Fusarium and Myrothecium Resistance in Twenty Dieffenbachia Cultivars

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

University of Florida
Institute of Food and Agricultural Sciences
Central Florida Research and Education Center Research Report

CFREC-Apopka Research Report RH-96-10

There are two major fungal pathogens of Dieffenbachia which can cause extensive damage during production: 1) Fusarium solani Sacc. causes a petiole rot (Chase et al., 1982); and 2) Myrothecium roridum Tode ex Fr. produces leaf spots (Chase et al., 1984). During Dieffenbachia production, frequent fungicide applications make it possible to grow disease susceptible plants; however, environmental concerns about extensive use, cost of chemicals, and legal ramifications make chemical use questionable. One way to reduce pesticide use is to develop disease-resistant plants.

Most Dieffenbachia cultivars originate from crosses of different cultivars or species or result from 'sports'. Breeding emphasis for Dieffenbachia focuses on development of new foliage colors and patterns, plant branching, size, growth rate, and overall aesthetic nature of the plant. Disease resistance is usually not a major emphasis in plant selection in breeding programs. Therefore, this research was undertaken to determine if resistance to these two important fungal pathogens exist within cultivars currently being produced.

Pathogen-free tissue-cultured Dieffenbachia plants were obtained from commercial tissue culture labs in 72-plug cell packs and 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. Four fungal isolates, two each, of F. solani and M. roridum were selected. 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 pathogencity 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 fungal species.

Fungal inoculum isolates were grown on Difco Potato Dextrose Agar (PDA), at 25 ± 1°C under cool white fluorescent lights on a 12 hours day / night cycle (49 ft-c). Fungal cultures were incubated 2 to 3 weeks for sufficient spore generation. Fungal spores were harvested from PDA plates by flooding the plates with sterile distilled water (SDW) and scraping with rubber spatulas. Before inoculations, spore concentrations were adjusted in SDW with the aid of a hemacytometer to 1 X 10⁶ conidia/ml. For M. roridum inoculations, the three uppermost expanded leaves of each plant were wounded using a cork with three insect pins imbedded in it, making a total of nine wounds per plant. No wounding was done with F. solani inoculations. Spore suspensions were applied to plant surfaces using hand sprayers until run-off. To maintain high humidity and aid in the infection process, plants were placed inside clear polyethylene plastic 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. Dieffenbachia plants which were inoculated with M. roridum were rated by counting total number of wounds which exhibited typical symptoms of M. roridum infection. To rate F. solani infections, total number of infected shoots per plant were counted and percent infection was determined. Re-isolations from representative symptomatic plants were made to verify presence of a causal disease agent in each experiment. Results from the four experiments with each pathogen were combined and data was compared using Tukey's LSD procedures.

Conclusions

None of the Dieffenbachia cultivars screened were immune to fungal pathogens, although several showed minimal infection levels. The cultivars Baccarat, Star Bright, and Parachute exhibited resistance to both fungal pathogens (Table 1). Of the remaining cultivars, 11 exhibited good resistance to Fusarium while 4 demonstrated resistance to Myrothecium.

Although no information exists regarding the genetic basis of disease resistance in Dieffenbachia, a previous report showed that resistance to Cylindrocladium root and petiole rot in Spathiphyllum was controlled by a single dominant gene (Henny and Chase, 1986). Such results are encouraging since both genera are members of the family Araceae. The results of this study will help in planning a breeding program whose goal is to produce disease resistant hybrids. Incorporating Bacara, Star Bright, and Parachute into a breeding program could lead to production of hybrids possessing resistance.

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

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References

1. Chase, A. R., and EL-Gholl, N. E. 1982. Stem rot, cutting rot, and leaf spot of Dieffenbachia maculata 'Perfection' incited by Fusarium solani. Plant Dis. 66:595-598.
2. Chase, A. R., and Poole, R. T. 1984. Development of Myrothecium leaf spot of Dieffenbachia maculata 'Perfection' at various temperatures. Plant Dis. 68:488-490.
3. Henny, R. J. and Chase, A. R. 1986. Screening Spathiphyllum species and cultivars for resistance to Cylindrocladium spathiphylli. HortScience 21:515-516.

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

1. ^aTotal number of infected shoots or fungal spots were counted for F. solani and M. roridum, respectively. Percent infection was then compared using Tukey's LSD procedures.