Resistance of Some Rex Begonia Cultivars to Xanthomonas campestris pv. Begoniae

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University of Florida, IFAS
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report, RH-92-18

A.R. Chase *

Begonias were first found infected with Xanthomonas campestris pv. begoniae in the late 1920's (1). Symptoms can be confined to leaves, stems and petioles or sometimes a systemic infection results in a wilt. Foliar lesions are initially small, circular, translucent and expand with age into chlorotic and finally necrotic areas. Infections usually occur through hydathodes or wounds by bacteria moving in water. Systemic infections apparently result from invasion of wounded roots (2).

Although removal of symptomatic leaves is sometimes recommended for control of bacterial diseases, it is not advisable on Rieger Begonias since the pathogen readily becomes systemic in this host. Removal and destruction of systemically infected plants are warranted in light of the poor chemical control achieved with the bactericides available today (2, 4). Reduction of free water on plant foliage will decrease development of new infections of leaves but may not reduce spread of systemic infections. High fertilizer rates have been found to decrease severity of Xanthomonas leaf spot and blight of Rex Begonia but the best rate for disease reduction resulted in stunting, decreased leaf size and marginal burning on the plant (3).

Xanthomonas campestris pv. begoniae has been found on Rieger Elatior, fibrous, tuberous, Lorraine Elatior, and Rex Begonias with the most severe problems found on Riegers. Rieger cultivars are moderately to highly susceptible to this pathogen (5). Since information on susceptibility of Rex cultivars has not been published, the following study was undertaken to evaluate the relative susceptibility of eight Rex Begonia cultivars to X campestris pv. begoniae.

Liner size plants (3-5 inches tall) were obtained from commercial producers and established in 4 inch pots containing Vergro potting medium. They were top-dressed with 1.5 g Sierra slow-release fertilizer (17-6-12 with minor elements). Eight cultivars were included in Test 1: Cleopatra, Dew Drop, Helen Teupel, Marion Louise, Mikado, Phoenix Red, Red Pride, and Tiger Kitten. Three tests were performed using ten plants each of these cultivars between 18 October 1990 and 16 June 1992. Two additional tests with seven other cultivars were performed between 14 January 1991 and 16 June 1992. The following cultivars were included in Test 2: Duarten, Her Majesty, Meteor, Pauline, Peace, Red Dot, and Vesuvius. Plants were placed on a mist bench (15 sec. mist every 30 min. from 0800 to 2000 hr daily) one day before inoculation with the bacterial pathogen. Disease severity was evaluated six weeks after inoculation and consisted of an estimate of percentage of leaf surface with symptoms of disease. Data from each set of tests were combined and analyzed for significant effects.

Differences between trials of Test 1 and Test 2 were noted for overall disease severity with the first trial in each Test having the most severe disease. The first set of eight Rex Begonia cultivars showed significant differences in susceptibility to Xanthomonas campestris pv. begoniae (Figure 1). The most susceptible cultivar was Mikado with Dew Drop, Red Pride, and Phoenix Red also highly susceptible. Cultivars with the lowest susceptibility in Test 1 were Marion Louise and Helen Teupel. In Test 2, Meteor had the highest disease severity ratings with Pauline and Duarten least susceptible (Figure 2).

An overall ranking of the fifteen cultivars combining the two tests is given in Table 1. When all 15 cultivars were compared degree of susceptibility did not appear to be affected by leaf color, size, shape, and number of leaves per plant. These tests show that significant differences in susceptibility are present in the cultivars of Rex Begonia currently grown. Growers should choose cultivars with the lowest susceptibility to X campestris pv. begoniae as well as the growth characteristics they require.

Table 1. Relative susceptibility of fifteen Rex Begonia cultivars to Xanthomonas campestris pv. begoniae.

Figure 1. Susceptibility of some Rex Begonia cultivars to Xanthomonas campestris pv. begoniae (Test 1). Click image for larger view. [113k]

Figure 2. Susceptibility of some Rex Begonia cultivars to Xanthomonas campestris pv. begonia (Test 2). Click image for larger view. [127k]

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

Literature Cited

1. Ark, P. A. and C. M. Tompkins. 1939. Bacteriosis of tuberous begonia. Phytopath. 29:633-637.

2. Harri, J. A., P. O. Larsen and C. C. Powell, Jr. 1977. Bacterial leaf spot and blight of Rieger Elatior begonia: Systemic movement of the pathogen, host range and chemical control trials. Plant Dis. Reptr. 61:649-653.

3. Shaw, D. M. and A. R. Chase. 1991. Effect of fertilizer rate on susceptibility of Mikado Begonia to Xanthomonas campestris pv. begoniae. Foliage Dig. 14(12):1-3.

4. Strider, D. L. 1975. Chemical control of bacterial blight of Rieger Elatior Begonias caused by Xanthomonas begoniae. Plant Dis. Reptr. 59:66-70.

5. Strider, D. L 1975. Susceptibility of Rieger Elatior begonia cultivars to bacterial blight caused by Xanthomonas begoniae. Plant Dis. Reptr. 59:70-73.