Cotton host resistance as a tool for managing Rotylenchulus reniformis in Louisiana

In 2022 and 2023, cotton fields were planted at two locations in Louisiana to evaluate the utility of new cotton cultivars with resistance to reniform nematode. The first field was located at the LSU AgCenter Northeast Research Station (NERS) in St. Joseph, Louisiana, United States (31.942155779069132, −91.22663016858824). The soil at the site is a Bruin silt loam (49% sand, 42% silt, 9% clay, 0.8% organic matter, 5.8 pH, 4.8 meq/100 g cation exchange capacity). The site has a history of moderate reniform nematode infestation, and the trial was planted in a location that was planted with soybean in the prior growing season. The second field was located at the LSU AgCenter Macon Ridge Research Station (MRRS) in Winnsboro, Louisiana, United States (32.13687481246169, −91.69727009027044). The soil at the site is a Gigger silt loam (35% sand, 48% silt, 17% clay, 1.3% organic matter, 5.4 pH, 6.3 meq/100 g cation exchange capacity). The site has a history of severe reniform nematode infestation and was also planted with soybean in the prior growing season. For both field locations, irrigation, fertilization, insecticide, and fungicide applications were followed according to commercial production practices for Louisiana.

At both field locations, the experimental design was a randomized complete block with five replications of each cultivar. In 2022, cultivars included: (1) Deltapine^® DP 1646 B2XF (susceptible), (2) Deltapine^® DP 2141NR B3XF (resistant), (3) PhytoGen^® PHY 332 W3FE (resistant), (4) PhytoGen^® PHY 411 W3FE (resistant), and (5) PhytoGen^® PHY 443 W3FE (resistant). In 2023, an additional susceptible cotton cultivar, PhytoGen^® PHY 340 W3FE, was also included. Cultivars were grown as small plots that were 10.7 m long by four rows wide. Rows were spaced 96.5 cm apart at NERS and 101.6 cm apart at MRRS. The trial was planted on May 19, 2022, and May 18, 2023, at the NERS field and on May 17, 2022, and May 23, 2023, at the MRRS field using a 420 seeds/plot planting density.

Stand count was measured at 14 and 28 days after planting (DAP). On each measurement date, the number of plants within a row was counted in four randomly selected 1.52-m subsections from within the center two rows of each plot. Data are presented as the number of plants per hectare. Plant vigor was measured at 56 DAP. Within each replicate, the most vigorous plot was assigned a value of 100%, and all subsequent plots within the replicate were assigned a vigor rating relative to the most vigorous plot. Data are presented as percent vigor.

Canopy coverage was measured at 56 DAP. Images were collected on a Samsung Galaxy S21 smartphone (Samsung Electronics, Suwon-si, South Korea) using a 1X zoom at a height of 1.52 m above the row bed and default image acquisition settings. Images were collected from four randomly selected locations within the center, two rows from each plot. Images were analyzed for canopy coverage using Canopeo software with vegetation type set to cotton (Patrignani and Ochsner 2015). Data are presented as percent canopy coverage.

The population density of soil nematodes was determined at planting, mid-season (56 DAP), and at harvest. Twelve randomly selected soil cores (20 – 25 cm in depth, 2.5 cm in diameter) were obtained from the center two rows of each plot. When plants were present (mid-season and harvest sampling dates), soil cores were removed approximately 5 cm from the stem. Soil cores from each plot were thoroughly mixed, then placed into a plastic bag and stored at 12 °C for a maximum of 14 days prior to subsequent processing. Nematodes were extracted from a 250-cm³ subsample of soil from each plot using a semi-automatic elutriator (Byrd et al. 1976) combined with the sugar flotation technique (Jenkins 1964). After collecting the nematodes over a 25-μM sieve, the nematode samples were transferred in water into plastic vials and stored at 12 °C for a maximum of two weeks prior to counting. The abundance of reniform nematode was determined using an inverted compound microscope. Data are presented as number of nematodes per 500 cm³ of soil.

Seed cotton was harvested on October 20, 2022, and October 13, 2023, at the NERS field and on October 2, 2022, and October 18, 2023, at the MRRS field. Yield data was collected from the center two rows of each plot using a two-row cotton picker. Data are presented as kilograms of seed cotton per hectare.

Data from the field trials were subjected to a one-way ANOVA in SAS OnDemand for Academics (SAS Institute Inc. 2014, Cary, NC) using the Mixed Model procedure. The experimental model was a randomized complete block with five blocks as random effects and cotton cultivars as fixed effects. Differences in treatment means were examined using Tukey’s HSD test (P < 0.05).

At the NERS field in 2022, seedling establishment did not differ among the five cotton cultivars at either measurement date (Table 1). Mid-season plant vigor ratings were greater for three of the resistant cotton cultivars, PHY 332 W3FE, PHY 411 W3FE, and PHY 443 W3FE, than DP 1646 B2XF. The cultivars PHY 332 W3FE and PHY 443 W3FE also improved crop vigor relative to DP 2141NR B3XF. Canopy coverage showed a similar trend to that of plant vigor, with all three of the resistant PhytoGen^® cultivars (PHY 332 W3FE, PHY 411 W3FE, and PHY 443 W3FE) displaying increased canopy coverage relative to that of DP 1646 B2XF. Despite differences in vigor and canopy coverage, no differences in seed cotton yield were observed among the five cotton cultivars, which yielded 2,899 – 3,256 kilograms of seed cotton per hectare at the NERS field. Reniform nematode soil population densities at the NERS field in 2022 averaged 5,472 – 8,832 nematodes per 500 cm³ of soil at the time of planting the field (Table 2). By the mid-season sampling date, reniform nematode soil population densities in the plots planted with DP 2141NR B3XF, PHY 411 W3FE, and PHY 443 W3FE were reduced by 39.3 – 56.8 % relative to that of plots planted with DP 1646 B2XF. At harvest, only PHY 411 W3FE and PHY 443 W3FE continued to maintain lower reniform nematode soil population densities relative to DP 1646 B2XF, resulting in 57.5 % and 54.9 % fewer nematodes in soil by the end of the growing season, respectively.

At the MRRS field in 2022, seedling establishment did not differ among the five cotton cultivars at either of the two measurement dates (Table 3). All four resistant cotton cultivars improved mid-season plant vigor ratings relative to that of DP 1646 B2XF; however, PHY 332 W3FE and PHY 411 also had higher plant vigor ratings than DP 2141NR B3XF. Canopy coverage was greater in the PHY 411 W3FE and PHY 443 W3FE plots than in plots planted with DP 1646 B2XF. No differences in seed cotton yield were observed among the five cotton cultivars, which yielded between 1,469 and 1,551 kilograms of seed cotton per hectare at the MRRS field. Reniform nematode soil population densities at the MRRS field in 2022 averaged 17,152 – 25,952 nematodes per 500 cm³ of soil at the time of planting the field (Table 4). By the mid-season sampling date, reniform nematode soil population densities in the plots planted with DP 2141NR B3XF, PHY 332 W3FE, and PHY 411 W3FE were reduced by 38.9 – 48.6 % relative to that of plots planted with DP 1646 B2XF. At harvest, only PHY 411 W3FE continued to maintain lower reniform nematode soil population densities relative to DP 1646 B2XF, resulting in 70.3% fewer nematodes in soil by the end of the growing season.

At the NERS field in 2023, seedling establishment did not differ among the six cotton cultivars at either measurement date (Table 5). Mid-season plant vigor ratings were greater for PHY 332 W3FE than the susceptible cultivar PHY 340 W3FE. Canopy coverage showed a similar trend, with PHY 332 W3FE displaying increased canopy coverage relative to that of PHY 340 W3FE and DP 1646 B2XF. Seed cotton yield was greatest with PHY 332 W3FE, which averaged 1,522 – 1,905 more kilograms per hectare of seed cotton relative to that of DP 1646 B2XF, DP 2141NR B3XF, PHY 340 W3FE, or PHY 443 W3FE. The cotton cultivar PHY 411 W3FE yielded 1,141 more kilograms per hectare of seed cotton than DP 2141NR B3XF and 1,265 more kilograms per hectare of seed cotton than PHY 340 W3FE. Reniform nematode soil population densities at the NERS field in 2023 averaged 320 – 2,624 nematodes per 500 cm³ of soil at the time of planting the field (Table 6). By the mid-season sampling date, reniform nematode soil population densities in the plots planted with either of the four resistant cotton cultivars were lower than plots planted with DP 1646 B2XF (59.6 – 70.1 % reduction) or PHY 340 W3FE (62.8 – 72.3 % reduction). At harvest, PHY 332 W3FE, PHY 411 W3FE, and PHY 443 W3FE continued to maintain lower reniform nematode soil population densities relative to DP 1646 B2XF or PHY 340 W3FE.

At the MRRS field in 2023, DP 2141NR B3XF had 31,271 and 30,592 more established seedlings per hectare at 14 DAP relative to that of PHY 340 W3FE and PHY 443 W3FE (Table 7); however, no differences in seedling establishment were observed by 28 DAP. All four resistant cotton cultivars improved mid-season plant vigor ratings relative to that of PHY 340 W3FE; however, only DP 2141NR B3XF, PHY 411 W3FE, and PHY 443 W3FE had higher plant vigor ratings relative to that of DP 1646 B2XF. Canopy coverage was greater in the PHY 332 W3FE and PHY 443 W3FE plots relative to that of plots planted with DP 1646 B2XF, DP 2141NR B3XF, or PHY 340 W3FE. No differences in seed cotton yield were observed among the six cotton cultivars, which yielded between 3,537 and 3,982 kilograms of seed cotton per hectare at the MRRS field. Reniform nematode soil population densities at the MRRS field in 2023 averaged 3,520 – 5,504 nematodes per 500 cm³ of soil at the time of planting the field (Table 8). No differences were observed in soil population densities at the mid-season sampling date. At harvest, reniform nematode soil population densities were lower in plots planted with PHY 332 W3FE and PHY 411 W3FE relative to that of plots planted with DP 1646 B2XF or PHY 340 W3FE.