In recent years, Root-Knot Nematode (RKN, Meloidogyne incognita) has been a leading yield reducer of soybeans across the region. The phase out of many fumigant options, reduced efficacy of nonfumigant chemical control options, milder winters, and extreme precipitation events that keep field soils saturated have all favored the proliferation of RKN in the sandy soils of DE and eastern shore
MD. Unfortunately, RKN also has a tremendous host range, including many vegetable crops, and rotation does not guarantee reduction of nematode populations for soybean production. Symptoms of RKN are not as prominent in corn, but preliminary soil samples in 2023 indicated that RKN reproduction on corn could be higher than anticipated. Nematode populations can be quantified at plant level by digging up individual plants to examine root systems or on a field scale by collecting representative soil samples from the base of plants for nematode enumeration. By tracking reproduction rates on multiple hosts, we seek to improve understanding of nematode reproduction rates and crop rotation recommendations for soybean production. This project will fund two months of support for a M.S. student focusing on management of root-knot nematode in soybeans to conduct host reproduction trials. This data will be shared at multiple extension events to help farmers make informed decisions regarding nematode management.

1. Track in-season nematode populations following the use of multiple host plants.
2. Share research findings through extension events and use findings to inform future management trials.

Objective 1. Track in-season nematode populations following the use of multiple host plants.
At the University of Delaware Carvel Research and Education Center, a field has been used for rootknot nematode research for over a decade. Within this field four host crops: soybean, corn, susceptible lima bean, lima bean with RKN resistance will be planted and observed for RKN reproduction. In addition to natural inoculum, plots will be inoculated at the time of planting by adding symptomatic root pieces from RKN infested tomato plants. Nematode samples will be collected from plots at the time of planting, mid-season and at the end of the growing season. At maturity for each host, plants will be dug, and root systems will be rinsed to remove excess soil. Roots will then be processed to extract nematode eggs and measure the reproductive potential of each host.

Objective 2. Share research findings through extension events and use findings to inform future management trials.
Findings from this project will be shared through the University of Delaware’s Weekly Crop Update, which reaches over 700 growers, consultants, and stakeholders and provides a platform to discuss disease concerns and other production issues. Data will also be shared through training events and extension presentations such as the 2025 Delaware Ag week and winter meetings.

Updated September 20, 2024:
To track soybean maturity across multiple hosts, a trial was established at the Carvel Research and Education Center in our RKN testing field. Ground was prepared and rows were made by leaving open the wheel press. Seeds were placed by hand and root-knot nematode inoculum was added before closing the trench. Four host crops: soybean, corn, susceptible lima bean, lima bean with RKN resistance were planted in June and have been observed for RKN symptoms. Baseline nematode samples were collected at the time of planting. At maturity for each host, plants will be dug, and root systems will be rinsed to remove excess soil. Roots will then be processed to extract nematode eggs and measure the reproductive potential of each host. We will also collect end of season soil samples to compare to root egg counts.

Updated January 2, 2025:
The host reproduction trial was completed in Georgetown, DE from June to September 2024. Nematode soil samples were collected at the time of planting and inoculation on June 6, 2024. Crops including RKN susceptible lima bean, RKN resistant lima bean, field corn, and RKN susceptible soybean. Each plot consisted of four rows with the middle two rows receiving inoculum. Minimal RKN populations and no SCN were recovered at the start of the trial. End of season soil samples were collected from all plots on September 16, 2024. Lima bean roots and corn roots were assessed for gall ratings and eggs were extracted and quantified. As final soil counts are received the final report will be submitted.

Updated April 10, 2025:

View uploaded report

Meloidogyne spp., known as root-knot nematodes (RKN), are a collection of over 100 species affecting a wide range of host plants. In soybean production, RKN typically ranks as the second most destructive pathogen, right behind Soybean Cyst Nematode. In 2019, the south saw a loss of 8.64 million bushels of soybeans due to RKN infection and destruction, while 2018 saw an even higher loss of 11.92 million bushels in the south to RKN (Allen et al. 2020; Allen et al. 2019). The primary root-knot species of concern for soybeans across the south and in DE and MD is southern root-knot nematode, Meloidogyne incognita.

RKN is characterized by the formation of galls on the roots, and several above-ground symptoms can also indicate infection. Symptoms are often noticed in patches or clusters throughout the field. Within these clusters, plants may appear wilted and or stunted, especially compared to other parts of the field (Mitokowski & Abawi 2011). These symptoms can resemble other diseases,
nutrient deficiencies, or other abiotic disorders. Surveys for soybean nematode populations were conducted from 2019-21 and RKN populations ranging from 4,000-9,000 nematodes per 500 cc were observed, particularly in fields with a history of vegetable rotation. A high level of infestation is considered >170 RKN per 500 cc, meaning many local fields are dealing with populations 25-53 times above threshold (Kessler and Koehler 2022). Crops like soybean and lima bean typically show more symptoms and yield loss from RKN than corn. However, in recent soil surveys we have noted elevated RKN populations in corn fields that did not show notable symptoms or yield loss. While we think of corn as a good rotation partner soybean production in fields with Soybean Cyst Nematode, the rotation may be more complicated for RKN. Limited RKN soybean host resistance is available for soybeans in appropriate maturity groups for the region. A comparison of nematode reproduction levels on various common host crops will improve our understanding of RKN accumulation and subsequent management recommendations.