<strong>Pre-breeding and breeding of potatoes for quantitative resistance to Phytophthora infestans on foliage and tubers and for diff erent utilization - problems, solutions and results</strong>

Abstract

This script informs about importance of late blight in potato, explains both types of resistance, and discusses factors infl uencing resistance expression. The state of research on pathogenicity of P. infestans as well as on resistance on foliage and tubers is reviewed. Epidemiological aspects in assessment of late blight resistance, its stability and the strategy of its use are comprehensively considered. Breeding for late blight resistance in past and present is analysed and reasons for mainly globally insufficient results are given on the background of about 45 years successful pre-breeding under long-day conditions at JKI Gross Luesewitz. Social aggravations for using polygenic resistance are mentioned.
The known methods of assessment of foliage and tuber blight resistance are rewieved and partly annotated. Higher demands are necessary in methods of assessment, which are mainly internationally insuffi ciently fulfi lled. Evaluation of genetic resources of tuber-bearing potato collections is critically estimated; international use of potential donors of resistance is listed for some countries. More eff ective search of sources of resistance is recommended.
The own strategy in assessment of resistance and breeding and own results in pre-breeding up to retirement of the author are presented on about a third of the pages. 33 pedigrees illustrate the breeding successions. 74 tables present results; 157 fi gures illustrate methodical details, show results or pre-breeding clones. The given results prove that quantitative blight resistance on foliage and tubers can be combined with all desired traits of potato including earliness and quality. This is just the opposite of the teaching of science and experience of majority of variety breeders. It is the intention of this script to change that by sharing information and communicate the know-how. The very extensive number of about 1400 references facilitates information in elder or ignored literature and may reduce repeated research.
Pre-breeding for quantitative blight resistance caused by minor genes requires another assessment practice of resistance and breeding strategy than for use of dominant major genes. Reasonable methods of large-scale screening of resistance were developed and suitable breeding conception was applied systematically at ZL Gross Luesewitz. The main problems of this prebreeding and their solutions are discussed. 197 accessions of 42 Solanum species were used by producing of seeds and starting selection of next parental generation over the years. Usually several genotypes of an accession were used as cross parents. As much as possible traits were tested several years besides blight resistance of foliage and tubers in wild potato clones too. The whole of the expression of all the traits decides to reject or to use a potential source of resistance or clones of its progeny as cross parent. Sources of resistance were exploited by crossing or by protoplast fusion. High susceptibility to virus diseases (PVY mainly, PLRV) frequently caused premature loss of wild clones or its progeny. Cross parents with very high resistance to viruses were used therefore at the beginning. Other undesirable negative ‘wild’ eff ects were reduced by following backcrossing and inter-crossing (resistant x resistant). 68 traits were considered in pre-breeding for quantitative late blight resistance; the methods of assessment of these traits and the scheme of selection are described. An important role played inter-crosses preferred of not related clones to resist loss of genes for blight resistance in meiosis. It is essentially to improve other traits by inter-crossing at the same time. Given results show that in our genetic broad based blight-resistant material enough variance for most traits was produced by suitable cross planning. Progress in polygenic or oligogenic traits occurred in little steps and requires planning of crosses beginning with wild species and beyond several generations. A series of at minimum eight cross steps will be necessary from the wild species to a new variety. It is necessary to decide the utilization of a blight resistant clone for processing, table potato or starch as early as possible.
The data of expression of 64 traits of 248 tetraploid and 61 dihaploid clones, grown in 2006, demonstrates the state of the progressed part of pre-breeding for late blight resistance at the ZL Gross Luesewitz. Slow, constant progress is shown in the most diffi cult part too, the combining with table potato quality. Some clones reached the level of good cultivars and give better preconditions to go on than in past. Above all, discolouration after cooking and uniformity of tuber size are to improve. Additionally a sub-programme is running to combine high starch content with blight resistance, earliness, resistance to Globodera pallida Pa2 and Pa3, G. rostochiensis Ro1 and Synchytrium endobioticum. Combining of suitability for processing and blight resistance has better results concerning French fries than crisps.
Three methodical variants to assess maturity were conducted in parallel, results are off ered. About 6% of the late blight resistant pre-breeding clones grown in 2006 were very early, 14% were early, 20% between early and second early, 31% second early, 19% second early to second late and 10% second late. The mathematical separation of the part of true late blight resistance of foliage from the eff ect of maturity (lateness) on disease progress is a primary condition of appropriate selection for foliage blight resistance. Sufficient genetic variance of resistance was ascertained in the pre-breeding material of ZL in JKI at Gross Luesewitz, even, if it was calculated as vertical distance of rAUDPC to the regression line with maturity (h² = 0.768).
Pre-breeding for blight resistance was conducted mainly on tetraploid level. Absence of male fertility in blight resistant dihaploids allowed only crosses resistant x susceptible up till now. So advantage of breeding on diploid level could not be exploited. Some clones were used by fusion on tetraploid level. Newest possibilities to introduce self-compatibility should be used.
69 tetraploid blight-resistant cross parents were handed over to cultivar-breeders since 1994, among them 27 usable for table purpose, 17 for processing and 18 clones for starch production. Six of 12 blight-resistant dihaploids were suitable for table potatoes, four for processing, and one for starch. Beyond that, seeds of 477 cross combinations with 100-900 seeds each were offered in 2005-2007. These parental clones are expected to become parents or grandparents of new varieties, which are expected to save 33% of current oomicide use in growing. The internationally leading state of ZL Gross Luesewitz in pre-breeding for quantitative blight resistance for temperate climate and long-day conditions should be maintained and used for cooperation with countries as China, South Africa, Australia, Argentina, Chile, Canada, Russia, and other.
Cooperation in research concerned broadening of basis of used sources of resistance and application of new methods (University Tuebingen) or introgression of marker-assisted selection for late blight resistance (CIP Lima). Acceleration of combining starch content with resistance of foliage and tubers and not late maturity was realized in a national project. Harmonizing of methods of late blight resistance assessment in Europe could be advanced in project EUCABLIGHT. Since decade’s foliage and tuber blight resistance of gene bank GLKS of IPK Gatersleben was tested by the same group of ZL at Gross Luesewitz.
Continuation of pre-breeding for quantitative late blight resistance of potato in departmental research of Federal Ministry of Food, Agriculture and Consumer Protection is legitimated by its potential contribution to environmental and consumer protection, by the breeding results up till now, by world-wide shortage of blight resistant pre-breeding material and cultivars, by necessity to counteract the loss of genes in potato breeding and by the demand on suitable potato material for basic and applied research. Conclusions for utilization of quantitative late blight resistance in future and recommendations are given for concentrated organization of pre-breeding for late blight resistance on potato in EU.