The effect of virulent and hypovirulent strains of Endothia parasitica on hydraulic conductance in American chestnut

Abstract

Measurements of hydraulic conductance per unit length (Kh) were made on stems of Castanea dentata (Marsh) Borkh. that were inoculated with virulent and hypovirulent strains of Endothia parasitica and on stems with naturally occurring (wild) cankers. For stem segments infected with virulent strains (virulent infections), Kh values were significantly lower (p < 0.01) than in healthy control stems or in stem segments immediately above or below the canker. Adjacent segments were not significantly different from controls at the 0.05 level of probability. For hypovirulent infections, the Kh of canker segments, adjacent segments, and controls were not significantly different from one another. Dye ascents and descents indicated the xylem conductive area was sharply reduced in stem segments with virulent, but not hypovirulent, infections. Stem segments with wild virulent infections showed not only a reduction in xylem conductive area, but often a missing or incomplete outer xylem growth ring. Segments with wild hypovirulent infections often showed an incomplete outer xylem growth ring, with the intact portion of the outer growth ring (or rings) often quite swollen and sometimes with anomalous vascular bundles. It is concluded that xylem dysfunction is strongly correlated with reductions in Kh and with death of the leaves distal to virulent infections.