Classical and Molecular Approaches to Breeding Horticultural Plants for Disease Resistance

colloquium was organized to review recent developments in classical and molecular approaches to breeding diverse horticultural plants for resistance to viruses, bacteria, and fungi. The reviews embody con- cepts, methods, strategies, and examples of current developments and successes that will interest breeders of all horticultural crops. A comprehensive classical breeding program for disease resistance involving cooperation between breeders and pathologists encom- passes most or all of the following areas: knowledge of the biology, strain, variation and virulence, and epidemiology of the pathogen; development of rapid inoculation methods with repeatable results; use of sets of differential varieties/lines (where available) to provide information on strain variation; easily applied and understood disease rating scales; identification of sources of resistance; knowledge of the genetics of resistance; development of effective selection and breeding schemes for resistance; and widespread evaluation of new lines along with controls in production areas to determine merit for release. while at the same time improving breeding methods and advancing knowledge in attaining that goal. This perspective needs to be imparted to graduate students, administrators of land-grant institutions, and funding agen-cies because of the continued erosion of plant breeding programs. It is imperative for the future well-being of humanity that we continue to train plant breeders to meet the stated goal. In some cases, sufficiently high levels of resistance to some pathogens of many crops have not been secured through classical breeding. In this category are physiological resistance to the fungus Sclerotinia sclerotiorum [(Lib.) de Bary], some gemini viruses, and bacterial pathogens. Molecular approaches alone, or in combination with classical breeding, may be the most useful approach to attain a desired resistance level. Breeding for virus resistance has proved to be long, difficult, and costly in some crops. Transgenic plants with high levels of resistance to some virus diseases have been developed in recent years through insertion of a gene controlling the virus coat protein into a host genome (see review by R. Grumet). This goal can be achieved in a short time in comparison with traditional breeding procedures. Grumet also discusses other strategies to develop transgenic virus-resistant plants. Classical breeding procedures have not been successful in introducing high levels of resistance to some bacterial pathogens into acceptable crop lines or varieties. The recent introduction of foreign genes that control specific peptides with antibacterial properties from nonplant sources into desired susceptible horticultural varieties may provide high levels of resistance to a broad spectrum of bacterial pathogens (see review by A.K. Kuehnle). This approach is particularly valuable where it is desired to maintain the original genotype of the variety, except for the introduced foreign gene, and where the classical backcross method is difficult to use or cannot be used to constitute the genotype due to incompatibility, as in apple ( Malus domestica Borkh.) (see review by S.M. Mehlenbacher). If an introduced foreign gene does not provide a high enough level of disease Genetic resistance offers one way to manage disease. When com- bined with the use of culture-indexed propagules and sanitation, it can be a powerful control. Classical breeding offers some genetic solu- tions, but little has been published regarding resistance to bacterial diseases of florist crops. This dearth contrasts sharply with the active resistance breeding for other horticultural and agronomic crops and with the progress made in fungal disease resistance in some flowers crops, e.g., Fusarium spp. in carnation and tulip, Puccinia horiana Henn. (white rust) in chrysanthemum, and Colletotrichum gloeosporioides Penz. (anthracnose) in anthurium (Aragaki et al., 1968; Sparnaaij, 1991). Introgression of resistance genes, either through tissue culture manipulations, use of molecular genetics, or by genetic engineering, are supplementary breeding approaches that appear prom- ising. Yet, despite the current and predicted importance of the floricul-ture industry to U.S. agriculture (1993 wholesale value approached $3 billion), little use of novel breeding strategies for (bacterial) disease control is evident in either the public or private sector. In the last several years, numerous papers describing genetic transformation systems for floricultural crops have appeared (Robinson and Firoozabady, 1993). Preceding and during this same period, numerous biological compounds have been identified with potent antimicrobial properties—research driven in by the biomedical and science The objective of this is to discuss previous and current research that uses novel genetic strategies to control bacterial disease in various cropping systems, with focus on integrating genetic engineering with nonplant resistance genes. A case study for controlling Xanthomonas spp. , species of which severely limit geranium and anthurium production, is given for anthurium, using genes derived from those of the Cecropia moth. species, their major effects on plant phenotype (often deleterious mutants), and the inability to score multiple morphological mutant traits in a single segregating population (Paterson et al., 1991). The development of markers based on allelic variants of specific enzymes (isozymes; Tanksley and Orton, 1983) and, most notably, of those based on variation in the length of DNA fragments obtained by digestion with restriction endonucleases (RFLPs; Botstein et al., 1980) has overcome these limitations. RFLP markers offer significant advantages over isozymes. They demonstrate more detectable loci and alleles, are phenotypically neutral, and can be scored at any stage of plant development. RFLP markers have been used extensively to “tag” useful genes in common bean ( Phaseolus vulgaris L., Degremont and Vallejos, Nodari lettuce ( Lactuca 1991), pepper ( Capsicum annuum potato ( Solanum tuberosum Barone and tomato ( Lycopersicon esculentum Mill., Klein-Lankhorts 1991a; Nienhuis Young 1988). However, development of RFLP markers involves a tedious, expensive, multistep process that requires considerable in-vestment in personnel, equipment, chemicals, and safety concerns if radioactive probes are used. The technology is not compatible with the needs of those breeders involved in resistance breeding and working with large populations and limited budgets. Recently, a of that the the of the with the that a of the of and of the the are in the seedling population. The to identify the of seedlings the that have potential as new cultivars. Evaluation is stepwise, with the number of selected genotypes decreasing in each step while the number of vegetatively propagated plants of In the first step, selection is based on the performance of the original seedling plant and is limited to highly heritable traits that can be evaluated quickly and inexpensively. In the second phase of evaluation, a small unreplicated trial is used to study the performance of selections more carefully. Typically, 10 plants of each selection are included. In the third stage of evaluation, advanced selections are grown in a replicated trial and compared to standard cultivars. other and a few progressive growers plant the best of these selections to see how they perform in their locations while yield are in progress. The very best selection is released as a cultivar.