Pathogenesis, Virulence and Population Genetics of <i>Puccinia emaculata</i>, the Causal Agent of Switchgrass Rust

The infection process of Puccinia emaculata on Panicum virgatum leaves was studied histologically. Appressoria were formed approximately 10 hr after inoculation and penetrations were observed over stomatal openings. Substomatal vesicles developed 3 days after inoculation (DAI) and at 5 DAI, both haustorial mother cells and haustoria were observed. At 11 DAI, uredinia were developing, but urediniospores were immature. Until 14 DAI, matured uredinia ruptured host epidermis and urediniospores were released for secondary infection. This clearly studied infection process will help understand resistance mechanisms of a resistant cultivar in future research.

Twelve agronomic and ornamental switchgrass cultivars were inoculated with 40 switchgrass rust isolates to study the different host resistance levels, rust virulence and their interactions by measuring germination percentage, latent period, the number of uredia and urediniospores produced per cm² of leaf surface. Ornamental switchgrass cultivars had longer latent periods and fewer numbers of uredia and urediniospores produced per cm². Furthermore, virulence variations were observed among 40 switchgrass rust. The results of this study will provide more information in durable horizontal resistance of switchgrass cultivars and find new resources for breeding more sustainable rust resistant switchgrass.

Six switchgrass rust isolates were sequenced. The 29 million paired-end sequence reads of length 100bp for a switchgrass rust isolate were de novo assembled. Due to the high contamination of the reads, only 622 rust contigs were confidently provided, which would be random samples of nucleotide pieces from the whole genome with GC content 47.33%. Variant detection was conducted by mapping sequence reads of the other five isolates on these rust contigs. The average single nucleotide variants and insertion/deletions density are 1.62 and 0.057 per kb, respectively.

The genetic relationship among switchgrass rust isolates collected from seven locations across southeastern US were analyzed using 22 SSRs. Linkages among loci were observed among all seven subpopulations with the standardized index of association significantly different from 0 (p< 0.01), indicating no sexual recombination in these locations. Moderate to high genetic differentiation was discovered among these subpopulations which demonstrated multiple introductions of genotypes of P. emaculata introduced to these areas.