The Rate of Decline of Sugarbeet Cyst Nematode in Central California Under Nonhost Crops May Impact Biofuel Production

Ten commercial fields in the San Joaquin Valley with a history of SBCN infestation were selected. Eight were in a “spring harvest” area (Fields A to H) known as Collegeville (N37°54.285′, W121°08.830″), and two were in a “fall harvest” area (Fields I to J) known as Eight Mile Road (N38°03.775″, W121°23.514′). In each field, 10 circular subplots located 30.5 meters apart (each with a 6-m radius) were established with reference to a permanent landmark. On each sampling date, 12 subsamples from each subplot were taken randomly with a shovel from the top 0 to 30 cm of soil and composited into a single sample of approximately 1 kg. Standard techniques were utilized to extract and count cysts from soil and eggs (Caswell et al., 1985; McKenry and Roberts, 1985). The extraction method is described in detail by Caswell et al. (1985). Briefly, 350 g of soil is air dried in paper bags. The soil is then thoroughly mixed, and cysts are separated from the soil using a modified Fenwick flotation can and caught on a 150-μm sieve. Cysts are washed onto tissue paper and air dried. Dried cysts are floated off the tissue onto filter paper in an ethanol-glycerine flotation apparatus and counted. Eggs are released from cysts using a tissue homogenizer (CEKA, Type UM, E. Bùhler, Tübingen, Germany) and then counted.

The standard extraction procedure for SBCN requires soil to be thoroughly dried prior to extraction. Because drying is not conducive to extraction of other nematode species, on the first sampling date for each field, nematodes were also extracted via elutriation followed by centrifugation (Byrd et al., 1976) to determine other nematodes which might be present. Fields were sampled approximately yearly for up to 6.3 years. Growers followed their normal crop rotation sequences which included corn (Zea mays), tomato (Lycopersicon esculentum), beans (Phaseolus vulgaris), wheat (Triticum vulgare), cabbage (Brassica oleracea), or Bok choy (Brassica rapa). A composite soil sample was analyzed by the University of California DANR Analytical Laboratory for physical and chemical properties. Data were evaluated by analysis of variance followed by Duncan’s New Multiple Range Test and regression (JMP, SAS Institute Inc.).

Field A was in cabbage when first sampled. Rotation crops included tomatoes (three times), cabbage, and wheat (two times) (Table 1). During the 6.3 years that populations were followed in this field, there was an overall decline from 4.35 eggs and 0.267 cysts to 0.41 eggs and 0.067 cysts per gram of soil, in spite of having cabbage in the rotation (Table 2). Eggs per cyst declined from 10.9 to 2.1. Between 2.8 and 3.8 years after first sampling, a slight but significant increase in cysts was noted when the field was cropped to tomatoes (P ≤ 0.05). The number of eggs and cysts more than doubled during this time. The yearly rate of egg decline in this field was 12.2% (y = 1.11005e – 0.29642x, r² = 0.702291, P ≤ 0. 0.0372, where x = years and y = eggs/gram of soil).

Field B had not been planted to sugarbeets for approximately eight years prior to first sampling but had been planted to Bok choy — a host of SBCN — for two years prior to first sampling. Bok choy is a 60- to 90-day crop and was the only one planted each year. Rotation crops included beans (two times), cabbage, wheat, and tomatoes (two times) (Table 1). Cysts, eggs, and the number of eggs per cyst were relatively low at the time of first sampling and remained at low levels until 3.8 years when the field was planted to tomatoes (Table 2). At this time, a significant increase in cysts, eggs, and eggs per cyst occurred (P ≤ 0.05). A regression equation fit on the sampling points prior to planting tomatoes indicates a yearly population decline of 34.6% for eggs (y = 0.10491 − 0.03632x, r² = 0.390959, P ≤ 0.1049054, where x = years and y = eggs/gram of soil).

At the time of first sampling, field C was just about to be planted to sugarbeets for the first time in eight years. Populations increased significantly under beets (P ≤ 0.05) and then declined during subsequent years (Table 2). Rotation crops included beans, tomatoes (three times), and wheat (two times) (Table 1). During the second planting of tomatoes at 3.8 years, a significant increase (P ≤ 0.05) in cysts but not eggs occurred. A regression line fit through all points except the first, which demonstrated a yearly decline rate for eggs of 12.4% (y = 0.76579e − 0.24778x, r² = 0.580384, P ≤ 0.1344, where x = years and y = eggs/gram of soil).

Field D was in sugarbeets at the time of first sampling. Rotation included fallow, beans (three times), corn, and wheat (two times) (Table 1). Populations of eggs and cysts did not change significantly during the course of the study (P ≤ 0.05) (Table 2). An increase in the number of eggs per cyst (P ≤ 0.05) occurred during a year when the field was fallow (1.8 to 2.8 years). A regression line fitted through all sampling points indicated a yearly decline in eggs of 20.7% (y = 0.30158e − 0.3138x, r² = 0.682169, P ≤ 0.0428, where x = years and y = eggs/gram of soil).

Fields E, F, G, and H were planted to sugarbeets the year prior to the first sampling and were farmed as a single unit during that time. One half of the field (G and H) had not previously been in sugarbeets for eight years. The other half of the field (E and F) had been planted to sugarbeets four years earlier. Following harvest of the sugarbeets, the grower elected to divide the field perpendicular to the original division and followed a different cropping pattern in fields E and H than in fields F and G (Table 1). Rotation crops in E and H were beans, tomatoes (three times), and wheat (three times). Rotation crops in F and G were tomatoes (four times) and wheat (three times).

During the 6.3 years that populations were followed in E, there was an overall decline from 1.08 eggs and 0.066 cysts to 0.21 eggs and 0.032 cysts per gram of soil (P ≤ 0.05) (Table 2). The number of eggs per cyst declined from 6.1 to 1.3 (P ≤ 0.05) (Table 1). A regression line fitted through all sampling points indicated a yearly decline in eggs of 15% (y = −0.135x + 0.8993, r² = 0.7633, P ≤ 0.0102, where x = years and y = eggs/gram of soil).

In field F, the number of eggs declined from 1.60 to 0.06 (P ≤ 0.05) while the number of cysts did not change significantly. The number of eggs per cyst declined from 6.1 to 0.4 (P ≤ 0.05) (Table 2). A regression line fitted through all sampling points indicated a yearly decline in eggs of 26.8% (y = 0.39149e − 0.46749x, r² = 0.797384, P ≤ 0068, where x = years and y = eggs/gram of soil).

Populations in G were barely at the detection level when first sampled (Table 2). A decline in eggs and eggs per cyst was not detected during the course of the study. There was an increase in cysts at 5.1 years when the field was in tomatoes (P ≤ 0.05).

Field H had detectable levels of cysts but not eggs when first sampled shortly after sugarbeets were harvested (Table 2). Sampling shortly afterwards at 0.3 years produced 0.67 eggs and 0.006 cysts per gram of soil and 6.4 eggs per cyst. These numbers declined over the next six years to non-detectable levels of eggs and of eggs per cyst (P ≤ 0.05). A linear regression line fitted through all but the initial sampling point indicated a yearly decline in eggs of 18.9% per year (y = −0.0899x + 0.4753, r² = 0.5869, P ≤ 0.0757, where x = years and y = eggs/gram of soil).

Sugarbeets had been recently harvested from Field I when it was first sampled. At that time, it had the highest number of eggs per gram and eggs per cyst of any of the fields sampled. Subsequently, it was fallow for three years of the study and in corn and sugarbeets for one year each (Table 1). There was an overall decline from 9.59 eggs and 0.204 cysts to 1.17 eggs and 0.151 cysts per gram of soil (P ≤ 0.05) (Table 2). The number of eggs per cyst declined from 25 to 4.7 during the same period (P ≤ 0.05). Populations did not increase during the time it was in sugarbeets (2.3 to 3.3 years). A regression line fitted through all sampling points indicated a yearly decline in eggs of 21.1% (y = −1.9965x + 9.4488, r² = 0.9223, P ≤ 0.0094, where x = years and y = eggs/gram of soil).

Field J was planted to tomatoes at the time of first sampling. It had been in sugarbeets the previous year for the first time in 20 years, when the grower reported a serious problem with SBCN. Following tomatoes, corn was planted three years in a row, then sugarbeets and wheat (Table 1). During the course of the study, the number declined from 1.51 eggs and 0.188 cysts to 0.32 eggs and 0.074 cysts per gram of soil (Table 2). Eggs per cyst declined from 5.1 to 1.9. A regression line fitted through all sampling points indicated a yearly decline in eggs of 24.8% (y = 0.41454e − 0.43177x, r² = 0.981131, P ≤ 0.0095, where x = years and y = eggs/gram of soil).

At the time of first sampling, no other plant parasitic nematodes were detected in six of the 10 fields. Three fields contained Pratylenchus sp., two each Meloidogyne sp. and Xiphinema sp., and one each Tylenchorhynchus sp. and Helicotylenchus sp (Table 1).

Fields in the Collegeville area (A to H) were either a silty clay or a silty clay loam, with levels of organic matter ranging from 1.4% to 2.4%. In the Eight Mile Road area, fields (I to J) were a loam with organic matter ranging from 3.9% to 6.2% (Table 3).

A significant finding of this study is that San Joaquin County’s rates of population decline are slower than they are in the Imperial and Ventura Counties. This validates grower experience that longer rotations between crops of sugarbeet are required. A slower decline rate necessitates a longer rotation between crops of sugarbeet impacting the frequency with which the crop can be grown. Additionally, we found that reproduction of SBCN on tomatoes, that had been previously shown in greenhouse trials, also occurs in grower fields.

The first recorded nematode pathogen of sugarbeets was SBCN, and it remains an important pathogen (Altman and Thomason, 1971; Cooke, 1993; Roberts and Thomason, 1981). SBCN is common and a significant problem in most areas of the world where sugarbeets are grown (Cooke, 1993). It is considered the third most important plant-parasitic nematode in the world (Bernard et al., 2017; Sasser and Freckman, 1987). The SBCN has hosts in a range of plant families; Steele (1965) investigated approximately 200 hosts in 98 genera from 23 out of 49 families. Of the agronomic crops that are known hosts, most occur within the Chenopodiaceae (sugarbeet, fodder beet, red beet, mangold, and spinach) and the Cruciferae (cabbage, kale, brussels sprout, broccoli, cauliflower, turnip, kohlrabi, mustard, and radish).

Over a period of several years, Roberts et al. (1981) sampled three fields infested with SBCN in the Imperial Valley and one on the Oxnard plain of California at 0 to 30- and 30 to 60-cm depths from two to five sites sampled in each field, with eight subsamples per sampling site. In their study, populations declined at the rate of 49% to 80%, leading to predictions of three- to four-year rotations for numbers to drop below the damage threshold of one to two eggs/gram of soil in these areas (Roberts and Thomason, 1981). In contrast, growers in the “spring harvest” area of the San Joaquin Valley reported needing eight to 10 years of rotation for SBCN to drop below damaging levels (F. Kegel, personal communication).

In England, Jones (1956) found the rate of decline of SBCN eggs to be 33% to 50% per year. In the Netherlands, Ouden (1956) obtained yearly egg decline rates for SBCN of 38% to 66%. In England, Moriarty (1963) reported SBCN egg decline rates ranging from 36% to 60% per year in one study and from 37% to 67% in another (Moriarty, 1961). In the current study, yearly rates of population decline could be measured in nine of the 10 fields examined and ranged from 12.2% to 34.6%. Rates of population decline were similar for “spring” and “fall harvest” areas.

Weed management during rotations could account at least partially for the apparent slow rate of decline of SBCN in this study. In the Imperial Valley and Oxnard Plain of California, it is common for two or even three rotation crops to be grown within a single year, with weed management being conducted for each crop. In the San Joaquin Valley, on the other hand, one crop per year during the spring and summer with a prolonged period of “fallow” during the fall and winter is common. During this “fallow” period, weed management is difficult because of the prolonged rainy season. Primary weeds in this area, which are reported hosts of SBCN, include yellow mustard (Brassica sp.), and shepherd’s purse (Capsella burs-pastoris). The population increase — occurring in field D during a year in which no crop was grown — could have occurred on weedy hosts. Longer rotations may also be required because overwintering of sugarbeets in “spring harvest” areas could lead to higher populations at harvest that would result in longer rotations being required.

Suppressive soils (loosely defined as fields that should have nematode problems but do not) have intrigued nematologists for many years (Westphal, 2005). Research has found that soils suppressive to SBCN frequently have one or more species of nematode parasitic fungi. Jaffee et al. (1991) found that H. rhossiliensis, which is parasitic on juveniles of SBCN, was present in California sugarbeet fields (including 80% of fields sampled in San Joaquin County), as was the ring trapping fungus Arthrobotrys dactyloides. Fungi parasitic on cysts and eggs of SBCN — including Hyalorbilia oviparasitica, Dactylella oviparasitica, and Brachypyoris oviparasitica — are also common in fields with a history of growing sugarbeets (Chen et al. 2021). These fungi could be a factor in the rates of decline seen in the fields studied. Tedford et al. (1993) suggest that the parasitic fungal populations are self-regulating in that they maintain a level below that which will eradicate the nematode population, thus ensuring that a continuous food source will be available.

Soil properties have been shown to affect nematode reproduction. For example, working with two soil types, Santo and Bolander (1979) demonstrated that Meloidogyne hapla reproduced best in a sandy loam soil while SBCN reproduced best in a silt loam. Cooke (1991) found that damage to sugarbeets was less in organic soils than in mineral soils. Fields in the present study were selected because growers had experienced significant damage to sugarbeets from SBCN. The fields in the Roberts et al. (1981) study on SBCN decline rates in southern California were both finer- and coarser-textured than in our study. Their fields had a clay content as high as 58.6% in one case and a sand content as high as 53.1% in another. The highest clay content in our fields was 45%, and the highest sand content was 37%. Although there are statistically significant differences in reproduction on various soil types, SBCN reproduces well enough in a wide range of soil types to cause significant economic damage to sugarbeet.

SBCN can produce more than 600 eggs/cyst under laboratory conditions (Raski, 1949), with several hundred per cyst not being unusual (Caswell and Thomason, 1991). Throughout the course of this study, on average, relatively few eggs were recovered per cyst. This is consistent with other samples processed from the San Joaquin Valley area over a period of years (B. Westerdahl, personal communication).

In fields that contained cabbage in rotation, the crop was typically planted in March and harvested in May or June, allowing minimal time for nematode reproduction to occur. Populations in Fields A and B declined in spite of cabbage in rotation.

Tomatoes are a host for SBCN in greenhouse trials (Lear and Miyagawa, 1972; Griffin and Waite, 1982). In this study, SBCN populations did not increase every time a field was planted to tomatoes, but increases were seen on three occasions, each time in a different field (A, B, and C).

The apparent population increase which occurred in field H several months following harvest of the sugarbeets could have been due to maturation of cysts on roots remaining in the soil following harvest, as has been demonstrated in microplot trials (Gardner and Caswell-Chen, 1997).

A shovel was utilized for sampling because soil types in this sugarbeet growing area typically contain substantial amounts of clay and silt (Table 3) and are difficult to sample with soil probes. Sampling was confined to the top 30 cm for two reasons: (i) because of soil texture, and (ii) because a previous study by Roberts et al. (1981) — in which samples from 0 to 30- and 30 to 60-cm depths were compared — indicated that highest numbers of cysts and eggs were found at the shallower depth.

This study demonstrates that the need for longer rotations in the San Joaquin Valley sugarbeet growing area is likely due to a combination of factors: (i) an apparent slower rate of population decline on rotation crops, (ii) weed hosts during rotations allowing populations to increase, (iii) population increases on tomatoes, and (iv) continued reproduction during winter months on overwintered sugarbeets. The possibility of a lower damage threshold in this area of California should be examined in future research.

A non-linear critical point model developed by Seinhorst (1965) has been used to relate yield of sugarbeets to initial populations of SBCN at the time of planting (Cooke and Thomason, 1979; Greco, Brandonisio and de Marinis, 1982; Cooke, 1984). Intraspecific competition among nematodes results in decreased damage per nematode as density increases. The model predicts that lower initial populations will result in higher populations at harvest than beginning with higher populations, leading to the subsequent need for increased lengths of rotation. Production losses owing to SBCN vary but can be as high as 60% of the crop (Ghaemi et al., 2020).

Based on crop rotation programs developed in England (Cooke, 1991) to reduce losses on sugarbeet due to SBCN, in California, processors, growers, County Agricultural Commissioners, and the University of California developed a program in which sugarbeets cannot be planted in non-infested fields more than two consecutive years and not more than four out of 10 years. In infested fields, sugarbeets can be grown only once every four years (Roberts and Thomason, 1981). Field infestation is monitored by an intensive sampling program conducted by processors and enforced through their contracts with growers. The success of this program is due to the natural decline of SBCN in the absence of host plants. Slower rates of decline in a region result in a longer length of rotation between sugarbeet crops, which could impact the economics of using sugarbeets for production of biofuel.

Interest in the use sugarbeets to produce bioethanol is increasing. The results of this study indicate that the potential impact of SBCN and other pests on the economics of bioethanol production should be assessed. Jiménez-Islas et al. (2021) conducted a bibliometric analysis of the Web of Science database to identify research related to sugarbeet as a biofuel. From 2003 to 2019, an exponential growth of publications was found, with Germany and the United States being the countries with the highest rates of increase. Growth can be attributed to the development of renewable energy and the relevance of global warming, energy security, and laws that promote clean energy.

Several studies have provided estimates of the amount of bioethanol that can be produced from sugarbeets. A 2006 USDA study calculated the yield of ethanol from the sucrose in a sugarbeet was 103.5 liters per 907 kg of root (wet weight) (Panella and Kaffka, 2010). In North Dakota, research suggests that 100.3 liters of ethanol can be produced from each 907 kg of sugarbeets (Farm Progress, 2010). A University of California, Davis study estimated 126.8 liters of ethanol could be produced from 907 kg of sugarbeets. (Zhang et al., 2011). A study by Iowa State University estimates 99.9 liters of ethanol could be produced from 907 kg of sugarbeets. Average sugarbeet yields in the United States are 49,307 kg/ha, with approximately 9,525 kg of sugar being produced per ha (Spreckels Sugar, 2012). Using an average estimate of 107.6 liters of ethanol per 907 kg of root and 49,307 kg of root/ha, this would yield an estimated 5,844 liters/ha of ethanol from a ha of sugarbeets.