York.

Von D. Jolley and Tim D. Davis are affiliated with the Department of Agronomy and Horticulture, Brigham Young University, Provo, Utah.

For additional information on rhododendron nutrition see "Phosphorus and Nitrogen Nutrition of Rhododendrons," by George F. Ryan, elsewhere in this issue and "Deficiency Symptoms of Rhododendron Studied," by C. R. Johnson and A. N. Roberts, Quarterly Bulletin American Rhododendron Society, Vol. 24: n2.

Volume 44, Number 2
Spring 1990

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