Xanthomonas and Erwinia Resistance in Twenty Dieffenbachia Cultivars

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D.J. Norman and R.J. Henny*

University of Florida/IFAS
Central Florida Research and Education Center
CFREC-Apopka Research Report RH-96-9

Dieffenbachia are among the most popular ornamental foliage plants in the United States, continually ranking in the top ten for annual wholesale value. Production of Dieffenbachia often requires frequent pesticide applications; however, environmental concerns about extensive chemical use and its associated liabilities as well as costs, make their recurrent use undesirable. One solution to reduce pesticide use is the development of disease-resistant plants produced by breeding. A prerequisite to such a breeding program is the identification of germplasm that contains genetic sources of resistance to major pathogens.

Therefore, this study was conducted to screen 20 popular Dieffenbachia cultivars for resistance to bacterial infections caused by Xanthomonas and Erwinia. Xanthomonas campestris pv. dieffenbachiae (McCulloch and Pirone) Dye causes a foliar blight while Erwinia chrysanthemi Burk causes a soft rot of leaves and stems.

The twenty Dieffenbachia cultivars used in this study (Table 1) were obtained from commercial tissue culture labs in 72-plug cell packs. Plants were potted into 10 cm pots (450 cm2) containing Vergo Container Mix A (Verlite Company, Tampa, FL 33610) amended with Sierra Plus Minors 17N-6P-12K (Grace/Sierra, 1001 Yosemite Dr., Milpitas, CA 95035) at the rate of 1.5 g/pot. Plants were allowed to grow for 2 months before they were inoculated. Two bacterial strains of X c. pv. dieffenbachiae and E. chrysanthemi were selected for this study. Criteria for selecting pathogens were as follows: 1) pathogens had to have been isolated from Dieffenbachia plants; 2) pathogens had to have been isolated from different locations in Florida to maximize their genetic variability; and 3) pathogens must have exhibited aggressive pathogenicity toward Dieffenbachia plants. Experiments were conducted in a randomized complete block design, having six blocks each containing twenty plants per block. A seventh block containing non-inoculated control plants was kept separate to limit possible cross contamination from inoculated plants. Each test of the 20 cultivars was conducted twice during warm weather from June to August and twice again during cooler weather from October to December, for a total of four separate tests for each pathogen species.

For production of bacterial inoculum, cultures of X c. pv. dieffenbachiae and E. chrysanthemi were grown for 48 hours at 28 ± 1°C on Difco Nutrient Agar (NA) amended with 5% sucrose. Bacteria were harvested from NA plates, suspended in saline (NaCl 8.5 g/l) and adjusted spectrophotometrically at A₆₀₀ to 1 X 10⁸ colony forming units / ml. Leaves and stems were sprayed with a bacterial suspension and enclosed in clear polyethylene bags for 24 hours. Inoculated Dieffenbachia plants were kept in a fiberglass house with temperatures maintained between 65 and 90°F and light at 1400 ft-c. In each experiment, non-inoculated plants (sprayed with saline) of cultivars were used as controls. To aid in bacterial infection, plants were placed in a fiberglass house equipped with overhead misters which misted plants for 15 seconds every 30 minutes during daylight hours. Percent of foliage exhibiting disease symptoms was visually estimated after 4 weeks and percentages were ranked using a pre-transformed rating scale (Little and Hills, 1978) as follows: 0 = no symptoms; 1 = 1-10%; 2 = 11-35%; 3 = 36-65%, 4 = 66-90%, and 5 = 91-100%.

Conclusions

The cultivars Camille, Compacta, Hilo, Parachute, Rebecca, and Triumph exhibited resistance towards both Xanthomonas and Erwinia. These cultivars showed minimal infection levels; however, none were immune to the pathogens (Table 1). Of the remaining cultivars, 7 exhibited good resistance to Erwinia and 3 showed resistance to Xanthomonas.

In this study, similar responses by closely-related cultivars indicates a probable genetic basis for resistance. For example, Camille and Compacta demonstrated resistance to both bacterial pathogens and these two cultivars are almost genetically identical. Sports from one variegation form (i.e. Camille to Compacta and visa versa) to the other are common in commercial production facilities that produce both cultivars via tissue culture. Likewise, one of the two least resistant cultivars, Princess, is a sport of Golden Sunset, another of the least resistant cultivars.

The majority of Dieffenbachia cultivars are propagated via tissue culture. A major factor in the growth of the tissue culture industry is to eliminate the problem of disease-induced losses encountered in long-used vegetative stock beds. Indexed tissue-cultured propagules have reduced the occurrence of pathogens in Dieffenbachia production (Chase et al., 1981); nevertheless, bacterial pathogens can eventually be introduced to production facilities by wind, water, contaminated soil, tools and workers. Development of resistant cultivars would further benefit Dieffenbachia production.

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*Assistant Professor of Plant Pathology and Professor of Environmental Horticulture, respectively. University of Florida, IFAS, Central Florida Research and Education Center, 2807 Binion Road, Apopka, FL 32703-8504.

References

1. Chase, A. R., Zettler, F. W., and Knauss, J. F. 1981. Perfection 137B, a pathogen-free selection of Dieffenbachia maculata derived through tissue culture. Fla. Agric. Exp. Stn. Circ. S-280. 7pp.
2. Little, T. M. and Hills, F.J. 1978. Agricultural experimentation design and analysis. John Wiley and Sons, New York, pp 350.

1. Table 1. Results of resistance testing of twenty cultivars of Dieffenbachia with two major bacterial pathogens affecting production. Means followed by different letters are significantly different (p = 0.5).

1. ^aPercent of foliage exhibiting symptoms was visually estimated and percentages were ranked using a pretransformed rating scale (Little and Hills, 1978) as follows: 0 = no symptoms; 1 = 1-10%; 2 = 11-35%; 3 = 36-65%; 4 = 66-90%; 5 = 91-100%. Cultivars were compared using Tukey's LSD procedures.