Click image for larger
view. Return to: MREC Home Page
Return to: MREC Research Index
University of Florida, IFAS,
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report, RH-91-9
D. Michael Shaw and A. R. Chase*
"One of the oldest known bacterial diseases of a foliage plant is caused by Xanthomonas campestris" (6). Because of the range of plants that X. campestris infects, serious losses occur in the ornamental industry yearly warranting extensive research regarding control. Xanthomonas diseases cause serious losses in the ornamental industry affecting many different commercially grown plants (5).
Xanthomonas campestris is a difficult disease to control; therefore, "an integrated approach to disease control should include as many factors as possible" (6). Several methods employed in combating X. campestris include bactericides, fertilizer rates and sources and cultural changes in the greenhouse environment. Bactericides are commonly used for disease control on some hosts but are not very effective and may be phytotoxic to the plant (4). Attempts to control bacterial diseases should include elimination of overhead irrigation (4) as well as manipulating fertilizer rates supplied to the host plant. Previous work has shown that by increasing rates of fertilizer it is possible to lower the rate of infection of X. campestris on a variety of ornamental plants (1, 2, 3). Furthermore, since X. campestris is a motile organism (flagella), adequate crop spacing can eliminate physical contact between plants, making it less likely for infection to pass from one plant to another.
Although increasing rates of fertilizer seems to reduce X. campestris infection, it can also have negative effects on the host plant. For example, higher rates may cause leaf burn, root damage, and growth inhibition. Other considerations that must be taken into account are the concern for ground water contamination due to fertilizer leachate and the cost of the fertilizer itself.
This report evaluates the effect of fertilizer rate on severity of Xanthomonas campestris on Begonia rex-cultorum cv. Mikado. A rate of fertilizer which is capable of reducing the rate of Xanthomonas infection without causing damage to the host plant would be ideal. With this type of control it may be possible to lower the amount of bactericides applied.
Materials and Methods
Rooted cuttings of Begonia rex-cultorum cv. Mikado were obtained from a local commercial foliage grower. Eighty plants were used in a test begun 21 January 1991 and terminated 22 April 1991. Plants were grown in a steam-treated medium consisting of one part Canadian peat and one part pine bark (1:1 v:v). Four rates of slow-release fertilizer (Osmocote 19-6-12, Grace-Sierra, Milpitas, CA 95035) were used: 1, 6, 11 and 16 grams per 6 inch pot. Light levels ranged from 1500 to 2500 ft-c. Plant height, number of leaves, leaf length, leaf width, top grade (plant quality), leachate soluble salts [using pour-through method (1)] were recorded as well as fertilizer damage and disease symptoms produced by X. campestris.
INOCULATION: Plants were placed in intermittent mist (15 sec/30 min for 12 hr day) 1 day prior to inoculation and were inoculated with a solution of Xanthomonas campestris pv. begoniae (1 x 107 colony forming units per ml) by a hand sprayer onto leaf surfaces. After inoculation, plants were placed in plastic bags for 24 hours, with mist treatment continuing until termination of the experiment.
Results and Discussion
Twenty-one days after fertilizer application plant height remained comparable for plants treated with 6, 11, and 16 g/6-inch pot. Forty-nine days after treatment, significant differences were seen, with plants receiving the highest rate of fertilizer (16g) being most stunted. The effects of fertilizer on total number of leaves were also substantial with that of lg having the most leaves to 16g having the least leaves (Figs. 1 and 2). Leaf length and leaf width did not show a significant difference statistically, but some trends were recorded (Figs. 3 and 4). It was shown that there were slight differences in plant quality due to fertilizer levels, where plant quality was based on a scale of 1-5 (l=dead, 2=not salable, 3=salable, 4=good, salable, 5=excellent quality plants) (Fig. 5). Leachate soluble salts increased as fertilizer rates increased with all soluble salts levels lower after 49 days as compared to 21 days (Fig. 6). The extent of fertilizer damage was significant in that plants receiving 1 or 6g showed the least amount of damage; those receiving 11 or 16g showed increasing levels of damage in order of fertilizer rate (Fig. 7).
Xanthomonas damage was significantly affected by fertilizer levels (Pr >F 0.0001). Plants receiving 16g showed less than 1% damage due to Xanthomonas while those receiving 1 or 11g showed significantly higher rates of damage. Plants receiving 6g Osmocote showed comparatively less damage than plants with 1 and 11g. This may be a result of insufficient replications, since disease severity rating varied widely. Past literature has shown that increasing to 20 or more replicates can eliminate these unusual dips, since the degree of
Summary
Increasing fertilizer rate is an extremely successful way to control Xanthomonas leaf spot, but cannot be recommended for Begonia rex-cultorum cv. Mikado. The extent of damage due to the rate of fertilizer far outweighed the beneficial effects. Because successful control of X. campestris on 'Mikado' Begonia required the highest rate of fertilizer tested (16g which is 3 times the recommended rate) it would not be cost effective and other cultural changes are recommended to control disease development. Apparently, successful control of Xanthomonas on 'Mikado' Begonia requires the highest rate of fertilizer (16g) utilized which is almost 3 times the recommended rate. It is doubtful that this would be cost effective and other cultural changes should be made to control development of Xanthomonas leaf spot on Begonia rex-cultorum cv. Mikado.
*Technical Assistant and Professor of Plant Pathology, respectively, CFREC-Apopka, 2807 Binion Rd., Apopka, FL 32703-8504.
Literature Cited
Figure 1. Effect of Osmocote 19-6-12 rate on plant height of Mikado Begonia.
Figure 2. Effect of Osmocote 19-6-12 on number of leaves on Mikado Begonia.
Figure 3. Effect of Osmocote 19-6-12 rate on leaf length of Mikado Begonia.
Figure 4. Effect of Osmocote 19-6-12 rate on leaf width of Mikado Begonia.
Figure 5. Effect of Osmocote 19-6-12 on top grade; evaluated on a scale from 5 (excellent salable) to 1 (dead).
Figure 6. Effect of Osmocote 19-6-12 on soluble salts of Mikado Begonia.
Figure 7. Effect of Osmocote 19-6-12 rate on damage of plant tissue. Based on a scale from 5 (dead) to 1 (none).
Figure 8. Effect of Osmocote 19-6-12 on percentage of damage caused by Xanthomonas blight.
Click image for larger
view. 
Click image for larger
view. 
Click image for larger
view. 
Click image for larger
view. 
Click image for larger
view. 
Click image for larger
view. 
Click image for larger
view.