Investigating the interactions between cover crops and the soybean cyst nematode through lab, greenhouse, and field studies

Abstract

<p>Throughout the past decade, the acres on which cover crops have been planted in agricultural systems in the United States has increased. In traditional row cropping, cover crops are sown shortly before or after the harvest of corn or soybean and grown through the fall to provide ground cover for approximately one third of the year during which the soil would otherwise be bare. Cover crops have many known environmental benefits, but the effects they may have on soybean cyst nematode (SCN), Heterodera glycines, are not well established. This dissertation describes experiments conducted i) in the laboratory and greenhouse to determine if cover crops have potential to serve as trap crops, ii) in the greenhouse to determine whether cover crops affect SCN population densities under controlled conditions and iii) in the field to assess how cover crops affect SCN population densities in multi-year, multi-location and rotation situations.</p> <p>The first set of experiments assessed the possibility that cover crop species may act as trap crops for SCN. A good candidate for an SCN trap crop might stimulate SCN hatching, attract hatched SCN juveniles, and/or would be infected by many nematodes. Root exudates and soil leachates (RE/SL) were collected from cover crop plants and used to test their effects on SCN hatching and chemotaxis. The hatching of SCN in crimson clover RE/SL was greater than in all other cover crop treatments as well as in the unplanted control. There were no cover crop RE/SL that had notable effects on SCN chemotaxis. In greenhouse experiments where root penetration by SCN was assessed, more SCN juveniles were recovered from roots of crimson clover than most other cover crop treatments. In total, these results suggest crimson clover has the most potential to act as a trap crop for SCN.</p> <p>A set of greenhouse experiments were conducted to determine if cover crops affect SCN population densities and subsequent SCN reproduction. After sixty days of growth by different cover crops, the SCN population density decreased numerically in all treatments. There were no differences in the amount of SCN population density decrease over the duration of the experiment for cover crop treatments compared to the non-planted soil control. When susceptible soybean plants were grown in leftover soil from this experiment, there were significantly fewer SCN females that developed on soybean roots following annual ryegrass 1, annual ryegrass 2, annual ryegrass 3, daikon radish, mustard 1, mix 1, and the tomato control compared to following the non-planted control. And in an experiment where cover crops were grown for 56 days followed by a 28-day exposure to Iowa winter conditions, there were no differences in SCN population density decreases for all cover crop treatments compared to the non-planted soil control. On soybeans grown in soil following cover crop growth, there were fewer SCN females that formed on soybean roots following seven of the treatments included all three annual ryegrass treatments, compared to the following the non-planted soil control. Overall, these results indicate that there may be an adverse residual effect of at least some cover crops on SCN reproduction.</p> <p>To expand the research to real-world field environments, experiments were conducted in 2016-2017 and 2017-2018 at the Iowa State University Muscatine Island Research Farm in Fruitland, Iowa, and at the Northern Research Farm in Kanawha, Iowa, with two experiments at each location designated by the annual crop rotation: corn-soybean and soybean-corn. Cover crop seeds were broadcasted into standing corn and soybean in late summer. Soil samples were collected at the time of cover crop seeding, again in late fall prior to a hard freeze, and once more in the spring prior to planting of corn or soybean to determine changes in SCN population densities each year. After two complete years of these experiments, there were no significant differences in the SCN population density changes observed among all treatments, sampling intervals, fields, locations, or years. Furthermore, there were no significant differences in the PCF means for any cover crop treatment compared to the non-planted control. None of the cover crops decreased SCN population densities under the conditions of these field experiments.</p> <p>The high spatial variability of SCN both within and among small plots may explain why the effects of cover crops detected in greenhouse and laboratory experiments were not apparent in the field. More work on cover crops, especially those with promise as trap crops, is warranted under highly-controlled conditions.</p>