Evaluating potential management strategies for control of plant-parasitic nematodes in soybean
Date
2023-12
Authors
Pennewitt, Monica Gayle
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Tylka, Gregory L.
Leandro, Leonor F.S.
Licht, Mark A.
Munkvold, Gary P.
Robertson, Alison E.
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Abstract
Soybean is an economically important crop, especially in Iowa, which is the second largest soybean-producing state in the U.S. Plant-parasitic nematodes, including the soybean cyst nematode (SCN; Heterodera glycines Ichinoe) and root-lesion nematode (RLN; Pratylenchus spp. Filipjev), threaten soybean production by negatively impacting plant health and reducing yields. Proper management by implementing multiple techniques is vital for growers to maintain profitability when faced with elevated nematode populations in their soils. This dissertation describes laboratory, growth room, and field experiments conducted to i) evaluate how host resistance gene combinations affect SCN population densities and the virulence of SCN populations, ii) assess the effects of inorganic and organic compounds, host root exudates, and root exudates in soil leachates (RESL) on Pratylenchus spp. hatching, iii) study the effects of a biological seed treatment on SCN and RLN biology, and iv) determine the efficacy of the Cry14Ab-1 Bt-toxin to manage Pratylenchus brachyurus in soybean.
Resistant soybean varieties are crucial for preventing yield loss from SCN. However, modern soybean varieties lack diversity in resistance, and most soybean varieties available to growers have resistance derived from PI 88788. Rotating genotypes with different host resistance gene combinations has shown promise in lowering SCN population densities and preventing increases in virulence. A microplot study evaluated the effects of continuously planting and rotating soybean genotypes containing various resistance gene combinations on SCN population densities and the virulence of SCN populations in the field. Increased SCN population densities were observed in all microplots over the four-year study, but most of the rotated genotype treatments had lower population densities than the continuously grown treatments. SCN populations with greater virulence were observed on Peking, PI 88788, and PI 90763 resistance sources, especially for genotypes that shared resistance genes with the resistance sources. The rotated treatments had lower egg population densities than most of the continuous treatments but had increased female indices on the resistance sources. The continuous use of any single genotype increased virulence on the respective resistance source. These results confirm the negative impacts on SCN populations and virulence from the overuse of a single resistance gene and showcase the need to utilize multiple SCN-resistant soybean genotypes within crop rotations.
Using compounds that affect nematode hatching could disrupt the life cycle and serve as effective management strategies against specific nematode genera. Although inorganic and organic compounds can affect the hatching of some plant-parasitic nematodes, the effects on Pratylenchus spp. are poorly understood. This study developed a method to collect large numbers of RLN eggs to conduct hatching assays, then used the method to assess the effects of inorganic salts, organic compounds, host root exudates, and root exudates in soil leachates (RESL) on the hatching of P. brachyurus, P. penetrans, and P. scribneri. Unhatched eggs were placed in micro-sieves with 22-µm-pore mesh, and the number of hatched second-stage juveniles (J2s) that emerged were counted daily over 14 days. There were no significant effects on P. penetrans or P. scribneri by any of the inorganic salts or corn root exudates. Phloretin significantly reduced the hatch of P. penetrans and P. scribneri. Corn RESL had the most effect on P. brachyurus hatching, which resulted in a significantly greater hatch percentage than the fallow treatment. The RESL treatments did not differ from each other. Results indicate that the manipulation of RLN hatching is not easily accomplished, and other forms of management should be prioritized.
A biological nematode-protectant seed treatment with heat-killed Burkholderia rinojensis (hkBr) strain A396 with fermentation media was used in experiments to determine its effects on the biology of SCN and two species of RLN, P. penetrans and P. scribneri. Hatching, chemotaxis, root penetration, and 30-day reproduction experiments were conducted to determine the effects of the seed treatment. Exudates from the fungicide base + hkBr treated seed decreased the hatching of SCN on one soybean variety. However, seed exudates collected from seed treated with only the base treatment decreased hatching for SCN on the second variety. There were no significant differences in percent hatch on either RLN species from the untreated, base, or base + hkBr treatments for any exudates or leachates collected. Fewer SCN eggs per female were observed on plants grown from base + hkBr treated seed in 30-day growth room assays. Chemotaxis of neither SCN nor RLN was affected by any of the exudates or leachates, but calcium chloride (CaCl2) successfully repelled RLN. Biological seed treatments have the potential to serve as an effective form of management against PPN’s, but under the conditions in our study very few differences were observed.
Management of RLN can be challenging as soybean lacks native resistance traits to RLN, but using transgenic plants to manage insects and other pests has become more common. Crystal (cry) proteins produced by the bacterium Bacillus thuringiensis have shown nematicidal effects on various plant-parasitic nematodes, including SCN. Growth room experiments were conducted to assess the efficacy of the Cry14Ab-1 toxin expressed by the GMB151 transgenic event in soybean on reproduction and life-stage development of P. brachyurus. There were significantly more P. brachyurus recovered from soybean roots without the Cry14Ab-1 toxin than soybean plants with the trait. Of the P. brachyurus life stages, there were significantly more fourth-stage juveniles and adults extracted from soybean roots than any other stage. Both treatments shared a similar distribution of nematodes per life stage, but more nematodes were extracted from soybean plants without the Cry14Ab-1 toxin. The use of the Cry14Ab-1 protein could provide an additional approach to nematode management and host resistance against P. brachyurus, as there are no RLN-resistant soybean genotypes currently available to growers.
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Plant Pathology
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