Efficacy of Different Nematicidal Compounds on Hatching and Mortality of Heterodera schachtii Infective Juveniles

Introduction

Chinese cabbage, Brassica rapa chinensis L. is one of the most highly valued economic crops and the leading vegetable in both production and consumption in Korea (Chang et al., 2008; Kim et al., 2014). It is ranked third after rice and fresh vegetables in annual production due to the high demand of the crop in the production of Kimchi; a Korean national dish and the most important processed food product in the country (FAO, 2013, Park et al., 2014). However, there is pressure exerted on Chinese cabbage production by pests and diseases especially in the highland areas (Kim et al., 2012, 2014). Besides insect pests and diseases caused by bacteria, fungi and viruses, an important plant pathogenic nematode, sugar beet cyst nematode (SBCN), Heterodera schachtii has recently been considered a serious pest causing severe damage and yield reduction in the Gangwon province, Korea (Lee et al., 2013; Kabir et al., 2015).

SBCN is a well-documented major pest of sugarbeet, causing severe yield losses (Griffin, 1987; Hafez and Seyedbagheri, 1997). The pest has a relatively wide host range and the symptoms include wilting, yellowing and eventual necrosis of the leaves. In the roots, the taproot may fail to develop and numerous root hairs are sometimes formed, on which white lemon-shaped females can be observed. Once fully grown, the females turn to brown cysts and dissociate from the root. The brown cysts can survive in irrigated fields for several years without a host (Hafez and Seyedbagheri, 1997). Management of SBCN involves integration of different strategies like prevention, cultural practices, resistant cultivars, trap crops and chemical control (Griffin, 1987; Turner and Rowe, 2006). Chemical control using aldicarb and 1, 3-dichloropropene is considered more potent depending on soil properties though their use is currently under restriction due to their side effects on non-target organisms and groundwater contamination (Holden, 1986; Ritter, 1990; Gray et al., 1992; Urwin et al., 2000). Aldicarb in particular was discontinued in 2015 (Cone, 2010). Non fumigants like imicyafos and fosthiazate are generally known to pose limited effect on non-target microbes though not widely used on SBCN (Wada and Toyota, 2008; Wada et al., 2011). Other alternatives in nematode control include abamectin, terthiophene and plant extracts from various plant species with nematicidal effect (Akhtar and Irshad, 1993; Zhang et al., 2010; Kang et al., 2012). However, limited information is known on the efficacy of these nematicidal compounds on SBCN.

Recent reports indicate that SBCN is rapidly spreading within the highland Chinese cabbage growing areas of Gangwon-do province (from 11.2 ha in 2011 to 70 ha in 2015) and cabbage production is greatly affected (Kweon et al., 2015). Efforts to understand the ecology of the pest in highland cabbage growing areas are underway through various laboratory and field studies (Kabir et al., 2015). However, current management practices are limited to prevention of spread and multiplication through proper disposal of infested waste soil and sanitation to avoid transfer of cysts to non-infested fields through farm machinery (Kweon et al., 2015). Thus, without a readily available reliable control strategy, the economic efficiency of Chinese cabbage under the increasing SBCN infestation in the main growing areas is under threat. Chemical control is considered as one of the most effective and reliable integrative method of SBCN control. However, no nematicides have been tested and registered for the control of SBCN in Chinese cabbage fields in Korea (KCPA, 2016). In addition, there is a need to detect more environmentally friendly non-fumigant nematicidal compounds in the control of SBCN due to the increasing global regulatory pressure on well- known effective nematicidal compounds (aldicarb and 1, 3-dichloropropene), This study therefore aimed at testing the efficacy of selected environmentally friendly nematicidal compounds for the control of SBCN on Chinese cabbage.

Materials and methods

Results

Effect of different nematicidal compounds on hatching of infective juveniles

Hatching rate of H. schachtii was significantly different depending on nematicidal compounds (Fig. 1). No hatching was observed in the first 3 days after treatment and the highest number of infective juveniles was recorded from cysts treated with terthiophene (Fig. 1).

Fig. 1.

Average number of infective juveniles of Heterodera schachtii recovered from cysts treated with nematicidal compounds. Cysts were soaked in each nematicidal compound with Chinese cabbage root exudates.

Hatching peaks were varied among different nematicidal compounds with the highest peak in the treatment of terthiophene on the 6^th day after treatment (Fig. 1). Highest hatching peaks for imicyafos and abamectin were observed on 7^th and 9^th day after treatment, respectively. Both Eclipta prostrata extract and fosthiazate treated cysts showed maximum hatching on the 11^th day after treatment (Fig. 1). There was hardly any hatching from the healthy cysts treated with fluopyram. Daily hatching rates from cysts were significantly different among nematicidal compounds (P ≤ 0.05), except on the last day when there were no significant differences among treatments (P=0.056). Total infective juvenile hatched populations were also significantly different among nematicidal compounds (df=6, 14, F=34.58, P<0.0001) (Fig. 2).

Fig. 2.

Total mean numbers of infective juveniles of Heterodera schachtii recovered from cysts treated with nematicidal compounds after 14 days. Error bars show standard error and letters indicate significant differences among means (Tukey’s HSD test, P<0.05).

In second experiment, initial treatment of the cysts with root extract solution increased hatching rates of infective juveniles. However, treatment of cysts with nematicidal compounds on the 5^th day resulted in varying hatching rates among nematicidal compounds (Fig. 3).

Fig. 3.

Percentage of hatching increase from cysts initially treated with root extracts for 4 days before synergistic treatment with nematicidal compounds and root extract solution. Bars (±SD) with the same letter in each day after treatment are not significantly different (Tukey's HSD test, P<0.05).

There was a positive correlation in percentage reduction of hatching rates among nematicidal compounds (Fig. 3). Hatching hardly occurred in the treatment of fluopyram as seen in Fig. 3 (almost 100% no hatching) and there was significant differencs in the number of infective juveniles hatched in the first two days after treatment with the nematicidal compounds (1 day after treatment; df=6, 14, F=37.79, P<0.0001 and 2 days after treatment; df=6, 14, F=7.06, P<0.0013). However, differences were not significant on the third day after treatment (df=6, 14, F=2.71, P<0.0586).

Acute toxicity of nematicidal compounds on infective juveniles

Treatment of cysts with various concentrations of the nematicidal compounds showed variations in the acute toxicity of compounds depending on concentrations. Imicyafos and fluopyram showed 100% control efficacy with LC₅₀ as low as 0.0178 and 0.0543 ppm, respectively (Table 1).

Table 1.

Lethal concentrations of nematicidal compounds treated against infective juveniles of Heterodera schachtii

Discussion

Hatching of SBCN was affected by treatment of nematicidal compounds at varying levels. Initial treatment of cysts with root extract accelerated the hatching rates and this was in agreement with the findings published by Kabir et al. (2015). There was a positive correlation in percentage reduction in hatching rates in the treatment of nematicidal compounds. Hatching was slowed down depending on the nematicidal compound.

Fluopyram showed the greatest inhibitory effect and acute toxicity on the infective juveniles. Hatching was hardly noticed and there was a 100% mortality of infective juveniles. Faske and Hurd (2015) showed the effect of fluopyram on motility of Meloidogyne incognita and Rotylenchulus reniformis. Fluopyram showed a strong effect on M. incognita. Their results also indicated that immobility could be reversible; a mode of action similar to aldicarb (Faske and Starr, 2006). However, this study demonstrated that fluopyram could be a nematistatic compound with irreversible mode of action on SBCN. This was supported by the lethal nematicidal effect on infective juveniles even at the lowest concentrations tested (LC₅₀=0.0543 ppm), a characteristic which can be vital along with its hatching inhibitory effect in the control of SBCN on Chinese cabbage.

Fosthiazate is an acetylcholinesterase inhibitor that negatively affects nematode hatching and movement (Woods et al., 1999). Thus, this could explain the slow progress of hatching compared to the control in our experiment. Additionally, treatment of cysts with fosthiazate does not permanently inhibit hatching. It only slows down the hatching rate and this is in agreement with the findings by Woods et al. (1999) on potato cyst nematodes. While as inhibition of acetylcholinesterase by oxime carbamates like aldicarb and oxamyl is reversible, the process is thought to be irreversible with fosthiazate (Richard, 1960; Yu et al., 1972) and this was supported by the limited number of hatched infective juveniles in comparison to the control.

Imicyafos on the other hand, equally showed a significant effect on infective juvenile hatching rates compared with the control. More notably, it had a high toxic effect on the juveniles with LC₅₀ of 0.0178 ppm. Imicyafos has been shown to have a similar significant effect on plant parasitic nematodes like Pratylenchus penetrans (Wada et al., 2011) and Meloidogyne spp. (Kim et al., 2015). Its mode of action is similar to that of fosthiazate. The low LC₅₀ shown by the two compounds strongly indicate the great potential of these organophosphates for the control of H. schachtii on Chinese cabbage.

Abamectin and terthiophene had limited effect on hatching of SBCN. Despite the fact that abamectin showed limited influence on hatching rates of SBCN, it is noted to be one of few non-fumigant nematicides with a true nematicidal effect as nematode paralysis is irreversible on treatment (Faske and Starr, 2006). According to our results, terthiophene had a nematicidal property on infective juveniles (LC₅₀=0.7574 ppm). These compounds did not adversely affect infective juvenile hatching but they could be an alternative in cyst nematode control program integrated with other management strategies.

Eclipta prostrata considerably reduced the hatching rates of infective juveniles and additionally had a nematicidal effect on infective juveniles (LC₅₀=183.97 ppm). This is in agreement with Zarina et al. (2003), who also showed similar results on inhibition of hatching and respective mortality of Meloidogyne javanica infective juveniles. Kim et al. (2015) showed that terthiophene could be isolated from the aerial parts of E. prostrata. However, given the fact that terthiophene showed limited effect on infective juvenile hatching, it is therefore possible that different compound (s) within the E. prostrata extract is involved in inhibiting hatching of SBCN. Probably, a closer analysis of the different phytochemicals constituting E. prostrata extracts could reveal the compound involved in hatching inhibition. Botanical nematicides are generally considered to be a secure alternative to synthetic nematicides (Dang et al., 2012), and with the increasing call to phase out or limit use of synthetic nematicides, the emergence of promising botanical nematicides could be a solution.

Wada and Toyota (2008) demonstrated that the application of imicyafos and fosthiazate has little effect on nontarget organisms like bacteria and fungi. The same findings were documented relating to their effect on natural soil nematode community (Wada et al., 2011). Thus, being non-fumigants and their selectivity to act against plant parasitic nematodes could make them better alternatives for the control of cyst nematodes. Fosthiazate is already used in the UK for control of potato cyst nematode (Woods et al., 1999). Efficacy of fosthiazate at higher application rates is comparable to soil fumigation using effective fumigants (Pullen and Fortnum, 1999). Therefore, fosthiazate could be one of the best alternatives to the environmentally harmful fumigants which are being phased out. This study, therefore, demonstrated an immediate potential control strategy which can be employed for the control of H. schachtii on Chinese cabbage. However, it is important to note that the findings were based on initial tests carried out under controlled laboratory conditions; thus, detailed comprehensive field investigations are required to test the efficacy and potential of these nematicidal compounds for control of SBCN.

Acknowledgments

This research was carried out with the support of the “Research Program for Agriculture Science and Technology Development (Project No. PJ010774)”, Rural Development of Administration, Republic of Korea.

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