Papilla Formation and Hypersensitivity at Penetration Sites and Resistance toPseudocercosporella herpotrichoidesin Winter Wheat

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Abstract
Seedlings of six winter wheat cultivars were inoculated with conidia of Pseudocercosporella herpotrichoides and the infection process followed. Penetration of coleoptiles and leaf sheaths occurred sooner in susceptible than in resistant cultivars. Lignified papillae, stain halos, and a hypersensitive reaction formed in epidermal cells in response to penetration in all cultivars. More papillae were found in first leaf sheaths of resistant than of susceptible cultivars; VPM, the most resistant cultivar, had the most papillae at penetration sites (100%) and Selection 101, the most susceptible cultivar, had the fewest papillae (66%). Fewer successful penetrations occurred in resistant than in susceptible cultivars at sites where papillae were present. However, penetration always occurred when papillae did not form, regardless of host resistance. The total number of successful penetrations (sites with and without papillae) increased with increasing host susceptibility from 2.3% in VPM to 74.3% in Selection 101. The hypersensitive reaction occurred only at sites with papillae, and there were fewer successful penetrations at sites with both papillae and the hypersensitive reaction that at sites with papillae alone in all cultivars except VPM. Histochemical tests indicated lignin was present in papillae of all cultivars. These data indicate that both lignififed papillae and hypersensitive reactions are involved in seedling resistance to P. herpotrichoides.