Biological suppression and natural population decline of Heterodera schachtii in a California field

Soil suppressiveness to Heterodera schachtii was demonstrated in a field at the research station of the University of California, Riverside. In two fi eld trials planted to Swiss chard (Beta vulgaris), introduced H. schachtii multiplied 2.7 and 1.7 times more in preplant metam sodium-fumigated plots than in nontreated plots in 1994 and 1995, respectively. In greenhouse expe riments, preplant treatments with metam sodium, methyl bromide, methyl iodi de, formaldehyde, and aerated steam reduced suppressiveness of soil against H, schachtii to undetectable levels. H. schachtii multiplied significantly less in nontreated soil than in treated soil on Swiss chard. At harvest, t he number of infective second-stage juveniles in suppressive soil was close to the lowest detection level, whereas high numbers were encountered in so ils initially treated. In a crop rotation trial with host crops of H. schac htii, introduced H. schachtii populations were monitored for five cropping periods over 30 months in initially fumigated versus nontreated suppressive field plots. In fumigated plots, H, schachtii population levels increased in the first and second cropping periods and then declined in the third cro pping period. In the fourth and fifth cropping periods, the nematode reprod uction factor in the initially fumigated plots was not significantly differ ent from that in suppressive plots.