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Effect of Upper-Profile Soil Water Potential on Numbers of Cysts of Heterodera glycines on Soybeans¹

Upper-profile (0 to 15 cm) soil water potential (SWP) was maintained at four levels in a greenhouse study to determine the effect of these soil moisture regimes on cyst formation by Heterodera glycines (Ichinohe) on soybeans [Glycine max (L.) Merr.]. Number of cysts increased significantly above initial levels in the wet (–0. I to –0.2 bar SWP) and optimum (–0.3 to –0.4 bar SWP) soll moisture treatments. The optimum soil moisture treatment had the highest number of cysts at termination. Root weight in the 0 to 15-cm zone was highest in the wet treatment and lowest in the dry treatment (–0.6 to –0.7 bar). Number of cysts in the 15 to 45-cm depth zone of all treatments (equally wet at –0.25 to –0.35 bars) increased significantly above initial levels and reached a level essentially equal to the number of cysts in the 0 to 15-cm zone of the optimum soil moisture treatment. No difference in root mass or number of cysts was measured in the deeper zone. Shoot weight and height were not significantly different among treatments, but a decrease in leaf area index with increasing dryness of the upper soil zone indicated moisture stress. The similar shoot weights among treatments indicated that the cyst levels in the wet and the optimally wet treatments may have reduced plant development, since the wetter soil should have increased shoot growth.

Key Words: Soybean cyst nematode • Vicksburg sandy loam • Glycine max (L.) Merr. • Shoot weight • Root weight

² Research agronomist, USDA-ARS; Soybean Production Research, P. O. Box 196, Stoneville, MS 38776; research nematologists, USDAARS Nematology Investigations, West Tenn. Agric. Exp. Sm., Jackson, TN 38301; and research agronomist, StoneviIle, Miss.

Received for publication August 24, 1981.