Abstract
Potato cyst nematodes (PCN), Globodera pallida and Globodera rostochiensis, are among the most damaging pests of potatoes. These species co-evolved with solanaceous plants in the Andean region of South America, the center of origin for the potato. In the U.S., infestations of PCN remain highly localized but pose a substantial threat to domestic and international trade. This dissertation 1) examines the phenotypic virulence spectrum of PCN populations collected from Peru compared to the U.S.; 2) characterizes the phylogenetic structure of native Andean PCN compared to introduced lineages; and 3) optimizes biofumigation strategies as alternative tools to control existing G. pallida infestations.Using a diverse panel of potato genotypes, standard differentials, breeding clones, and commercial varieties, this study quantified the virulence of 10 Globodera populations from Peru alongside U.S. reference populations (G. pallida Pa2/3 from Idaho and G. rostochiensis Ro1 from New York). While the U.S. reference populations showed expected virulence profiles, the Peruvian populations displayed highly complex, diverse phenotypes. A distinct "Pa1-like" G. pallida lineage (PER10) was identified that was avirulent on the H2 differential clone P55/7 and partially overcame Pa2/3 resistance in the commercial cultivar ‘Maria Huanca’. Furthermore, specific Andean Globodera populations demonstrated high virulence, completely overcoming the partial resistance of the H3 breeding clone 12674ab1 with reproduction rates significantly higher than the domestic U.S. populations. Conversely, resistance in clone ‘NY121’ was effective against the Andean G. rostochiensis population (PER26). These findings expose a significant virulence gap between Peruvian and U.S. populations, demonstrating that current U.S. potato germplasm is highly vulnerable to specific Andean lineages and emphasizing the importance of strict quarantine enforcement and the discovery of novel resistance sources.
This research also utilized mitochondrial and nuclear markers to analyze the phylogenetic structure and haplotype networks of novel PCN populations from the Cajamarca, Puno, and Cusco regions of Peru, and G. pallida, G. rostochiensis, and G. ellingtonae populations from the U.S. These new sequences were integrated with U.S. references and global datasets from GenBank. Maximum likelihood phylogenies and statistical parsimony networks revealed deeply structured PCN population profiles within native Andean ranges that contrasted sharply with severe bottlenecks in PCN populations introduced from outside of Andes. Globally introduced G. pallida lineages collapsed into a single dominant haplotype tracing to the southern Peruvian Andes (Cusco/Puno), whereas native populations exhibited profound regional north-south geographic partitioning. However, G. rostochiensis exhibited relatively lower genetic diversity, reflecting a narrow global genetic base. However, because G. rostochiensis was less represented in this dataset than G. pallida, further sampling of G. rostochiensis native populations is required to fully characterize its diversity. Inclusion of a broader set of new populations from Peru firmly anchored G. ellingtonae as a basal sister clade to G. pallida. Although COI and ITS phylogenies did not directly correlate with traditional pathotype classifications, this phylogeographic mapping underscores the persistent threat of secondary invasions and the necessity of building diagnostic and regulatory frameworks informed by native diversity.
While characterization of native PCN diversity is crucial, management of current G. pallida infestations requires sustainable alternative control strategies to navigate the phase-out of synthetic fumigant nematicides. To address this, the nematicidal efficacy of two biofumigation formulations was evaluated against G. pallida egg viability, hatch, and reproduction. The formulations included powdered Brassica juncea seed meal extract (SME) and TerraMG™, a two-part (A and B) liquid formulation. Brassica juncea SME alone provided consistent control, reducing G. pallida progeny cysts by 97% to 100% in greenhouse experiment and 72% to 78% under field conditions. The efficacy of the liquid TerraMG™ system (Part A + B) was rate dependent. The higher application rate (393 L/ha) yielded greater reduction in G. pallida egg viability, egg hatch and reproduction compared to the lower rate (140 L/ha). TerraMG™ Part A alone offered the least suppression among TerraMG™ treatments due to a lack of myrosinase enzyme source. However, supplementing Part A with Sinapis alba seed meal (SM) as a biological myrosinase source successfully triggered sinigrin hydrolysis, achieving control similar to the standard Part A+B formulation. Combining S. alba SM with the B. juncea SME did not consistently suppress G. pallida compared to B. juncea alone, likely due to lower application rate of S. alba SM and timing of application.
In conclusion, the findings of this dissertation demonstrate that the potato industry remains highly vulnerable to secondary introductions of complex PCN lineages from the center of origin that are capable of breaking available resistance sources. To counter this threat, this work underscores the necessity of strict quarantine enforcement, pyramiding of diverse resistance genes to manage the virulence reservoir present in these native populations, and discovering new sources of resistance to introduce into potato germplasm. This research also validates standardized powdered B. juncea SME and liquid TerraMG™ formulations as highly effective alternatives for controlling G. pallida. Ultimately, by studying the genetic and virulence diversity of PCN and alternative control strategy for G. pallida, this dissertation provides an integrated framework to safeguard potato industry from current and future nematode threats.