Population dynamics of Heterodera schachtii Schm. and yield response of susceptible and resistant sugar beet (Beta vulgaris L.) after cultivation of susceptible and resistant oilseed radish (Raphanus sativus L.)

Populationsdynamik von Heterodera schachtii Schm. und Ertragsreaktion von anfälligen und resistenten Zuckerrüben (Beta vulgaris L.) nach dem Anbau von anfälligem und resistentem Ölrettich (Raphanus sativus L.)

Materials and Methods

Fig. 1. Experimental design of the field trials, 2001–2005. Planting strips of the oilseed radish were re-rando­mised at each location.

All trials were conducted on fields with 3-year rotation of sugar beet. Following winter barley (Hordeum vulgare L.) or winter wheat (Triticum aestivum L.; only at Borsum 2005 and at Kelz 2005), oilseed radish (Raphanus sativus L. var. oleiformis Pers.) ‘Adagio’ (nematode-resistant), ‘Siletta Nova’ (susceptible) or a 50/50 mix of both were grown as a cover crop in the year prior to sugar beet (Beta vulgaris L. ssp. vulgaris var. altissima Doell).

Oilseed radish was planted in July/August (Tab. 1) at 25–30 kg ha^–1 after site-specific tillage and seedbed preparation and application of quick-acting N-fertiliser (calcium ammonium nitrate or urea-ammonium nitrate solution, 50–60 kg N ha^–1). The oilseed radish stands were cut once or twice during the vegetation period and finally ploughed down. A drought period in June 2003 caused low emergence and poor establishment of the plant stands at Ottbergen, Niedernjesa and Geroldshausen. Therefore, the 2004 sugar beet trials at these sites were cancelled, and oilseed radish was sown in May 2004 at Obernjesa and Borsum. At the other sites, oilseed radish development was good or adequate.

Two sugar beet varieties were grown, one susceptible (‘Macarena’) and one resistant to H. schachtii (‘Paulina’). For all trials in 2002–05, sugar beet seeds from the same seed lots were freshly coated every year and treated with standard amounts of the fungicides thiram and hymexazol and the insecticides imidacloprid and tefluthrin.

In all trials, sugar beets were sown after site-specific seedbed preparation and across the tillage direction. Seeding distance was 6–11 cm within and 45 cm between the rows. The stands were thinned to a plant distance of 21–25 cm as early as possible to avoid a trap effect of the sugar beet plants. Crop protection was carried out according to best local practice. In all 6-row plots, three of the four central rows were harvested (5.40–11.25 m²) with on-site available machinery. Root yield and concentrations of sucrose, K, Na and amino-N were determined in tarehouses in the particular growing regions or at the Institute of Sugar Beet Research Göttingen. The beets were washed, weighed and brei samples were prepared. Quality analysis was carried out using an automatic beet laboratory system (Venema, Groningen, NL) according to standard procedures (ICUMSA, 1994; Kubadinow and Wieninger, 1972; Burba and Georgi, 1975, 1976). White sugar yields were calculated according to the standard equations of quality assessment in Germany (Buchholz et al., 1995; Märländer et al., 2003).

The sugar beet at Geroldshausen 2003 were frost-damaged in mid April (BBCH 10–12) followed by drought later in the season. Consequently, yield was far below average and the data were excluded from overall yield analysis (see below). In Koslar 2003, beets had to be re-sown in late April due to technical problems caused by cover crop residues on the soil surface and a patchy crop stand after sowing on 24 March.

The heterogeneous distribution of cyst nematodes in the field requires a high number of soil samples and an adequate amount of soil to assess the actual nematode density (Müller, 1983a, b). Before oilseed radish sowing, the initial population density of H. schachtii (Pi_OR) was determined from one mixed soil sample (20–24 cores) per experimental field. The initial population density in sugar beet (Pi_SB) was measured plotwise and also regarded as the final population density under oilseed radish (Pf_OR). Soil samples were taken within 8–10 days after sugar beet sowing to prevent an earlier hatching of the juveniles. The final nematode population in sugar beet (Pf_SB) was determined in soil samples taken shortly before harvest. At each sampling date, four cores were taken from each row (i.e. 24 samples per plot) in 10 cm distance to the beet to a depth of 30 cm. Each core contained ca. 250 g of soil, amounting to a composite sample of ca. 6 kg soil per plot. The composite samples were mixed and stored in plastic bags at 5°C until analysis. Three aliquots were processed for each plot (see below). Reproductive rates (r) were calculated plot-wise for sugar beet (r_SB = Pf_SB/Pi_SB) and trial-wise for oilseed radish (r_OR = Pf_OR/Pi_OR; identical reference area).

The number of eggs and juveniles (E + J) in soil was determined by a standardised procedure in the labora­tories of the former BBA at Münster (for trials in South Lower Saxony) and Elsdorf (trials in Rhineland), at Bavarian State Research Centre for Agriculture (LfL) at Freising (trials in Franconia) and Plant Protection Office (PSA) Hanover (trials in the Hildesheim Plain). In an annual round robin test, systematic differences between the four laboratories were detected (data not shown). In the overall analysis of white sugar yield and Pi-levels, values were adjusted using laboratory specific correction factors. These were calculated for each laboratory and year as the deviation from the mean value by all laboratories in the round robin test.

According to the different techniques of the respective laboratories, cysts were either extracted by density centrifugation or a modified Oostenbrink elutriator (EPPO, 2013). Cysts were extracted from a defined quantity of soil (depending on the laboratory 200 to 300 g). After cysts were crashed by a modified revolving grinding mill, the number of eggs + juveniles was determined at minimum dilution of 30 mL and at higher dilutions if the sample exceeded 20 cysts. The viable eggs and juveniles were counted under a microscope in calibrated counting chambers with 1 mL volume. The results were converted to counts per 100 g of soil.

Results

The reproductive rate in sugar beet (r_SB) was higher in the susceptible variety than in the resistant one at all locations (Tab. 3, Fig. 2) and highest when Pi_SB was low (Fig. 3). Varietal differences in r_SB were greater after resistant oilseed radish (i.e. at low Pi_SB) than after sus­ceptible oilseed radish (i.e. at high Pi_SB) (Tab. 3). In the

susceptible variety, reproductive rates > 1 were found at all environments except Ottbergen 2002, where Pi_SB was about 900 E + J · 100 g soil^–1 (Tab. 2, Tab. 3). Contrastingly, a population increase was found at Koslar 2003 at Pi_SB beyond 2300 E + J · 100 g soil^–1 resulting in a maximum Pf_SB of 7636 E + J · 100 g soil^–1 in the susceptible variety (data not shown). In the resistant sugar beet variety, reproductive rates > 1 were found at eight environments in the resistant oilseed radish treatment, at four of these environments in the susceptible oilseed radish treatment as well (Tab. 3). Comparing all sites, equilibrium density of the population was at a maximum at Niederembt 2004 and at Kelz 2005 being close to 1000 E + J 100 g^–1 soil (Fig. 2b and 2c) whereas reproductive rates were con­siderably lower at Niederembt 2004 than at Kelz 2005 (Tab. 3). At all other environments, population growth could be detected at Pi_SB below 400 E + J 100 g^–1 soil and lower (Fig. 2d).

Fig. 2. Population density of eggs and juveniles of Heterodera schachtii shortly after sowing (Pi) and shortly be­fore harvest (Pf) of a nematode-susceptible and a nematode-resistant sugar beet variety. Thirteen environ­ments, Germany 2002–2005 (a), Geroldshausen 2002 (b), Niederembt 2004 (c) and Kelz 2005 (d); n = 5–8.

Fig. 3. Relation between initial population density (Pi) of Hetero­dera schachtii and reproductive rate in a susceptible and a resi­stant sugar beet variety. Thirteen environments, Germany 2002–2005. *, **: significant at p ≤ 0.05 and p ≤ 0.01; n = 5–8.

Fig. 4. Initial population densi­ty (Pi) of Heterodera schachtii and root yield of a susceptible and a re­sistant sugar beet varie­ty. Thirteen environments, Germany 2002–2005; n = 5–8.

Averaged over all trials, in plots following susceptible oilseed radish and the 50/50 mix, white sugar yield was higher in Paulina than in Macarena, whereas Macarena yielded higher than Paulina following resistant oilseed radish (Tab. 4). Differences between the sugar beet varieties were greater in the susceptible (significant at four locations) than in the resistant oilseed radish treatment (significant at two locations). Average yield gain following resistant compared to susceptible oilseed radish was 0.89 t white sugar ha^–1 in Macarena and 0.36 t ha^–1 in Paulina. The 50/50 mix of the oilseed radish varieties was intermediate. However, this effect was not consistent at all environments. Significantly higher yield in Paulina than in Macarena could be realised at low initial popu­lation densities of H. schachtii (Geroldshausen 2002, Ottbergen 2003; both sites without significant variation in Pi_SB). At other environments with high Pi_SB, Paulina yielded lower than Macarena in all oilseed radish treatments (Niedernjesa 2003 and Koslar 2003).

The effect of increasing population densities of H. schachtii on white sugar yield was demonstrated by calculating expected values for white sugar yield at Pi_SB levels of 500, 1500, 2500 and 3500 E + J 100 g^–1 soil (Fig. 5). With increasing Pi, expected white sugar yield decreased in both sugar beet varieties, but much more rapidly in the susceptible than in the resistant one.

Fig. 5. Influence of population density (Pi) of Hetero­dera schachtii on white sugar yield of a suscep­tible and a resistant su­gar beet variety (expected values). Twelve environments, Germany 2002–2005. Asterisks indicate signi­ficant differences bet­ween the varieties at a given Pi level (Tukey, p ≤ 0.05).

Discussion

Differences in Pi_SB between oilseed radish treatments tended to be larger after sowing in July than after sowing in August and the four highest population densities were achieved after sowing in July. This is in accordance with previous studies demonstrating that changes in the population density of H. schachtii are highly depending on the sowing date of the cover crop (Koch and Gray, 1997; Müller and Steudel, 1983). In the 2004/05 trials, population densities decreased or remained on a low level at all environments, even after susceptible oilseed radish sown already in May (Borsum and Obernjesa). So, this whole season-long crop did not seem to provide any benefit for nematode population density adjustments.

Reproductive rates > 1 were observed in both sugar beet varieties. Multiplication of the nematode in resistant sugar beet can be attributed to incomplete transmission of the resistance gene during the seed production process (Müller, 1999; Niere, 2009). For example, the transmission rate in Paulina was indicated at 92% (Niere, 2009), meaning that 8% of the plants will be fully susceptible to infection by H. schachtii. Müller et al. (1995) demonstrated that with a share of 7% of susceptible plants in a resistant variety, multiplication of H. schachtii can occur. While this biological phenomenon presumably impacted our study, current resistant sugar beet varieties have higher transmission rates than Paulina (Niere, personal communication), and suppress population densities more strongly (Krüssel and Warnecke, 2014).

The transmission rate substantially controls the host specific equilibrium density of a variety, at which r = 1 (Schlang and Müller, 1996). In the resistant variety, equilibrium density was approximately 300 E + J 100 g^–1 soil or lower which is in accordance with values reported by Heinrichs (2000) for another nematode resistant sugar beet variety. However, great differences between environments became obvious as maximum equilibrium density was 1000 E + J 100 g^–1 at Niederembt 2004. The reason for this variation remains unclear. As seeds from the same lot were used in all thirteen trials, different equilibrium densities at the environments cannot be due to differences in transmission rate. No environmental factor – soil or weather conditions – distinguishing trials with high and low equilibrium densities was identified on the basis of the available data. Environment-specific differences in equilibrium density were thus presumably due to an unknown variance of biological reproductive patterns (e.g. seedling penetration, larval emergence or fitness and virulence) between certain local nematode populations as they were described by Griffin (1981) and Lange et al. (1993).

In summary, although a general pattern was noted with overall reproductive rates being as expected, great differences between environments occurred and neither r_OR nor r_SB were predictable due to unknown environmental factors.

Resistance to pests or diseases in agricultural crops often comes along with a yield penalty in the absence of the pathogen (Brown, 2002). Schlang and Müller (1996) confirmed this effect for nematode resistant sugar beet hybrids. In the present study, the resistant variety did not necessarily yield lower at low Pi_SB than the susceptible one but site-specific differences in yield response occurred. A positive yield effect of the resistant compared to susceptible oilseed radish as cover crop became ob­vious at most environments, especially in the susceptible variety and where great variation in nematode population had been achieved (e.g. Koslar 2002, 2003). At Niedernjesa 2003, however, yield response was relatively light despite high variation in Pi_SB, presumably caused by severe drought stress that can limit the pest activity (Steudel et al., 1981). Due to these heterogenous and partly controversal findings, a general relation between Pi_SB and yield response valid for all environments could not be derived. An unpredictable relation of H. schachtii population density to sugar beet yield was reported before (Steudel and Thielemann, 1970, 1979) although close relations were found at single environments (Arndt, 2002; Deumelandt et al., 2010; Heijbroek et al., 2002).

In both varieties, other environmental factors beyond population density must have influenced yield response to H. schachtii. In some cases, the deep and heavy soils at the trial sites may have buffered the nematode’s influence which is more severe on light soils (Santo and Bolander, 1979), and a certain annual effect became evident in 2003 with low precipitation rates. Furthermore, especially at sites with high population densities of H. schachtii, parasitic fungi can prevent a yield effect of the nematodes (Balke, 2001; Steudel et al., 1990). The relation between nematode population density and white sugar yield may have been closer if population density below the topsoil had also been measured. Steudel et al. (1989) found high densities of H. schachtii in 30–40 cm soil depth and Westphal (2013) demonstrated that deep occurring H. schachtii in 30–60 cm depth can decrease white sugar yield of sugar beet.

Finally, data interpretation was additionally complicated by laboratory specific differences in the measured nematode population densities. In further studies, all samples should thus be analysed by the same laboratory to get more homogeneous data allowing for a more precise evaluation.