Development of methods to study plant-parasitic nematode chemotaxis and the effect of ILeVO and VOTiVO seed treatments on the soybean cyst nematode, Heterodera glycines
Nematode-protectant seed treatments for controlling the soybean cyst nematode, Heterodera glycines, have been introduced for integration into existing nematode management plans. Several seed treatments labeled for H. glycines control are currently available, with more expected in the near future. This dissertation describes i) tools to study nematode chemotaxis to chemicals and roots and ii) assessment of two commercial seed treatments, ILeVO (active ingredient: fluopyram) and VOTiVO (active ingredient: Bacillus firmus I-1582), on important H. glycines life processes.
The first objective was to develop and test two novel microfluidic chips to study chemotaxis of plant-parasitic nematodes to chemicals (“basic chip”) and roots (“root chip”). Proof-of-concept experiments were conducted with both chips. Several ionic solutions were tested with the basic chip on second-stage juveniles (J2s) of H. glycines and the root-knot nematode, Meloidogyne incognita. There was a differential response of the two nematode species to a number of the ionic solutions. Custom software was written to assist in tracking nematodes in the root chip using a flatbed scanner. The attraction of H. glycines J2s to 3-day-old soybean seedlings (cv. Williams 82) was successfully captured using the root chips over an 18-hour period.
The second objective was to assess the effects of ILeVO and VOTiVO seed treatments on several H. glycines life processes. In greenhouse experiments, ILeVO consistently reduced H. glycines reproduction on soybean roots. Seed exudates from ILeVO-treated seeds inhibited hatching and motility of H. glycines J2s, relative to the untreated control. Conversely, radicle exudates from seedlings grown from ILeVO-treated seeds reduced H. glycines hatching modestly in only one experimental run did not affect motility of J2s. In a growth chamber experiment, ILeVO significantly reduced H. glycines root penetration compared to untreated plants at two nematode inoculation levels. In another root penetration experiment, soybeans grown from treated or untreated seeds were inoculated with H. glycines J2s at soil depths of 2.5, 5, or 7.5 cm. ILeVO reduced nematode root penetration only at the 2.5-cm inoculation depth. Nematode behavioral changes in response to exudates from treated and untreated seeds then were assessed using custom software. ILeVO seed exudates significantly reduced speed of H. glycines J2s after two hours of exposure. A flatbed scanner then was used to study the movement of H. glycines populations exposed to seed treatments. Treated and untreated seeds were incubated in well plates for one hour before removal with clean forceps and insertion of H. glycines J2s. The percentage of the H. glycines populations that moved over a critical threshold distance (300 μm) in the wells were evaluated every hour for 24 hours using another custom software program. ILeVO significantly reduced population movement relative to the untreated control. And H. glycines J2s in wells that had a seed, treated or not, had significantly less movement compared with the blank control. Overall, the results suggest that ILeVO affects multiple life processes of H. glycines. VOTiVO did not consistently affect any of the variables measured in the experiments.