DORMANCY OF HETERODERA-GLYCINES IN MISSOURI

A 2-year study was conducted in field microplots to determine the rela tive importance of soybean phenology and soil temperature on induction of dormancy in Heterodera glycines in Missouri. Four near-isogenic so ybean lines differing for maturity date were planted in microplots inf ested with a race 5 isolate of H. glycines. Soil temperature was monit ored at a depth of 15 cm. Eggs of H. glycines, extracted from cysts co llected monthly from each microplot, were used in hatching tests and b ioassays to determine dormancy. Egg hatching and second-stage juvenile (J2) infectivity rates decreased sharply from their highest levels in midsummer (July-August) to a low level by October of each year and re mained low (< 10% hatching and < 0.2 J2/cm root) until May or June of the following year. The patterns of numbers of females and eggs in the bioassays were similar. The decreases were not related to soil temper ature and did not differ consistently among soybean isolines. The mono phasic changes in all nematode responses with peak midsummer rates sug gest that H. glycines produces one primary generation per year in cent ral Missouri. Changes in hatching rates and the timing of minimum and maximum rates suggested that H. glycines eggs exhibit more than one ty pe of dormancy.