Evaluation for resistance to late blight (Phytophthora infestans (Mont.) de bary.) of potato (Solanum tuberosum) clones in an early generation selection program

University of Minnesota Master of Science thesis. October 2009. Major: Applied Plant Sciences. Advisor: Christian A. Thill. 1 computer file (PDF); ix, 104 p.Late blight (LB) is one of the most devastating diseases for potato worldwide. It is caused by the pathogen Phytophthora infestans which has re-emerged more aggressively because of the A2 mating type. The US-8 strain of late blight is difficult to control due to its resistance to metalaxy, the most common used fungicide. This disease affects potato foliage as well tubers. Resistance to diseases is generally evaluated late in breeding process. Selection to LB is done after selection for other traits has been done, when the population has been reduced. Early generation selection (EGS) allows for an increased number of desirable genotype to be evaluated. EGS strategy could be utilized to select for resistance to LB using minituber (Mt) or transplant seedling (Ts) populations. Progenies from 22 tetraploid families from parents with different levels of resistance were combined and evaluated. Mt and Ts progenies were planted at the LB disease nursery at Rosemount, MN in June 2002 and 2003 respectively. Area under disease progress curve (AUDPC) was calculated based on visual evaluations of plant percent necrosis. Families were divided in to six mating categories based on parent’s resistance. Differences were observed in families within mating categories and across propagule sources. Mt populations had a mean AUDPC of 10.25 with 9.74% resistant genotype. Mean AUDPC in Ts populations was 11.02 and 4.74% of the genotypes were selected. However, the method of propagation had no influence for the proportion of selected genotypes. Thus, EGS for LB resistance is possible when experimental material is evaluated in field conditions. Maternal effects were statistically different indicating a great contribution of resistance to both progenies. Resources such as human, money, time, genetic material among others are important in breeding programs, usually because a large number of genotypes are evaluated and many are discarded in the first year. Thus, it will be beneficial to know how many visual assessments are required for selection in EGS. A total of nine and ten visual evaluations were taken in 2002 and 2003, for Mt and Ts, respectively. Data subsets were formed by combining the total number of assessment days (AD), and average number of interval days (ID) between given assessments. Ts progenies had higher mean AUDPC scores than Mt. Three or more AD: ID combinations for Mt, and four or more AD: ID combinations for Ts were highly correlated with the final disease ratings, respectively. Low correlations with the method of propagation sources were observed for AD: ID combinations. Thus, a minimum of three and four visual assessments are required for Mt and Ts populations in an EGS for LB evaluation in a breeding program. Year-to year variation in disease scores of families in Mt and Ts was different to the similar response of the families as a population when propagation sources were compared. Thus, low correlations observed across Mt and Ts populations indicate the independence of propagation type. This independence in propagation need to be considerate when evaluations for LB resistance are done in first progenies assessed in the field. LB disease is partially controlled by agrochemical fungicides such as metalaxil that are expensive and harmful for the environment. After selecting genotypes with resistant to LB it is desirable to see the resistance remain stable across years. Late blight resistance of 75 resistant clones selected from single hill plots was evaluated across years and two seasons as four hills per genotype were considered in disease environment. Agronomic performance included total tuber number (TTN), total tuber weight (TTW) and specific gravity (SG) were evaluated as four hills per genotype in disease and non-disease environments. Decrease in resistance among the clones based on Relative area under disease progress curve (RAUDPC) was observed from 2002 (22.22) to 2004 (58.91). However, after three years of evaluation, up to 20% of the resistance originally identified hold over years. Differences in yield and yield components were observed within disease environment and across disease and non-disease environment. RAUDPC had a low correlation with yield and its components; a similar result was observed for (SG). Thus, selection for yield and yield components should be made later in the breeding process if resistance to LB is a priority