A major project studying the role of fertilizer in bacterial disease severity has been conducted at the Central Florida Research and Education Center in Apopka over the past ten years. Many diseases of foliage plants, caused by Xanthomonas campestris pathovars, are less severe when plants receive higher than recommended rates of a balanced fertilizer whether in a slow-release or liquid form. Additional research on some plants showed that nitrogen was the most important nutrient, although potassium and phosphorus also can affect disease severity. The following research was conducted to determine effects of nitrogen and potassium on severity of Xanthomonas leaf spot of Ficus benjamina caused by X. campestris pv. fici.

Ficus benjamina cuttings were established in steam-treated potting medium consisting of Canadian peat and pine bark (1:1 by volume). The medium was amended with 7 lbs dolomitic lime and 1 lb of Micromax per cubic yard. Single superphosphate was added as a top dressing, prior to testing, at about 1 9/6 inch pot. Three experiments were performed between May and December 1989. In each experiment, ten plants in 6 inch pots were used per treatment. Treatments included three rates of a slow-release nitrogen (39:0:0 - 3, 6, or 9 g/pot) in combination with three rates of potassium (0:0:46 - 2.5, 5.0, and 7.5 g/pot). Plants were irrigated by hand two or three times a week as needed. Maximum light levels were 3000, 2500, and 2200 ft-c for experiments 1, 2, and 3, respectively. Minimum and maximum temperatures were about 60 to 95°F depending upon time of year. Leachate for electrical conductivity and pH measurements was obtained monthly by adding water to the surface of each pot until about 4 oz of leachate collected in a beaker beneath the pot. Plants were grown for two months following the fertilizer application. Plant height was recorded at test initiation and completion. In experiments 2 and 3, top quality was rated also on the following scale: 1 - dead; 2 = poor, unsalable; 3 - moderate, salable; 4 = good, salable; and 5 - excellent, salable.

One day prior to inoculation, intermittent misting was initiated and continued until test completion. Plants were inoculated with the a suspension of Xanthomonas campestris pv. fici adjusted to 1 x 10 bacteria/ml. The suspension was sprayed onto leaves and plants were covered with a plastic bag for one day. The number of lesions per plant was recorded 7 to 14 days after inoculation.

Leachate electrical conductivity increased as nitrogen level increased, but was not consistently affected by increases in potassium level (Table 1). Leachate pH was affected by nitrogen level but results were inconsistent (Table 1). Increasing nitrogen level decreased plant height in the first experiment but had no effect on height in experiments 2 and 3 (Figure 1).

Top quality slightly increased as potassium level increased in experiment 2 and increased as nitrogen level increased in experiment 3 (Figure 2).

The number of lesions per plant decreased as nitrogen level increased in each of the three experiments (Figure 3). Potassium level affected disease severity in experiment 1 only with increased potassium levels decreasing number of lesions. Disease severity was reduced significantly when nitrogen level was increased. Intermediate rates of nitrogen should reduce disease severity significantly without damaging the host. Although the highest rate of nitrogen did not appear to reduce top quality very much in these experiments, lower rates should be employed to minimize fertilizer cost, potential for ground water contamination and damage to the crop under other growing conditions. Additional information on the role of fertilizer in severity of Xanthomonas leaf spot of Ficus can be found in the paper listed below.

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

1. Chase, A. R. 1988. Effect of fertilizer rate on susceptibility of Ficus benjamina to Xanthomonas campestris pv. fici. Proc. Fl. State Hort. Soc. 101:339-340.

^aSignificant effects were evaluated using analysis of variance and denoted as significant at the 5% (*) or 1% (**) level or not significant (ns).

Figure 1. The effect of nitrogen and potassium levels on height of Ficus benjamina 2 months after fertilizer application.

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Figure 2. The effect of nitrogen and potassium levels on top quality of Ficus benjamina 2months after fertilizer application. Top quality was rated on the following scale: 1 = dead; 2 = poor, unsalable; 3 = moderate, salable; 4 = good, salable, and 5 = excellent, salable.

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Figure 3. The effect of nitrogen and number of lesions on Ficus benjamina caused by Xanthomonas campestris pv. fici.

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