Effect of an Alltech soil health product on entomopathogenic nematodes, root-knot nematodes and on the growth of tomato plants in the greenhouse

Four EPN strains of the family Steinernematidae and Heterorhabditidae were cultured, maintained, and stored at 9°C in the enviroCORE laboratory facilities, Institute of Technology Carlow, Ireland. These were: an Irish isolate Steinernema feltiae [strain 12(1); Boyle, 2007], Steinernema feltiae (e-nema), Steinernema carpocapsae (e-nema), and Heterorhabditis bacteriophora (Andermatt Biocontrol UK).

Entomopathogenic nematodes were reared in Galleria mellonella (Lepidotera: Pyralidae), sourced commercially from Live Foods Direct (Sheffield, UK). Standard size petri dishes (100 × 15 mm) were inverted and the lids were lined with two sheets of Whatman filter paper. Five G. mellonella were placed onto the filter paper, the base was placed on top. Approximately 1 to 1.5 ml of a dense IJ suspension was applied to the filter paper until it was moist. The dish was lightly sealed with parafilm to prevent the filter paper drying out. Plates were then incubated in the dark, at 21°C for three to seven days until mortality occurred.

Once insect mortality had occurred, nematode IJ were recovered using White traps (White, 1927). Freshly hatched juveniles were then stored at 9°C until required and were used for experimentation no more than two weeks post emergence.

All the treatment studies were conducted in 96-well plates. Freshly cultured EPN, not older than two weeks post emergence, were used for the study. EPN were kept in room temperature for 30 min before conducting the experiments to check their viability. The four EPN strains were concentrated by transferring the cultures into sterile and clean falcon tubes. The falcon tubes were kept upright, to allow the IJ to settle down at the bottom of the tube. Slowly, the supernatant was decanted to obtain approximately 10  IJ/100 μl. In all, 100 μl of water containing approximately 10 juveniles and 100 μl of the various product (ACS 5075 obtained from Alltech^® Inc) dilutions were added in each well to obtain the required concentration. In case of 100% treatment concentration, the product was directly added without any dilutions and the juveniles were picked up individually by observing under the stereoscope using micropipettes. The concentrations of product tested were 0, 10, 20, 40, 60, 80, and 100% in a total volume of 200 μl in each well of the 96-well plate. The Experiments were conducted to determine the lethal concentration of the product for each EPN strain. Furthermore, concentrations within the range of 0, 2, 4, 6, 8, and 10% were used to determine the sub-lethal concentration (LC₅₀) of the product for each EPN strain. All the experiments were conducted with 10 replicates for each treatment concentration and the untreated (control), which received water. Each set of experiments was carried out three times.

After 24-hr incubation at 20 to 22°C, survival and mortality were calculated by counting the motile and immotile IJ using an Olympus Stereo Microscope (SZX7). The mortality of immotile juveniles was ensured by gently touching them with a needle. Another set of plates with the same concentration range was analyzed after 48-hr incubation. It was observed that the mortality level was similar to that of 24-hr incubation. Therefore, data recorded after 24-hr incubation were considered for statistical analysis. All these bioassays were repeated two more times.

Tomato roots infected with M. javanica and M. incognita were a kind offer from Dr E. A. Tzortzakakis, Hellenic Agricultural Organization – DEMETER, Greece. Three- to four-week-old tomato seedlings were freshly infected with five to seven egg masses/plant around the root, for rearing and increasing the nematode population for further studies. All the plants were maintained in a plant growth room at 32 ± 2°C. Organic pesticide was sprayed on the leaves and stem to avoid white flies, spider mites, and insect infection. Tomato roots were assessed for nematode galling damage approximately after a period of 8 to 12 weeks. Egg masses were excised from the roots with the help of sterile scalpel and forceps and were stored at 9°C to cease hatching before application of the actual treatment. All PPN work, in culturing and in experimentation, was carried out under strict quarantine and containment conditions.

Experiments were conducted to collect nematode inoculum by hatching juveniles from the egg masses collected from infected tomato roots. The protocol was a slightly modified version of the one followed by Coyne and Ross (2014). The egg masses were removed carefully using a scalpel and fine tweezers. They were placed on moist filter paper, which was in turn placed in a plastic container with 20 ml of distilled water. The container was incubated in dark at 20°C for 48 hr for juveniles to emerge out into the water.

Five concentrations of product were prepared in 5 ml water, i.e., 0% (control), 0.5%, 1%, 2%, and 3% and placed in 35 mm sterile petri dishes. Five similarly sized egg masses were randomly picked up and placed in each petri dish containing respective treatment concentrations. Egg masses placed in sterile water were considered the control. These petri dishes were incubated in dark at room temperature and checked for hatching at regular intervals of 24 hr for three days. After three days of treatment, the suspensions in each treatment were thoroughly mixed and placed in a counting dish each, to record the number of juveniles hatched using Olympus Stereo Microscope (SZX7).

Freshly hatched RKN juveniles (J2) obtained three days after incubating egg masses at room temperature were used to study the effect of product on juvenile mortality. The study was conducted in a 96-well plate. In total, 100 μl of inoculum containing approximately 10 juveniles and 100 μl of the product dilutions were added to each well to obtain the required concentrations, i.e., 0.5, 1, 2, and 3%, respectively. In total, 100 μl inoculum containing 10 juveniles with only water was used as control and 10 replicates were run for each concentration.

After 24-hr incubation at 20 to 22°C, in the dark, mortality was calculated by counting the motile and immotile juveniles using Olympus Stereo Microscope (SZX7) and expressed as a percent relative to the control. The mortality of immotile juveniles was ensured by transferring them into separate petri dishes containing water without product and were observed after 24 hr. Mortality was further confirmed after that. Another set of plates with the same concentration range was analyzed after 48-hr incubation, and the mortality level was similar to that of 24-hr incubation. Therefore, data recorded after 24-hr incubation were considered for statistical analysis. All these experiments were repeated two more times.

Tomato (Solanum lycopersicum) seeds were germinated under environmentally controlled glasshouse conditions at 32 ± 2°C, 70 ± 10% relative humidity (RH) and natural day/night cycle. The seeds were first grown on garden soil with the following characteristics: pH-7.6 ± 0.02, electrical conductivity- 540 µs/cm, clay (%) 8 ± 0.02, silt (%) 19 ± 0.5, sand (%) 73 ± 3.9, Na 12.58 (mg Kg⁻¹), P 2.8 (mg Kg⁻¹), K 30.59 (mg Kg⁻¹). The seedlings attained a height of around 6 to 10 cm before taken for treatments. They were carefully removed from the soil bed and were transplanted into the individual soil pots. Seedlings were grown in plastic pots with 1 kg soil mixture with a composition of soil: compost ratio being 1:1 ratio.

Treatment ACS 5075 was in range of 0, 1, 3, 5, and 7% prepared in water. Product dilutions were prepared in a total volume of 100 ml and were poured into the plastic pot saucers. These pot saucers were placed beneath the tomato plants growing pots. Each pot had several perforations in the bottom allowing the roots of the tomato seedlings to take up the solutions from the saucers that were placed under the pots. Product treatment was applied only once at the start of the experiment. Tomato plants were regularly irrigated with equal volumes of water. Treatments including control pots were conducted in triplicates, after the treatment duration of 1 month, seedlings were harvested to measure the growth parameters.

Three tomato seedlings of same age and variety were randomly selected for treatment study. The treatment was conducted as described above. The treated and control seedlings were irrigated at the same time with equal volumes of water and were monitored regularly.

The effects of product on growth of tomato seedlings were studied in terms of percent increase in shoot length (SH) and number of leaves (NL) from the day of treatment (0th day) up to 30 days.

Shoot height (SH) (cm) and number of leaves (NL) of plants were recorded. SH (cm) was measured using meter rule and NL by visual counting.

The chlorophyll content (Total Chlorophyll (TC), Chlorophyll_a (Chl_a), and Chlorophyll_b (Chl_b)) was determined to study the effect of treatment on tomato plants (Pulavarty and Sarangi, 2018). Approximately 250 mg of leaf samples were weighed and macerated with 10 ml of 80% acetone using a mortar and pestle. The contents were centrifuged at 3,000 rpm for 10 min. The supernatant was collected, and its volume was made up to 25 ml using 80% acetone. Light absorbance was measured at 480, 510, 645, 652, and 663 nm by UV–visible spectrophotometer (UV-1800, Shimadzu UV spectrophotometer). Chl_a was calculated using the formula: (12.7 × OD at 663) − (2.69 × OD at 645) × v/1,000 × weight of the sample taken in gm. Chl_b content was calculated by: (22.9 × OD at 645) − (4.68 × OD at 663) × v/1,000 × weight of the sample taken in gm. TC content was calculated by: {(ABS₆₅₂ × 1,000)/34.5) × (Final volume of supernatant/1,000 × weight of leaf sample taken in gm)} = mg chlorophyll g⁻¹ FW (Pulavarty and Sarangi, 2018).

All the experiments were statistically designed and analyzed. The experiments were arranged in a completely randomized factorial design (CRD). Dose-response bioassays were performed in 96-well plates with 10 replicates for each treatment including control (only water with no product). These bioassays were repeated two more times. Mortality data were recorded manually for each treatment and were subjected to probit regression analysis using IBM SPSS version 23 (Tzortzakakis, 2017). The software transforms the sigmoid dose-response curve into straight line by converting mortality units into probits and analyzing them against the log of concentration. The regression slope provides the sub-lethal concentration (LC₅₀) at which 50% mortality was monitored. The mean values obtained from the plant experiments were separated based on least significant differences (LSD) obtained from analysis of variance (ANOVA) using IBM SPSS version 23. The plant experiments were also repeated two more times.

There were no statistically significant different results recorded among the experiments carried out over repeated times in any type of bioassay and the results were reproducible.

Effect on S. feltiae (SB 12(1)) and S. feltiae (e-nema)

ACS 5075 product had a significant and strong effect on S. feltiae (SB 12(1)) and S. feltiae (e-nema). In total, 100% mortality was noted at concentrations above 10%. In case of S. feltiae (SB 12(1)), survival was 100, 95.6, 50.2, 9.5 and 0%, for the control, 4, 7, 8, and 10% product concentrations, respectively (Fig. 1A), whereas survival was 100, 97.07, 48.5, 9.5, and 0% for control, 4, 7, 8, and 10% product concentrations, respectively, for S. feltiae (e-nema) (Fig. 1B). There was no significant difference in survival percentage of IJ for the 4% concentration when compared to that of control conditions. However, survival decreased with the increase in product concentrations; 2- to 2.06-folds and 10-fold decreases in survival at 7 and 8% ACS 5075, respectively, when compared to control were observed. At 10%, ACS 5075 no S. feltiae survived. The use of probit analysis determined 6.71 and 6.73% as LC₅₀ concentration for S. feltiae (SB 12(1) and S. feltiae (e-nema), respectively.

Figure 1:

The seedlings treated with 3% product had the highest increase in shoot length (91.6%) followed by 1% (82.8%), 5% (77.3%), control (72.9%), and 7% (64.4%) treatment concentrations, respectively, however these values were not significantly different from control (Fig. 2A).

Figure 2:

A similar but significant effect (p-value = 0.00) was observed in terms of number of leaves (Fig. 2B). The seedlings treated with 3% product had the highest increase in number of leaves (186.1%) followed by the control (155.4%), 1% (152.2%), 5% (144.8%), and 7% (85%), respectively, whereas 1 and 5% treated seedlings had no significant effects compared to control seedlings. At 7% ACS 5075, there was a slight decline (1.8 fold) in plant growth in terms of number of leaves when compared to those in the control plants.

Total chlorophyll (TC) content and the Chl_a/Chl_b ratio were significantly affected (p-value = 0.000) in treated plants compared to ratios in the untreated (Fig. 2C). Tomato plants treated with 3% product had the highest TC (5.07 mg g⁻¹) and Chl_a/Chl_b ratio (1.52). In the cases of the untreated and the 1, 5, and 7% treated plants, TC and Chl_a/Chl_b ratios were 3.77, 3.98, 3.63, and 3.51 mg g⁻¹ and 0.98, 1.32, 1.08, 1.18, respectively. There was a 1.35- and 1.06-fold increase in TC content in 3 and 1% treated plants when compared to the TC content in the control plants (Fig. 2C). However, TC content and Chl_a/Chl_b ratio (Fig. 2C) in 5 and 7% treated plants were slightly reduced when compared to those in the untreated plants.

A significant (p-value = 0.02) effect on both egg hatching and juvenile mortality in both the species of RKN was noted. In the case of M. javanica, 9.6-fold and 31.6-fold reductions in egg hatching were observed with 0.5 and 1% ACS 5075, respectively, treated egg masses when compared to that of control after 72 hr of treatment. At concentrations above 1%, the hatching process completely ceased. Similarly, there was 100% survival in the untreated, but this declined to 10% in the 0.5% ACS 5075 treated juveniles and 0% in the case of all other concentrations above 0.5% after 24 hr of treatment (Table 1).

In the case of M. incognita, the product had a strong and significant (p-value = 0.012) effect on both egg hatching as well as juvenile survival (Table 1). At all the concentrations above 0.5%, no egg hatching or juvenile survival was recorded compared to control after 24 hr of treatment.