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Anatomical and Physiological Mechanisms of Resistance to Brown Leaf Spot in Smooth Bromegrass

USDA-ARS, Pasture Systems and Watershed Management Research Lab., University Park, PA 16802

^* Corresponding author.

Smooth bromegrass (Bromus inermis Leysser) germplasm PL-BDR1 and two seed increase generations derived from it are highly resistant to brown leaf spot caused by Pyrenophora bromi (Died.) Drechs. [Drechslera bromi (Died.) Shoemaker, conidial state = Helminthosporium bromi Died.)]. Lesions are about one-half the width and one-tenth the area of spots on susceptible cultivars. Physiological or anatomical mechanisms of resistance have not been identified, but they could have a bearing on stability of resistance and on forage quality components. This study was conducted to determine whether certain resistance mechanisms identified in other leaf spot diseases of grasses operate in PL-BDR1 germplasm. The three resistant generations were compared with susceptible cultivars Belmont, Radisson, and Saratoga for fungal penetration, halo formation, and leaf vein architecture. Susceptible leaves supported higher numbers of penetration attempts and successful penetrations. No effect of plant line was found for appositional wall formation (=papilla formation), initial colonization rate, distance between parallel veins, proportion of large vascular bundles, or girdering of vascular bundles. Resistant lines had wider vascular bundles than susceptible cultivars indicating a possible role for strength of the vascular framework in restricting lateral spread. Extensive cell necrosis and chlorotic halo formation in susceptible cultivars indicated possible involvement of a fungal phytotoxin. Resistant lines had few halos, and reduced sensitivity to toxins may be involved in resistance of PL-BDR1 to brown leaf spot.

Received for publication December 7, 1994.

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