Breeding For Blight Resistance Research Articles

Half-sib progeny evaluation and selection of potatoes resistant to the US8 genotype of Phytophthora infestans from crosses between resistant and susceptible parents

The objectives of this study were to evaluate the use of potato (Solanum tuberosum L.) late blight (Phytophthora infestans (Mont.) de Bary) resistant parents in cultivar development and identify superior clones possessing moderate to high late blight resistance combined with acceptable maturity and tuber quality. Ninety-five crosses were made between eight unadapted parents with reported late blight resistance (B0718-3, Bertita, Bzura, Greta, Libertas, Stobrawa, Tollocan and Zarevo) and susceptible parents (cultivars or advanced breeding clones) adapted to North American growing conditions. A total of 408 field selected clones were assessed for late blight resistance in the greenhouse and in the field using a mixture of US8 P. infestans isolates (A2 mating type, metalaxyl resistant) that overcame all known R-genes except R8 and R9. Clones with ≤ 10% infected foliar area in the greenhouse test or ≤ 0.30 RAUDPC (relative area under the disease progress curve) value in the field in 1998 were re-tested in 1999. A total of 118 (29% of 408) putative late blight resistant clones were selected. The eight late blight resistant parents differed in both the ability to transmit late blight resistance and in the level of resistance transmitted to the progeny. The Tollocan and B0718-3 families (half-sib progeny) had the greatest degree of resistance and frequency of resistant clones. Scott-Knott cluster analysis ranked 79 clones (67% of 118) in the high and moderate late blight resistant groups. Among these 79 clones, 19 clones had vine maturity equal to or earlier than mid-season combined with acceptable tuber quality. Further selection in 2000 resulted in eight advanced selected clones (six from Tollocan and two from B0718-3 families) with the same level of resistance as the parent combined with vine maturity and tuber quality equivalent to Atlantic, a standard cultivar for chip processing in North America. The results indicate that this breeding approach can be used to select parents for late blight resistance breeding and to identify superior clones with high levels of late blight resistance and marketable vine maturity and tuber quality.

Identification of Resistance Genes Effective Against Rice Bacterial Blight Pathogen in Eastern India.

Breeding for bacterial blight resistance in rice requires an understanding of the contemporary pathogen populations in the locations where resistance genes are to be deployed. We characterized 450 strains of Xanthomonas oryzae pv. oryzae collected from three states of India using polymerase chain reaction fingerprinting and virulence analysis. This pathogen collection was differentiated into 17 haplotypes (12 lineages at 80% similarity level). Significant differences in the distribution of haplotypes were observed among regions. Virulence analysis of the pathogen collection revealed nine pathotypes. Among the populations from three regions, the Orissa population was the most diverse, consisting of 11 out of 17 haplotypes and five out of nine pathotypes detected in the total collection. Representative pathotypes were used to evaluate seven near-isogenic lines carrying individual bacterial blight resistance genes (Xa3, Xa4, xa5, Xa7, Xa10, xa13, and Xa21) and gene pyramids. Pathogen strains compatible to individual genes were present in detectable frequencies, although no single strain could overcome all resistance genes. Gene combinations Xa4 + xa5, xa5 + Xa21, and Xa4 + xa5 + Xa21 conferred a broad spectrum of resistance to all the strains evaluated, supporting the strategy of pyramiding appropriate resistance genes.

Constitutive hydrolytic enzymes are associated with polygenic resistance of tomato to Alternaria solani and may function as an elicitor release mechanism

Foliar resistance to early blight disease of tomato, caused by the necrotrophic fungus Alternaria solani, is inherited in a complex quantitative manner. Our previous studies revealed that three moderately-resistant tomato breeding lines with different sources of early blight resistance, all possessed higher constitutive and more rapid accumulation of PR proteins, including specific antifungal isozymes of chitinase and β-1,3-glucanase than susceptible genotypes. In the present study, additional early blight resistant tomato breeding lines and susceptible genotypes were investigated for their constitutive levels of PR proteins. All resistant lines possessed higher constitutive levels of chitinase and β-1,3-glucanase than susceptible genotypes supporting earlier reported findings. Constitutive and pathogen-induced expression of PR genes during early stages of the interaction was found to be much higher in the highly resistant breeding line NC 24-E than a susceptible line.In vitro elicitor release experiments were performed with purified A. solani cell walls treated with constitutive total enzyme preparations isolated from resistant and susceptible plants. Enzyme preparations from resistant and susceptible genotypes differed in their ability to produce hypersensitive response (HR) elicitors from fungal cell walls: the enzyme preparations from resistant breeding lines released HR elicitors, while enzyme preparations from susceptible lines did not. Additionally, experiments with a partially purified preparation of basic tomato chitinases demonstrated that these enzymes were able to release HR elicitors from germinating spores of A. solani, but not mature, intact cell walls. The possibility that constitutively produced hydrolytic enzymes may act as an elicitor-releasing mechanism in resistance to early blight of tomato is discussed.

Differential induction of pathogenesis-related proteins in tomato by Alternaria solaniand the association of a basic chitinase isozyme with resistance

Accumulation of pathogenesis-related proteins is thought to play a role in pathogen-induced plant defense responses. Although early accumulation of hydrolytic enzymes such as chitinase and β-1,3-glucanase has been associated previously with genetically-inherited and induced systemic resistance, their role in resistance in tomato (Lycopersicon esculentum)to the phytopathogenic fungus Alternaria solaniis not yet understood. Here we describe the accumulation patterns of specific isozymes of pathogenesis-related proteins in the resistant tomato genotypes 71B2, NC EBR-1, NC EBR-2 and the susceptible cultivar Piedmont. Western blot analysis demonstrated that four isozymes of chitinase (26, 27, 30, and 32kDa) were induced in all genotypes upon challenge with A. solani,but only resistant lines had significantly higher constitutive levels of the 30kDa isozyme as well as total chitinase activity. In addition, the 30kDa chitinase isozyme was found to accumulate to significantly higher levels in resistant lines during pathogenesis than the susceptible genotype. Two isozymes of β-1,3-glucanase (33 and 35kDa) were detected in all genotypes, but a slightly higher constitutive level was detectable in all resistant lines when compared to the susceptible. Similar accumulation patterns of these isozymes were observed in all genotypes during the course of pathogenesis. Purified preparations of acidic and basic tomato chitinase and β-1,3-glucanase isozymes were tested for their antifungal activity against A. solani in vitro.Results presented in this study indicate that only basic isozymes of chitinase and β-1,3-glucanase were inhibitory to A. solaniwhereas, no inhibitory activity was observed with the acidic isozymes. The results of this study suggest that a higher constitutive level of chitinase and β-1,3-glucanase and the induction pattern of a 30kDa chitinase isozyme in early blight resistant breeding lines is related to genetically-inherited resistance of tomato to A. solani.

Role of Natural Seed Infection by the Web Blight Pathogen in Common Bean Seed Damage, Seedling Emergence, and Early Disease Development

The web blight pathogen, Thanatephorus cucumeris (Rhizoctonia solani), was seedborne in both white- and pigmented-seeded genotypes of common bean (Phaseolus vulgaris) and was associated with blemishes and discoloration on the seed coat. The pathogen was consistently isolated from blemished seeds of six genotypes grown in fields where the predominant AG type of the pathogen was AG-I-IB (microsclerotial). Ninety-five percent of the isolates from seeds were AG-1-IB, and the remainder were AGI-IB (macrosclerotial), AG-2, and AG-4. The pathogen was isolated less frequently from unblemished seeds. T cucumeris was isolated from up to 17% of unblemished seeds from black-seeded genotypes, including the reported web blight resistant breeding line HT-7719. All seedborne isolates were pathogenic to bean seedlings. When seedborne in genotypes with white, red mottled, and black seed, T cucumeris significantly (P < 0.05) reduced seedling emergence and establishment. Isolates AG-1-IB and AG-2-2 added to the soil caused only slight injury to bean seedlings, whereas soil isolate AG-4 was very detrimental to seedling emergence and growth.

Treasure-digging for blight resistance in potatoes

After a historical survey of the breeding for blight resistance in potatoes the author concludes that neither the genes for hypersensitivity from Solanum demissum, nor those for field resistance as occur in the common potato can offer a final solution for the blight problem. Therefore other sources are necessary.

I. The breeding of resistant varieties of potatoes

1. An introductory historical survey of the breeding for late blight resistance is given. 2. The results of the investigation on specialization problems and on methods of selection for resistance for the interval of 1936–1945 (Muncheberg/ Mark) and 1947–1948 (Voldagsen/Hann.) are discussed. 3. The assortment of commercial varieties must be divided into two groups: (I) Pure tuberosum varieties, which are highly susceptible with the exception of very few varieties with some degree of resistance. (II) “Hybrid varieties” with different degrees of resistance, which may be used as differential varieties in specialization researches. 4. Some more wild species could be classified with regard to their behavior to late blight. Their genetics of resistance and their suitability for hybridization is discussed. A report is given concerning the results of the testing of selected hybrid clones for the interval 1947–1948.