Development, selection and characterization of summer patch (Magnaporthe poae Landschoot and Jackson) resistant somaclones of Kentucky bluegrass (Poa pratensis L.) genotypes

To develop a somatic embryogenic system for Kentucky bluegrass, crowns, embryo axes, leaf, and endosperm tissues of twelve genotypes were cultured on Murashige and Skoog medium containing various concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) and 6-benzylaminopurine (BAP). Callus induction and shoot regeneration varied significantly by genotype and type of explant. In all genotypes leaf and endosperm tissues were recalcitrant. Embryo axes and crown pieces regenerated an average of two shoots per culture on medium containing 0.3 mg/l 2,4-D and 0.2 mg/l BAP. The effects of growth regulators and dark incubation on regeneration of cultured explants were studied using two genotypes. Increasing the concentration of 2,4-D and BAP to 0.6 mg/l and 0.3 mg/l, respectively, plus incubating the cultured explants for three weeks, significantly increased both somatic embryogenesis (from 29% to $\ge$90%) and the average number of shoots per culture regenerated (from 2 to $\ge$5). Regenerated plants varied in growth characteristics, chlorophyll pigmentation, resistance to powdery mildew (Erysiphe graminis), and summer patch (Magnaporthe poae). Variants highly resistant to powdery mildew and summer patch were selected, and the resistance was expressed through several cycles of vegetative propagation. There was no relationship between resistance to summer patch and that of powdery mildew.Physical effects and patterns of root colonization by M. poae under stressed and non-stressed moisture regimes were studied in ten genotypes of Kentucky bluegrass. Genotypes varied significantly in root length, dry root weight, and disease incidence under stressed or non-stressed moisture regimes. Under moisture stress, fungal colonization significantly reduced root length and root dry weight in all genotypes, and caused severe rotting of vascular and crown tissues. Regardless of moisture status, ecto- and endotrophic root colonization by the fungus was in non-uniform (patchy) patterns.A rapid in vitro bioassay to rapidly identify variants resistant to M. poae was developed by transferring tissue culture-derived plantlets onto four day old cultures of M. poae growing on half strength potato dextrose agar. Cultures were incubated under light or in the dark for fourteen days. Under light, M. poae showed more ectotrophic root colonization and lower disease severity than in the dark. Resistance to M. poae was associated with the degree of colonization of the crown tissues. Resistant variants consistently showed a lower degree of fungal colonization and rotting of the crown tissues. The variants were isolated, vegetatively propagated, and further subjected to two cycles of in vivo screening, resulting in the selection of variants highly resistant to M. poae.U of I OnlyETDs are only available to UIUC Users without author permissio