Impact of Globodera ellingtonae on yield of potato (Solanum tuberosum)

Inoculum of G. ellingtonae was produced at the Oregon State University Central Oregon Agricultural Research Center (OSU-COARC), Powell Butte, OR by planting potato Russet Burbank into 22-liter pots buried in the soil containing G. ellingtonae cysts in May 2015. Plants were allowed to grow for four months, receiving irrigation and fertilizer. The pots were collected and soil was emptied onto a tarp, dried, and passed through a 4-mm sieve. The soil was mixed and subsamples (100 g) collected to determine G. ellingtonae initial population densities (Pi). Cysts were extracted from ten 100 g soil subsamples using a USDA cyst extractor (Ayoub, 1980), collected and counted. The number of eggs/cysts was determined by crushing all the collected cysts within the subsample with a rubber stopper on a 60- over a 500-mesh sieve (Zasada et al., 2013). The eggs that were retained on the 500-mesh sieve were washed into a 50 ml polystyrene tube. Eggs were enumerated by counting two 1 ml aliquots using an inverted microscope. The average cyst density of the soil was 0.6 (±0.04) cysts/g soil, the average egg density was 169 (±13) eggs/g soil, and the average number of eggs/cyst in soil was 255 (±26). Soil without G. ellingtonae was collected on a part of the farm at Powell Butte, OR where no G. ellingtonae had been found. Collected soil was dried and passed through a 4-mm sieve. Both the infested and uninfested soil is a Redmond ashy sandy loam.

The experiment was conducted in 2017 in microplots in Corvallis, OR. The experimental unit was a 5-liter pot, and pots were placed into beds that were 2.4 × 1.2 × 0.3 m. The bed frames were constructed with wood and lined with pond liner. Soil containing G. ellingtonae was mixed with non-cyst-containing soil to achieve varying Pi (Table 1). The total weight of soil in the pots was 4,800 g. During the process of mixing the soils, water was added to the mixture to create an initially moist environment. The pots were placed into the beds and the remaining area around the pots was filled with soil to help insulate the pots. A single-drop potato Russet Burbank tuber was planted approximately 6 cm deep in each pot. Two trials were conducted each occupying two beds with the second trial established a week after the first trial. The experimental design was completely randomized for both trials with treatments replicated six times.

The pots were watered daily or as needed. The plants were fertilized with Osmocote Smart-Release Plant Food (Scotts Miracle-Gro; Marysville, OH) at planting and then one more time approximately 3-weeks-after emergence with 20N-20P-20K fertilizer (Peters; Allentown, PA). Over the course of the experiments, any plant health issues (insect or pathogen damage, chlorosis, necrosis) were noted. After the majority of the plants within a trial had senesced, at 16 and 15 wk in trials 1 and 2, respectively, the trials were terminated. At this time, the tops of the plants were removed and placed into a 70°C dryer for a week and then weighed to obtain aboveground biomass. Each pot was then lifted out of the ground and the contents of the pot emptied into a container. The contents of the pot were laid out to dry for a week in a greenhouse. The tubers were collected and the soil was mixed and a 100 g subsample of soil collected for G. ellingtonae extraction as described above. The tubers were counted and each tuber was weighed individually.

A two-way analysis of variance (ANOVA) with trial, Pi and trial × Pi was used to test for difference in mean yields, aboveground biomass, tuber number, and individual tuber weight, followed by a Tukey’s honest significant difference (HSD) test for pairwise comparisons; since trial was not significant in the model, data from the trials were combined for presentation. To test for a relationship between Pi and tuber yield and Pi and aboveground biomass, linear regression with log₁₀ transformed Pi (x + 1) was performed in concordance with the relationship between Pi and yield proposed by Oostenbrink (1966); data were combined from the trials for linear regressions. Although various models have been proposed to describe the relationship between Pi and yield for PCN, the loglinear equation of Oostenbrink (1966) was used as it is simpler, does not require estimation of unknown parameters such as minimum yield and damage threshold, and has been determined to fit as well or better than other proposed relationships at mid-range Pi (Mulder et al., 1997). Host status and reproductive ability were measured by eggs/cyst, final density of eggs/g soil (Pf), and reproduction factor (RF) calculated as Pf/Pi, with differences tested by two-way ANOVA and Tukey’s HSD. Data were transformed to log₁₀ (x + 1) when necessary to meet the assumptions of ANOVA. All statistics were performed using JMP 13 (SAS Institute, Cary, NC).

Field trials were carried out in 2013 (two trials), 2014 (two trials), and 2015 (one trial) at OSU-COARC, Powell Butte, OR (Table 1). The field used in all years and trials had been in a long-term four-year potato rotation. It was planted to barley (Hordeum vulgare) and Austrian winter pea (Pisum sativum) in 2007, to potato in 2008, and was fallow in 2009 and 2010. At the end of summer 2010, the field was planted to a winter oat (Avena sativa) cover crop and in all subsequent winters was cover cropped with winter wheat (Triticum aerstivum). Each year, trials were placed in an area of the field where pretreatment soil sampling did not recover G. ellingtonae cysts. The soil at Powell Butte, OR is a Redmond ashy sandy loam.

In each year, an area approximately 20-m wide × 45-m long was prepared for potato planting. Treatments of varying Pi (Table 1) were applied to individual potato plants using cysts reared as described above. In 2014, a trial was conducted to evaluate additional potato cultivars from different maturity classes two early (‘Russet Norkotah’ and ‘Yukon Gold’), one mid (‘Ranger Russet’), and two late (‘Umatilla Russet’ and ‘Alturas’) maturing cultivars were used. Yukon Gold is a yellow table stock cultivar, Désirée is a red table stock cultivar, and all of the other cultivars are russeted types used primarily in processing. In all trials, Pi/cultivar combinations were replicated at least seven times. All trials were arranged in a completely randomized block design in a single (Russet Burbank and Désirée trials) or multiple (cultivar and Ranger Russet trials) rows. Additional rows of potatoes were planted on either side of experimental rows to serve as an environmental buffer.

Prior to establishing treatments, the entire area was treated with the pre-emergence herbicide S-ethyl dipropylthiocarbamate (Eptam, Gowan Company, Yuma, AZ, USA) at a rate of 4.1 liter/ha and 1,344 kg/ha triple 16 (16-16-16 N-P-K) prior to shallow cultivation. To plant and inoculate potatoes, a 30-cm diam. × 30-cm deep hole, with the top of the hole at the base of the furrow, was excavated with a 30-cm diam. gasoline-powered auger. In-row spacing between plants was 76 cm with rows spaced 86 cm apart. To establish treatments, uninfested field soil was thoroughly mixed with pre-measured inoculum soil to produce the desired level of eggs/g soil for that Pi. Globodera ellingtonae inoculum soil with average egg densities of 219, 117, and 320 eggs/g soil and 259, 114, and 245 eggs/cyst were used in 2013, 2014, and 2015, respectively. A total volume of ~9,000 cm³ of mixed soil (with or without G. ellingtonae inoculum) was returned to augured holes. Cut tuber pieces, ~56 g, treated with mancozeb (Maxim MZ; Syngenta Crop Protection, Greensboro, NC) were then placed on top of the soil in the furrow. The area surrounding the experimental area was planted to Russet Burbank for a total of 16 rows. After hand planting, the tubers were treated with imidacloprid (Alias 4F; Adama, Aventura, FL) at 0.44 liter/ha and azoxystrobin (Quadris; Syngenta Crop Protection) at 0.9 liter/ha applied in a 15-cm band over the seed, and then the furrows were closed with additional soil using a tractor-mounted disk hiller. Potatoes were planted May 20, 2013, May 9, 2014, and May 14, 2015. The trial area was sprinkler irrigated and managed with cultural practices common in central Oregon. To control early blight (Alternaria solani), plants were treated with boscalid (Endura; BASF, Florham Park, NJ) at 0.33 liter/ha and azoxystrobin at 0.88 liter/ha based upon a growing degree day model used in the region (Gent and Schwartz, 2003). Weeds were additionally managed post-plant with pendimethalin (Prowl; BASF) at 2.34 liter/ha. Potatoes were monitored weekly for vigor including stunting and chlorosis.

Potatoes were harvested approximately 18-weeks-after planting. Aboveground biomass was collected 2 wk prior to harvest by clipping stems at surface level. Dry weights were determined after drying in a 50°C oven for at least a week. At harvest, tubers from each plant were hand-dug and individually weighed. At the same time tubers were collected, an approximately 500 g soil sample was collected near the root zone of each plant and placed in plastic bags. Soil samples were air dried on trays in a greenhouse for at least 1 wk and cysts extracted and enumerated as described above.

A one-way analysis of variance (ANOVA) was used to test for differences in mean yields between, inoculum levels in Trials 1, 2, 3, and 5 followed by a Tukey’s HSD test. To test for a relationship between Pi and yield, linear regression with log₁₀ (Pi + 1) was performed. Linear regressions were performed separately for Trials 1, 2, 4, and 5. A Welch two sample t-test was used to compare differences in mean yields between treatments for Trial 4. Host status and reproductive ability were measured by RF and Pf with differences among Pi densities tested by ANOVA and Tukey’s HSD. Data were log₁₀ ( x + 1) when necessary to meet the assumptions of ANOVA. All statistics were performed using JMP 13.

Simple linear regression revealed that there was a positive correlation between logPi and tuber yield (Fig. 1); this correlation was significant (P ≤ 0.002). There was also a positive correlation between logPi and aboveground biomass (P ≤ 0.011, R ² = 0.08) (data not shown). The number of tubers produced per plant and the average individual tuber weight were not affected by increasing Pi (P ≥ 0.25) (data not shown). Number of tubers produced per plant ranged from 5 to 9 and average individual tuber weight ranged from 31.3 to 48.7 g.

Figure 1:

No cysts were recovered from the non-inoculated control in either trial (data not shown). Pf increased with increasing Pi (Table 2). While there was a significant difference in the number of G. ellingtonae eggs/cyst, for most Pi a similar number of eggs/cyst were produced (Table 2). RF values were significantly lower with higher Pi compared to lower Pi (Table 2).

Tuber yields of Russet Burbank averaged 1,580 (±129) and 2,505 (±108) g/plant across all Pis in Trial 1 and Trial 3, respectively. The average tuber yield of Désirée in Trial 2 was 2,087 (±112) g/plant and of Ranger Russet in Trial 5 was 2,347 (±94) g/plant. Average per plant tuber yields for Russet Burbank were significantly different between Trial 1 and Trial 3, and between Désirée (Trial 2) and Russet Burbank (Trial 5) (P ≤ 0.01). Simple linear regression revealed that there was a negative correlation between logPi and tuber yield for Trials 1, 2, 3, and 5 (Fig. 2); however, the negative correlation was only significant for Russet Burbank in Trial 1 (P ≤ 0.015) and Ranger Russet in Trial 5 (P ≤ 0.005). In all of the experiments, a Pi of 80 eggs/g soil was included (Table 3). For the cultivar trial (Trial 4), no significant differences were observed between yields of inoculated (Pi = 80 eggs/g soil) and non-inoculated plants for any of the potato cultivars (P > 0.05) (Table 3). The only difference between yield of inoculated (Pi = 80 eggs/g soil) and non-inoculated plants was observed for Russet Burbank in Trial 1 and Ranger Russet in Trial 5 (P ≤ 0.05); there was a 43 and 19% reduction in yield of potato when G. ellingtonae was present compared to when the nematode was not present, respectively (Table 3). Comparing the Pf from the 80 eggs/g soil treatment among the potato cultivars (Trial 4) revealed significantly higher Pf for Russet Burbank than for three cultivars: Alturas, Umatilla Russet, and Yukon Gold (P ≤ 0.05) (Table 3). Likewise, the same significant differences between Russet Burbank and those three cultivars were observed for RF values (P ≤ 0.001). As another indicator of host quality, we compared the mean final eggs/cyst between all cultivars. All cultivars had significantly fewer eggs/cyst than Russet Burbank inoculated with 80 eggs/g soil (Table 3).

Figure 2:

Final egg densities (Pf) tended to increase with increasing Pi except for Russet Burbank in Trial 1, where the highest Pf was observed at Pi = 20 eggs/g soil (Table 4). Although, in general, higher Pf were observed at higher P _i , the highest RF values generally occurred at the lowest Pi (Table 4). This trend was significant (P ≤ 0.05) in all of the field trials except for Russet Burbank in Trial 3. In all of the other years/cultivars the lowest Pi (10 or 40 eggs/g soil) had RF values at least five times greater than the corresponding highest Pi (80 or 320 eggs/g soil). There was some variability in egg/cyst across the Pi in Russet Burbank in Trial 3 and Ranger Russet in Trial 5. In general, there were fewer eggs/cyst at the highest Pi in these trials compared to at least one of the lower Pi (Table 4).