Purple deadnettle is an obligate winter annual weed and an alternate host of soybean cyst nematode (SCN). Field experiments were conducted using microplots to determine (1) the effect of purple deadnettle planting date and (2) the effect of purple deadnettle removal time on SCN egg population density in continuous no-tillage soybean. A population change factor (PCF) to assess treatment effects on SCN population densities was calculated by dividing the SCN population density at each sampling time (Pf) by the initial population density before treatments were applied (Pi). In the planting date experiment, purple deadnettle seeded on October 3, 11, and 18 resulted in PCF values ranging from 7.28 to 11.41, which were three- to fivefold higher than the PCF values for the weed-free control or purple deadnettle seeded on September 6, 20, and 27. Self-thinning of purple deadnettle seeded in September may have resulted in higher levels of SCN mortality compared to later seeding dates. In the removal time experiment, purple deadnettle removal at 2 and 4 wk after emergence (WAE) resulted in PCF values of 1.19 and 1.54, respectively, which were similar to the PCF for the weed-free control. In contrast, PCF values for purple deadnettle removal times of 6 WAE or later ranged from 2.46 to 5.44. Field and greenhouse experiments provided evidence that completion of the SCN life cycle on purple deadnettle was prevented if the weed was removed before the accumulation of 380 soil degree days within the 5 to 30 C range. Results from the removal study also suggested that completion of the first SCN generation on purple deadnettle in the fall was the primary factor causing increases in egg population density, since PCF values did not continue to increase significantly beyond the 6 WAE removal time.
Nomenclature: Purple deadnettle, Lamium purpureum L. LAMPU; soybean, Glycine max L. Merr; soybean cyst nematode, Heterodera glycines