One of the most serious bacterial diseases of Aroids is Xanthomonas blight caused by X. campestris pv. dieffenbachiae. This pathogen affects most members of the Aroid family including Aglaonema, Anthurium, Caladium, Dieffenbachia, Philodendron, and Syngonium. The disease first appears as tiny water-soaked spots on the edges or in the middle of leaves. These spots rapidly enlarge to form typical necrotic areas with chlorotic borders and can be confined to leaf margins only or appear anywhere on the leaf. On Syngonium, symptoms are frequently limited to interveinal areas while on Philodendron scandens subsp. oxycardium (heart-leaf phily) symptoms are present on leaf edges giving rise to the common name red-edge disease on this plant. Dieffenbachia are also affected by Xanthomonas blight and show a combination of marginal, interveinal and distinctly separate lesions nearly always with a bright yellow halo surrounding the necrotic areas.

In the past, control of Xanthomonas blight on ornamentals has been- marginal at best when bactericides were employed (1,2). Research on a wide variety of foliage plants has indicated that use of higher than recommended rates of fertilizer can reduce plant susceptibility to pathovars of X. campestris. For some of the plants tested, such as Syngonium (3), the host is not adversely affected by increased fertilizer levels, while on other plants, such as Anthurium (5), the host is damaged when higher than recommended rates of fertilizer are employed. The following report summarizes research performed to evaluate the potential for use of fertilizer to reduce severity of Xanthomonas blight on Dieffenbachia maculata cv. Camille.

Plant production. Rooted cuttings of Dieffenbachia maculata cv. Camille, obtained from commercial producers, were employed in two tests performed between 3 October 1989 and 28 March 1990. Plants were grown in 6 inch pots using a steam- treated medium consisting of Canadian peat and pine bark (1:1, by volume). Light levels ranged from 2000 to 2600 ft-c. with temperatures between 65 and 90°F. Ten plants were used for each of eight rates of Osmocote 19-6-12 (applied as a top- dressing): 1, 4, 7, 10, 13, 16, 19, and 22 g/6 inch pot. The recommended rate for dieffenbachias under these conditions is about 4 g (6). Plants height, number of leaves and leachate soluble salts [using the pour-through method (7)] were recorded at the beginning of each trial and again just prior to inoculation (about 8 weeks after fertilization). Plant quality was also recorded after 8 weeks on the following scale: 1 (dead), 2 (poor, unsalable), 3 (marginal, salable), 4 (good, salable), and 5 (excellent, salable).

Inoculation. One day prior to inoculation, plants were subjected to intermittent misting (15 sec/30 min for 12 hr/day). They were then inoculated by spraying with a suspension of the pathogen (X. campestris pv. dieffenbachiae - 1 x 10⁸ cells/ml) onto leaf surfaces with a pump action hand sprayer and immediately covering with a plastic bag for 24 hours. The percentage of the leaf area with symptoms of Xanthomonas blight was recorded about 3 weeks after inoculation.

Leachate soluble salts increased as fertilizer rate increased and ranged from 500 to 6500 mhos/cm. Increasing the Osmocote rate also affected height, number of leaves and plant quality of Camille. Rates between 4 and 13 g Osmocote gave the best growth although higher rates of fertilizer reduced height, leaf number and plant quality only slightly (Fig. 1 - 3). Only plants receiving the lowest rate of fertilizer (1 g) showed substantially reduced growth. The percentage of the leaf area with Xanthomonas blight decreased in a linear fashion as the rate of Osmocote increased (Fig. 4). Plants receiving 22 g Osmocote had only 30% the level of disease as those fertilized with the 1 g rate. These results are the same as those obtained for the majority of other ornamentals tested for fertilizer affects on Xanthomonas diseases including ficus, ivy, schefflera, and syngonium.

Ideally, fertilizer levels which reduce disease significantly should not result in plant damage, excessive costs (due to high rates), or significantly increased potential for ground water contamination. Fertilizer levels which reduced Xanthomonas blight of Dieffenbachia, by at least 50% (13 g), did not significantly reduce plant growth but represent a three-fold increase in the recommended rate of fertilizer for this plant. An additional consideration is that fertilizer rates used to decrease one disease do not increase the potential for other diseases. Previous research on Dieffenbachia showed that increased fertilizer levels resulted in increased severity of Myrothecium leaf spot disease (4). It is therefore unlikely that increasing the rate of fertilizer applied to Dieffenbachia to reduce Xanthomonas leaf spot would be beneficial in the overall production of the plant.

*Professor of Plant Pathology, Central Florida Research and Education Center - Apopka, 2807 Binion Road, Apopka, FL 32703-8504.

1. Chase, A. R. 1989. Aliette 80WP and bacterial disease control - Xanthomonas. Foliage Digest 12(11)1-3.

2. Chase, A. R. 1987. Update on controlling three bacterial diseases of foliage plants. Foliage Digest 9(8): 1-2.

3. Chase, A. R. 1987. Effect of fertilizer rate on susceptibility of Syngonium podophyllum 'White Butterfly' to Erwinia chrysanthemi and Xanthomonas campestris. Foliage Digest 10(8): 15-16.

4. Chase, A. R., and R. T. Poole. 1985. Host nutrition and severity of Myrothecium leaf spot of Dieffenbachia maculata 'Perfection'. Scientia Horticulturae 25:85-92.5.

5. Chase, A. R., and R. T. Poole. 1986. Effects of host nutrition on growth and susceptibility of Anthurium scherzeranum to Xanthomonas leaf spot. Nurserymens Digest 20(6):58-59.6.

6. Conover, C. A., and R. T. Poole. 1984. Light and fertilizer recommendations for production of acclimatized foliage plants. Foliage Digest 7(8):1-6. 7.

7. Wright, R. D. 1986. The pour-through nutrient extraction procedure. HortScience 21(2):227-229.

Figure 1. Effect of Osmocote 19-6-12 rate on height of Dieffenbachia maculata cv. Camille.

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Figure 2. Effect of Osmocote 19-6-12 rate on number of leaves on Dieffenbachia maculata cv. Camille.

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Figure 3. Effect of Osmocote 19-6-12 rate on plant quality of Dieffenbachia maculata cv. Camille. Plant quality was rated on a scale from 1 (dead) to 5 (excellent, salable).

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Figure 4. Effect of Osmocote 19-6-12 rate on percent disease on Dieffenbachia maculata cv. Camille caused by Xanthomonas campestris pv. dieffenbachiae.

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