Screening sesame (Sesamum indicum) for resistance to multiple root-knot nematode species (Meloidogyne spp.)

Root-knot nematodes (Meloidogyne spp.; RKN) are among the most damaging groups of plant-parasitic nematodes. They parasitize almost every species of higher plant by penetrating the host’s roots and inducing the formation of galls or knot-like swellings along the plant roots (Gill & McSorley, 2011; Moens et al., 2009). These galls are detrimental to plant growth since they interfere with the crop’s ability to absorb water and nutrients (Khan et al., 2017). Additionally, their feeding can cause early senescence, wilting, premature defoliation, stunting, and total crop loss (Khan et al., 2017; Moens et al., 2009; Perry & Moens, 2011). Under ideal environmental conditions and depending on the RKN species and host species, the life cycle of most RKN is completed in 3–6 weeks (Subbotin et al., 2021). This allows for several generations to develop during one cropping season, which can lead to a rapid increase in population and severe crop damage (Moens et al., 2009). The major species of RKN reproduce by parthenogenesis, where the progeny are “clones” of the female adult; the production of males is lower, and they do not have a sexual role (Bert et al., 2011).

Historically, RKN management has been achieved with chemical nematicides, yet the extensive use of these chemicals poses a potential hazard to the environment and human health, which has led to a decline or ban of many products (Noling & Becker, 1994; Perry & Moens, 2011). Therefore, development and implementation of economically feasible and environmentally safe alternatives is needed (Fernández et al., 2001; Noling & Becker, 1994). Crop rotation systems may introduce resistant or poor hosts that may either inhibit pathogen growth through failing to meet the nutritional needs of the pathogen, producing toxic compounds detrimental to the pathogen, or supporting antagonistic microbial communities in the rhizosphere (Rodriguez-Kabana & Canullo, 1992; Travedi & Barker, 1986). The magnitude of nematode control attained through crop rotation will vary with the year, cultivar, and resistance of the plant species, associated weed hosts, environmental conditions, and the length of the rotation (Travedi & Barker, 1986).

The introduction of sesame (Sesamum indicum) into a cropping system has proven to reduce RKN levels (Couch et al., 2017). The nematicidal effect of sesame is attributed to the lignans sesamin and sesamolin, which are natural antioxidants found only in this crop (Ashri, 1998; Bedigian & Harlan, 1986; Couch et al., 2017). Sesame is a relatively new crop in the US, with its production centered in Texas, Oklahoma, Arizona, and Kansas (Grichar et al., 2011; Gloaguen et al., 2018; Langham, 2007).

There is a need and interest to explore other suitable areas for sesame production, particularly the southeast United States due to its warm, long growing season and greater annual precipitation compared to the Southwest (Couch et al., 2017; Gloaguen et al., 2018). Nematode management is critical in North Carolina (NC), where major crops such as sweetpotato (Ipomoea batatas (L.) Lam), cotton (Gossypium hirsutum L.), tobacco (Nicotiana tabacum L.), and soybean (Glycine max L. Merr) are susceptible to various RKN species, including the southern root-knot (M. incognita), northern root-knot (M. hapla), peanut root-knot (M. arenaria), and the guava root-knot (M. enterolobii) (Thiessen & Rivera, 2019; Quesada, 2018). While these RKN species pose a threat to crop production in NC, considerable attention has been given to guava root-knot nematode in recent years as it is relatively new and causes significant damage to the state’s sweetpotato industry (Schwarz et al., 2021; Ye et al., 2013). North Carolina is the largest sweetpotato producing state, with 32,375 ha with a production value of $225,005,000 in 2022 (USDA, 2022). The guava root-knot nematode causes severe yield loss by affecting root quality and tonnage, thus reducing the overall marketable value of the produce (Brito et al., 2020; Moens et al., 2009).

With the introduction of sesame into North Carolina’s agronomic rotations, it is necessary to evaluate the crop’s potential to suppress nematode populations utilizing the cultivars proposed for this region. The objectives of this study were to evaluate sesame resistance to M. incognita, M. hapla, M. arenaria, and M. enterolobii, as well as to identify which sesame cultivars may provide higher resistance.

Nematode resistance trials were conducted at the North Carolina State University Method Road Greenhouse Complex, in Raleigh, NC. Seven commercially available sesame cultivars (Table 1) were screened for resistance to four RKN species) including Meloidogyne incognita, M. arenaria, M. hapla, and M. enterolobii. Seeds were provided by Equinom Ltd. (Givat Brenner, Israel) and Sesaco Corporation (Austin TX, USA). The tomato (Solanum lycopersicum L.) cv. Rutgers was used as a susceptible control. Three sesame seeds were planted in a 3.8 cm diameter × 20 cm deep (approximately 76 cm³ in volume) plastic cone container filled with a steam-sterilized 1:1 (v:v) sand-to-soil mixture, with a composition of 88% sand, 9% clay, 3% silt, and 0.20% organic matter. Seven days after planting (DAP), each container was left with one emerged seedling by carefully removing any other emerged seedlings. Each container was fertilized with 5 g of slow-release, balanced fertilizer (14-14-14 Osmocote Smart-Release Plant Food, The Scotts Company, Marysville, OH). The greenhouse was maintained at 25 °C to 28 °C, and plants were watered twice a week in the first 49 DAP and once a week from day 50 to harvest.

Thirty DAP, containers were inoculated with 1,000 RKN eggs by pipetting the egg solution into a 1.5 cm deep hole made into the soil at the base of each plant, which allowed the sesame plant to develop its root system. Eggs were collected from the four RKN species individually from cultures maintained on ‘Rutgers’ tomato following the NaOCl extraction method described by Hussey and Barker (1973). Species identity of each RKN isolate is confirmed yearly through species-specific PCR primers and conducting the North Carolina Differential Host Test. To produce the initial inoculum, roots from each infected tomato were soaked in a 10% bleach solution for 40 seconds, gently massaging egg masses from the roots. The solution was then poured through a set of sieves (from top to bottom, 250 μm, 75 μm, 25 μm) and rinsed with water to remove residual bleach from the eggs. Eggs were rinsed to the bottom 25 μm sieve with water, then collected in a 50 mL centrifuge tube to a final volume of 35 mL with water. To ensure a clean sample to aid in rapid counting of inoculum density, a sucrose centrifugation step was included. Each tube was topped to 50 mL with a 70% sucrose solution and centrifuged at 400 g for 5 min. After centrifugation, the supernatant was sieved (25 μm) a second time and transferred into a clean 50 mL tube. Eggs were quantified by taking three 100 μl subsamples and counting the eggs using an inverted microscope (Nikon TMS, Nikon Instruments Inc., Melville, NY, USA) at 40x magnification. Egg concentrations were calculated and expressed per mL solution.

Figure 1:

Plants were assessed for root gall severity following a 0–10 scale adapted from Zeck (1971). (a) example of a root gall severity score of 0 in a sesame root system and (b) root gall severity score of 5 in a ‘Rutgers’ tomato root system.

Sixty days post inoculation, the sesame and tomato control plants were cut at the crown to separate the roots. Each root system was rinsed with cold water to remove soil and fresh root weight was recorded. The trial was conducted twice and arranged in a complete randomized design with six replicate plants per cultivar × RKN species combination per trial. Four plants from each treatment group were given a visual root gall severity score based on the percentage of the root system galled using a scale of 0–10, with 0 representing 0% root galling, 6 representing 50% and 10 representing 100% (Zeck, 1971; Figure 1). RKN eggs were extracted from the same four individual root systems using the NaOCl method as described above. Three 1 mL subsamples were taken from each sample to quantify egg count. The average of these three subsamples was multiplied by the final extraction volume to estimate the total number of eggs per root system (final population, Pf).

Reproductive factor (RF) was calculated for each sample using a ratio of the final population (Pf) and the initial population (Pi; herein, 1,000). Reproductive factor values were used to determine the susceptibility or resistance of the sesame cultivars to the RKN species. An RF value less than 1.0 indicates a poor host or non-host resistance, while values greater than 1.0 indicate susceptibility (Ferris et al., 1993). An RF value equal to one indicates a maintenance host, in which the population of nematodes is neither increasing nor decreasing.