Genetic mapping, identification, and characterization of a candidate susceptibility gene for powdery mildew in Cannabis sativa

Authors: STACK, GEORGE - Cornell University; CALA, ALI - Cornell University; QUADE, MICHAEL - Cornell University; TOTH, JACOB - Cornell University; MONSERRATE, LUIS - Cornell University; WILKERSON, DUSTIN - Cornell University; Carlson, Craig; MAMERTO, ALLEN - Salk Institute Of Biological Studies; MICHAEL, TODD - Salk Institute Of Biological Studies; CRAWFORD, SETH - Oregon Cbd Seeds; SMART, CHRISTINE - Cornell University; SMART, LAWRENCE - Cornell University

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/2023
Publication Date: 1/29/2024
Citation: Stack, G., Cala, A., Quade, M., Toth, J., Monserrate, L., Wilkerson, D., Carlson, C.H., Mamerto, A., Michael, T., Crawford, S., Smart, C., Smart, L. 2024. Genetic mapping, identification, and characterization of a candidate susceptibility gene for powdery mildew in Cannabis sativa. Molecular Plant-Microbe Interactions. 37(1):51-61. https://doi.org/10.1094/MPMI-04-23-0043-R.
DOI: https://doi.org/10.1094/MPMI-04-23-0043-R

Interpretive Summary: One of the most important foliar diseases of hemp is the fungus, powdery mildew (PM). PM infestation can drastically reduce a plant's ability to photosynthesize, often results in premature defoliation, and can reduce post-harvest marketable quality of hemp inflorescences. PM can be managed by three different approaches: fungicide application, environmental modification, and using resistant cultivars. Identification and incorporation of genetic PM resistance into elite hemp cultivars is imperative for the implementation of comprehensive and robust integrative pest management programs. In this study, we characterized a novel source of PM resistance using genetic mapping, sequenced the gene that confers PM susceptibility, and developed simple molecular markers to screen hemp plants for PM resistance. This research contributes improved knowledge of PM disease resistance in hemp as well as new molecular tools for rapid release of resistant cultivars by public and private breeding programs.

Technical Abstract: Powdery mildew (PM) in Cannabis sativa is most often caused by the biotrophic fungus Golovinomyces ambrosiae. Based on previously characterized variation in susceptibility to PM, biparental mapping populations were developed by crossing the most resistant evaluated cultivar, ‘FL 58’, with a susceptible cultivar, ‘TJ’s CBD’. F1 progeny were evaluated and displayed a range of susceptibility. Subsequently, two of the F1 progeny were self-pollinated to generate two F2 populations. During the 2021 growing season, 706 individuals from the F2 populations were inoculated with PM and surveyed for disease severity. At the end of the survey period, 25% of the progeny in both F2 populations had disease ratings of 0, while the remaining 75% showed a range of susceptibility. The F2 populations, as well as the parents and selected F1 progeny, were genotyped using a single nucleotide polymorphism (SNP) array and a consensus genetic map was produced. Disease traits mapped to a major effect quantitative trait locus (QTL) on Chromosome 1 (Chr01), with other smaller effect QTL on five other chromosomes. The most significantly associated marker on Chr01 was located near CsMLO1, a gene previously predicted to be involved in PM susceptibility through in silico analysis of existing C. sativa genome assemblies. Further investigation of CsMLO1 in ‘FL 58’ revealed a 6,767 bp insertion in the genomic sequence compared to the susceptible parent. These data support the hypothesis that ‘FL 58’ has a mutated MLO susceptibility gene that confers resistance to PM in homozygous genotypes.