Progress 07/15/04 to 07/14/07
Outputs
1. Collecting and testing Pythium and Fusarium isolates: Three Pythium myritolum isolates were obtained from decaying roots from a field production site and two greenhouses, and they all were highly virulent and caused severe root rotting and leaf losses within 10 days after inoculation. Nine Fusarium solani isolates were collected from rotting tubers of four cultivars; they all caused tuber rot, but varied considerably in aggressiveness. Three isolates were most aggressive and used in disease screening. 2. Optimizing inoculation techniques for F. solani: All nine isolates responded similarly to temperature (13, 18, 23, 28, and 33C ) in mycelial growth, with optimal growth being at 30.5C, but caused most severe tuber rot at low temperatures (13 and 18C). To avoid potential cold damage from 13C, 18C was chosen as the incubation temperature for inoculated tubers. Three inoculation techniques were developed for evaluating or screening caladium for resistance to Fusarium
tuber rot. 3. Identifying sources of host plant resistance to Pythium root rot: Out of 42 commercial cultivars inoculated with three Pythium isolates, seven cultivars showed a moderate level of resistance (partial resistance) to Pythium root rot. Pythium infection also caused leaf discoloration, epinasty, wilting, and collapse. A linear relationship existed between the root rot and leaf loss severity on Pythium-inoculated plants. 4. Identifying sources of host plant resistance to Fusarium tuber rot: Out of 17 commercial caladium cultivars inoculated with three isolates, five cultivars were resistant and two moderately resistant, respectively. A significant interaction was detected between isolates and cultivars. 5. Inheritance of caladium host plant resistance and ornamental traits: The distribution of root rot resistance among the 42 cultivars was continuous rather than discrete, indicating that the resistance is likely to be a quantitative trait. Most of the root rot-resistant
cultivars are fancy-leaves and white in color. To determine if there is any possible genetic linkage among leaf color, leaf shape, and Pythium or Fusarium resistance, segregating populations were developed. Progeny of these populations were propagated to produce tubers or plants for Fusarium or Pythium inoculation. It was determined that a single locus with three alleles controls the main vein color, a single locus with two co-dominant alleles controls the leaf shape, and leaf shape and vein color are inherited independently. The genetic relationship between tuber rot or root rot resistance and foliar characteristics was scheduled to be analyzed later. 6. Utilizing sources of host plant resistance to develop elite breeding lines: Four resistant cultivars were crossed with 15 major cultivars possessing desirable leaf or tuber characteristics. Approximately 100,000 progeny were generated and subjected to three screening schemes to identify resistant breeding lines. More than 100
advanced breeding lines were identified being disease resistant. Field and greenhouse tests for plant, foliar and tuber characteristics are under way.
Impacts
Fusarium tuber rot and Pythium root rot are the most destructive and economically important diseases in caladium, representative of a group of ornamental and food aroid plants widely grown in the tropical and subtropical U.S. and in the Caribbean region. The disease resistant cultivars identified in this study can be used as an integral component for developing an economical and effective long-term control of these diseases. The availability of these sources of host plant resistance, aggressive isolates, resistance assessment techniques, inheritance information, and advanced breeding lines will facilitate the development of new resistant cultivars. The information may apply also to genetic improvement of disease resistance in other important ornamental and/or food crop aroids.
Publications
- Deng, Z., B.K. Harbaugh, R.O. Kelly, T. Seijo, and R.J. McGovern. 2004. Evaluation of caladium cultivars for resistance to pythium root rot. HortScience. 39:772.
- Deng, Z., B.K. Harbaugh, R.O. Kelly, T. Seijo, and R.J. McGovern. 2004. Resistance of nineteen major caladium commercial cultivars to pythium root rot. University of Florida / IFAS, EDIS Extension Fact Sheet ENH 996, 4 pp. http://edis.ifas.ufl.edu/EP251.
- Deng, Z. and B.K. Harbaugh. 2005. Mode of inheritance for leaf shape and main vein color in caladium. HortScience. 40:1003.
- Deng, Z. and B.K. Harbaugh. 2005. Resisting root rot: University of Florida pathologists and breeders are evaluating and breeding caladium cultivars with resistance to pythium root rot. Ornamental Outlook. 14:20-21. Deng, Z., B.K. Harbaugh, R.O. Kelly, T. Seijo, and R.J. McGovern. 2005. Pythium root rot resistance in commercial caladium cultivars. HortScience 40:549-552.
- Deng, Z., B.K. Harbaugh, R.O. Kelly, T. Seijo, and R.J. McGovern. 2005. Screening for resistance to Pythium root rot among twenty-three caladium cultivars. HortTechnology 15:631-634.
- Deng, Z., B.K. Harbaugh, R.O. Kelly, T. Seijo, and R.J. McGovern. 2005. Screening for resistance to pythium root rot among twenty-three caladium cultivars. University of Florida / IFAS, EDIS Extension Fact Sheet ENH1008, 3 pp. http://edis.ifas.ufl.edu/EP259.
- Deng, Z. and B.K. Harbaugh. 2005. Inheritance of leaf shape and main vein color in caladium. University of Florida / IFAS, EDIS Extension Fact Sheet ENH1006, 7 pp. http://edis.ifas.ufl.edu/EP257. Deng, Z. and B.K. Harbaugh. 2006. Independent inheritance of leaf shape and main vein color in caladium. Journal of the American Society for Horticultural Science 131(1):53-58.
- Deng, Z., N.A. Peres, and B.K. Harbaugh. 2007. Host plant resistance to fusarium tuber rot in commercial caladium cultivars. University of Florida / IFAS, EDIS Extension Fact Sheet ENH1081. http://edis.ifas.ufl.edu/EP336.
- Goktepe, F., T. Seijo, Z. Deng, N.A. Peres, and B.K. Harbaugh. 2006. Techniques for evaluating caladium cultivars for host resistance to Fusarium tuber rot. HortScience. 41(4):1001-1002.
- Goktepe, F., T.E. Seijo, Z. Deng, B.K. Harbaugh, and N.A. Peres. 2007. Toward breeding for resistance to fusarium tuber rot in caladium: inoculation technique and sources of resistance. HortScience. 42(5):1135-1139.
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Progress 10/01/05 to 09/30/06
Outputs
1. Optimizing inoculation techniques for Fusarium solani: The relationship of temperature to mycelial growth of F. solani in vitro and to disease development in caladium tubers (in vivo) was investigated. Isolates responded similarly to temperature and growth was optimal at 30.5 C. However, tissue rotting in inoculated tubers was greatest at much lower temperatures. Isolates varied significantly in virulence of causing tuber rot, and three highly virulent isolates were identified. 2. Identifying sources of host plant resistance: Tubers of 16 commercial caladium cultivars were inoculated with three aggressive F. solani isolates. Cultivars were ranked for susceptibility to each isolate and the total rank sum used to place cultivars into four categories: resistant, moderately susceptible, susceptible, and highly susceptible. Four cultivars showed a moderate level of resistance to three isolates. 3. Inheritance study: Cultivars with different levels of resistance to
Pythium root rot and Fusarium tuber rot were crossed to develop segregating populations. More than 100 progeny of these populations were propagated and grown to produce tubers and plants for Fusarium and Pythium inoculation. Meanwhile, the inheritance of foliar characters including leaf shape and vein color were studied and found to be controlled by single genes with two or three alleles. Moreover, fifty-nine major cultivars and species accessions were analyzed using DNA markers. A low level of diversity was exhibited in cultivars, while a high level of diversity was present among seven species accessions. A small percentage of DNA fragments was present in cultivars and absent in the species accessions, while a high percentage of DNA fragments was present in the species accessions and absent in cultivars. Cultivars shared a higher level of similarity at the molecular level, formed a large group in cluster analysis, and concentrated in the scatter plot from a principal coordinate
analysis. There is a limited amount of molecular diversity in caladium cultivars, but a great repertoire of unique genes in species accessions that could be used to enhance the diversity in future cultivars, reduce potential genetic vulnerability, and improve disease resistance in cultivars. 4. Utilizing disease resistance in breeding: the identified resistant cultivars were used as breeding parents, crosses were made, approximately 10,000 progeny were screened, and more than 20 disease-resistant selections have been made.
Impacts
Fusarium tuber rot and Pythium root rot are the most destructive diseases of caladium. They can cause substantial reduction of tuber yield in the field and loss of tubers or tuber quality during storage. Little research on this topic had been done and little information was available prior to this project. The availability of these sources of host plant resistance, virulent isolates, resistance assessment techniques, and information about trait inheritance and diversity will greatly facilitate the development of new disease-resistant cultivars highly demanded for caladium tuber and plant production. The information gained and techniques developed should be applicable to some other important food and ornamental aroids.
Publications
- Deng, Z. and B.K. Harbaugh. 2006. Independent inheritance of leaf shape and main vein color in caladium. Journal of the American Society for Horticultural Science 131(1):53-58.
- Deng, Z. 2006. Disease Resistance Gene Analogs: Isolation, Identification and Applications. In: J.A. Teixeira da Silva (eds). Floriculture, Ornamental and Plant Biotechnology: Advances and Topical Issues (1st Edition). Global Science Books, London, UK.
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Progress 10/01/04 to 09/30/05
Outputs
(1) Identifying host plant resistance: By artificial inoculation of root balls of tissue culture-derived plants, we identified seven caladium cultivars that have resistance to Pythium root rot. The results were published in HortScience and HortTechnology. To identify sources of resistance to Fusarium tuber rot, we have screened 20 commercial cultivars or lines by assessing disease severity on naturally infected tubers or artificially inoculated tubers. One cultivar was highly resistant, while four cultivars were somewhat resistant, to Fusarium, based on naturally infected tubers. The observed difference in resistance among these cultivars was quite in agreement with caladium grower experience in their storage facilities. Five additional F. solani isolates were collected from diseased tubers, five artificial inoculation procedures were tested, and 18 inoculation experiments were conducted. The effects of a number of factors, including tuber chilling, inoculation sites,
inoculum delivery tools, humidity, temperature, exogenous ethylene, etc., on development of tuber rot, were investigated. These techniques are being used to confirm the observed resistance in caladium, examine the possible existence of F. solani races, and screen breeding selections. (2) Inheritance study of disease resistance: Four commercial cultivars of different resistance levels (Candidum, Gingerland, Red Frill, and White Christmas) were chosen and cross-pollinated. Nine populations with a total of 450 progeny were developed. Progeny are being increased and tubers being produced. Resistance of these progeny are to be evaluated in 2006. (3) Utilizing disease resistance in breeding: The identified resistant cultivars have been used as breeding parents, more than 500 crosses have been made, and 100,000 progeny have been developed. More than 2,500 selections, including 500 advanced selections, have been made based on leaf and tuber characteristics. Three rounds of Fusarium screening
were completed on 1000 selections, which resulted in identification of more than 10 promising lines with Fusarium tuber rot resistance.
Impacts
The identified sources of resistance to Pythium root rot and Fusarium tuber rot, better understanding of the inheritance of disease resistance and other important traits, and the generation of numerous populations and breeding lines will greatly facilitate genetic improvement of caladium resistance to soil-borne diseases and other characteristics critical to the caladium and related industries.
Publications
- Deng, Z., B.K. Harbaugh, R.O. Kelly, T. Seijo, and R.J. McGovern. 2005. Pythium root rot resistance in commercial caladium cultivars. HortScience 40:549-552.
- Deng, Z., B.K. Harbaugh, R.O. Kelly, T. Seijo, and R.J. McGovern. 2005. Screening for resistance to Pythium root rot among twenty-three caladium cultivars. HortTechnology 15:631-634.
- Deng, Z. and B.K. Harbaugh. 2005. Resisting Root Rot. Ornamental Outlook 14:20-22.
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Progress 07/15/04 to 12/31/04
Outputs
Significant progress has been made toward the objectives of this project. We identified three pathogenic and very aggressive isolates of Pythium myriotylum isolates and completed evaluation of major commercial caladium cultivars for their resistance to Pythium root rot. The majority of these cultivars were found to be susceptible or very susceptible to this disease, but seven cultivars in four leaf shape/color groups were moderately resistant to the disease. We compared the Pythium root rot resistance of seven cultivars with their mid-parent values, and the comparison revealed that the resistance score for the majority of the cultivars were below the midparent values, but one of the cultivars, Florida Blizzard, possesses resistance similar to its resistant parent. This indicates that the identified resistance could be passed onto progeny. The continuous distribution of root-rot disease scores among 44 cultivars indicates that the Pythium-root rot resistance may be a
quantitative trait with a polygene control. Populations have been developed for study of the inheritance of Pythium root rot resistance. The modes of inheritance for two important traits, leaf shape and main vein color, were studied, and they were found to segregate normally and be inherited independently. Controlled crosses have been made to transfer the identified root rot resistance to other leaf shape/color groups. Tens of thousands of progeny from crosses with identified resistant cultivars as parents were screened for leaf characteristics; selections are to be screened for tuber production, and disease resistance.
Impacts
The identified sources of resistance to Pythium root rot, better understanding of the inheritance of disease resistance and other important traits, and the generation of numerous populations and breeding lines will greatly facilitate genetic improvement of caladium resistance to soil-borne diseases and other characteristics critical to the caladium and related industries.
Publications
- Deng, Z., B.K. Harbaugh, R.O. Kelly, T. Seijo, and R.J. McGovern. 2004. Screening for resistance to Pythium root rot among twenty-three caladium cultivars. HortTechnology (in review).
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