Hypoxylon mammatumAscospore Infection ofPopulus tremuloidesClones: Effects of Moisture Stress in Tissue Culture

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Abstract
Tissues from six aspen clones were cultured to produce plantlets from dormant buds. Plantlets of 1-2 cm were moisture stressed by adding various concentrations of mannitol to the growth medium. Inoculation of unwounded plantlets with ascospores of Hypoxylon mammatum resulted in visible signs of mycelium after 3-4 days. After 10 days, mycelial growth on controls and moderately stressed plants remained superficial; in contrast, highly stressed plants were invaded by the mycelium and exhibited necrotic lesions at the site of inoculation. The level of moisture stress needed for mycelium invasion and lesion development varied (-0.45 to 1.2 MPa) among the clones. Clonal differences observed could be applied in basic physiological studies or in aspen breeding programs for hypoxylon canker resistance.