Distribution and management of Diaporthe spp. and plant-parasitic nematodes affecting soybeans in Delaware and Maryland
Date
2022
Authors
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Publisher
University of Delaware
Abstract
Soybean (Glycine max L. Merrill) is a major agronomic commodity for the Mid-Atlantic region, with over 190,500 and 571,500 metric tons (MT, 1000kg) harvested in Delaware (DE) and Maryland (MD), respectively in 2020 (USDA NASS 2020). In annual disease loss estimates, Diaporthe species and plant parasitic nematodes cause over 168,736 MT (6.2 million bu) of yield suppression in soybean across both states. The Diaporthe complex includes multiple Diaporthe species causing diseases such as stem canker, pod and stem blight, and Diaporthe/Phomopsis seed decay. Due to similarities in pathogen signs and disease symptoms, misdiagnosis is common, and no molecular characterization of species distribution or abundance has been conducted in the Mid-Atlantic region. In this thesis, a two-year fungal survey was conducted to characterize Diaporthe species present within stem and root tissue of soybeans in DE and MD. This survey identified four species D. longicolla, D. aspalathi, D. sojae, and a novel Diaporthe pathogen, D. ueckerae, not yet reported within the region. D. longicolla accounted for 97% of samples. Koch’s Postulates confirmed the pathogenicity of D. ueckerae and the association of zone lines with D. longicolla. Greenhouse aggressiveness trials were conducted to compare D. longicolla, D. aspalathi, and D. ueckerae symptom development on three soybean hybrids. D. longicolla had the highest disease severity score within each hybrid and D. aspalathi had the lowest. In addition to fungal pressure, nematodes are of great concern to the region. Soybean cyst nematode consistently ranks at the top yield-limiting pathogen of soybeans within the region and the US. In this thesis, a three-year nematode soil survey was completed to update data on the distribution and population levels of SCN while also monitoring for other nematodes of concern. Across the 311 samples, nine nematode genera were recovered. Of economic concern, 53.4% of samples had SCN, 18.3% root-knot nematode, and 57.2% lesion nematode. In addition to survey work, two seed treatment trials were conducted to evaluate the efficacy of chemical and biological seed treatments for management of SCN. Soil samples were collected at planting and prior to harvest, with mid-season evaluation of female SCN on roots. Differences in percent emergence, initial SCN soil counts, on-root SCN females, and yield were observed across both trials. Findings from this project expand characterization of regional soilborne pathogens of soybean and provide a foundation to develop improved management approaches.
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Keywords
Diaporthe, Nematode, Root-knot nematode, Seed treatment, Soybean, Soybean cyst nematode