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Leaf and Root Growth of Water-Stressed Kentucky Bluegrass Infected by Ustilago striiformis or Urocystis agropyri¹

The relative growth of the shoot vs. roots has proven to be a useful parameter in investigations of drought tolerance. In addition, overall plant growth has been used as a measure of tolerance to water stress. Kentucky bluegrass (Poa pratensis) infected by Ustilago striiformis (Stripe smut) or Urocystis agropyri (Flag smut) exhibits greater mortality than healthy plants during periods of water stress and is suggestive of decreased drought tolerance as a result of infection. The purpose of this study was to compare healthy Kentucky bluegrass ‘Merion’ and plants systemically infected by Ustilago striiformis or Urocystis agropyri for total, leaf and root growth, and for root-shoot ratios in nutrient solution and in nutrient solution amended with polyethylene glycol to provide increasing levels of osmotically-induced water stress. Infection by U. striiformis increased total and leaf dry weight, and decreased root weight and root-shoot ratios of plants grown in nutrient solution. Plants infected by U. agropyri exhibited decreased total, leaf, and root weights, and root-shoot ratios. Total, leaf, and root weights of healthy, U. striiformis-and U. agropyri-infected plants decreased, and root-shoot ratios increased with decreasing osmotic potentials of nutrient solutions. The increases in total and leaf weights of U. striiformis-infected plants compared with healthy plants grown in nutrient solution were lost as osmotic potentials decreased. Healthy plants maintained higher root-shoot ratios than infected plants at all stress levels. The decreases in root-shoot ratios of infected plants represent a morphological basis for reduced stress tolerance and subsequent increased mortality of infected plants during periods of water stress.

Key Words: Disease • Flag smut • Kentucky bluegrass • Leaf smuts • Nutrient solution • Polyethylene glycol • Stripe smut • Turf • Water stress

² Postdoctoral research associate, Agronomy and Soils Dep., Washington State Univ., WWREC, Puyallup, WA 98371; and professor of horticulture, agronomy, and plant pathology, Dep. of Horticulture, Iowa State Univ., Ames, IA 50011.

Received for publication December 12, 1983.