Anju Kamra

Persistence and Nematicidal Efficacy of Carbosulfan, Cadusafos, Phorate, and Triazophos in Soil and Uptake by Chickpea and Tomato Crops under Tropical Conditions

The productivity of chickpea, Cicer arietinum (L.), and tomato, Solanum lycopersicum (L.), is adversely affected by root-knot nematode, Meloidogyne species. Nematode-resistant chickpea and tomato are lacking except for a few varieties and therefore grower demand is not met. The available nematicides, namely, carbosulfan, cadusafos, phorate, and triazophos, were, therefore evaluated for their efficacy and persistence in soil and crops to devise nematode management decisions. In alluvial soil, cadusafos was the most persistent nematicide followed by phorate, carbosulfan, and triazophos in that order. The percent dissipation of cadusafos was greater (P < 0.05) in chickpea than in tomato plots, which influenced its half-life in soil. Nematicide residues were differentially taken up by chickpea and tomato plant roots with active absorption continuing for up to 45 days. Cadusafos and triazophos were absorbed to greater extent (P < 0.05) in tomato than in chickpea. The translocation of residues to shoot was highest by day 15 for cadusafos and at day 45 for other nematicides, with carbosulfan residues translocated the most. Nematicide residue concentrations in shoots never exceeded those in roots, with residues in both roots and shoots persisting beyond 90 days. Nematicide residues in green seeds of chickpea and tomato fruits were all below the Codex/German MRLs of 0.02, including the Indian tolerances of 0.1 microg/g in fruits and vegetables. Cadusafos was found to be the most effective nematicide followed by triazophos against Meloidogyne incognita and reniform nematode, Rotylenchulus reniformis . Application of cadusafos (Rugby 10 G) or, alternatively, spray application of triazophos (Hostathion 40 EC) in planting furrows, both at 1.0 kg of active ingredient/ha, followed by light irrigation is recommended for the effective control of M. incognita and R. reniformis infestations on chickpea and tomato.

In vitro nematicidal activity of a terrestrial cyanobacterium, Synechococcus nidulans, towards plant-parasitic nematodes

The nematicidal activity of a terrestrial cyanobacterium, Synechococcus nidulans, was investigated. Extracts of S. nidulans cultures collected at weekly intervals for 5 weeks were sonicated and tested against second-stage juveniles (J2) of Meloidogyne incognita. Extracts of 2-week-old cultures caused the maximum immobility (94.2%) and mortality (29.3%) of J2, compared with controls (medium and water). This extract was tested in vitro against infective stages and hatch of M. graminicola, Heterodera cajani, H. avenae and Rotylenchulus reniformis. Extracts of sonicated S. nidulans caused a mean immobility in the range of 91.3-98.4% in infective stages of the nematodes, with no significant difference with an increase in exposure time from 24 to 72 h. The greatest mean percentage mortality was observed in M. graminicola (31.5%) followed by M. incognita (29.3%), H. avenae (20.9%), and R. reniformis and H. cajani (both 17.4%) with a significant increase with the period of exposure from 24 to 72 h. No significant differences in mortality were observed between M. graminicola and M. incognita and between H. avenae and H. cajani. The percentage hatch inhibition over control (water) was greatest in M. incognita (94.2%), followed by H. avenae (91.6%), H. cajani (72.3%) and M. graminicola (70.6%), and least in R. reniformis (58.6%).

Nematicidal efficacy, enhanced degradation and cross adaptation of carbosulfan, cadusafos and triazophos under tropical conditions

Nematicides need to be applied in each cropping season but repetitive applications can reduce their persistence and efficacy due to the unpredictable phenomena of enhanced biotransformation and cross adaptation. Experiments were conducted to ascertain the number of times the nematicides carbosulfan, cadusafos and triazophos can be applied effectively in the field. Incubation studies checked their degradation rates and cross adaptation and bioassays assessed their efficacy against Meloidogyne incognita infecting Solanum lycopersicum . Monitoring nematode populations at the middle of seven consecutive tomato crops following nematicidal treatments at a recommendable rate of 1.0 kg a.s. ha –1 revealed a linear decrease in efficacy with successive seasons. The chemicals remained effective up to the fourth application when 53-62% reduction of M. incognita in soil was still achievable, which decreased significantly to 14-33% by the seventh application. The nematicides were more effective against endoparasitic ( M. incognita and Rotylenchulus reniformis ) than ectoparasitic ( Helicotylenchus dihystera , Hoplolaimus indicus and Tylenchorhynchus vulgaris ) nematodes. Bioassays revealed 13-18% more invasions of second-stage juveniles of M. incognita into roots of tomato grown in soil pre-treated seven times with nematicide than in similar soil with no history of nematicide use; invasion and soil population were positively correlated. Root galling of field-grown tomato increased from the first to the seventh application. In bioassays, tomato root galling was greater in unsterilised field soil (1.4-2.1) than in similar soil/sterilised soil (1.0) compared with 2.4-2.9 in untreated control soil. The decrease in efficacy was attributable to accelerated microbial degradation of nematicides due to repeated use in each cropping season. Carbosulfan, cadusafos and triazophos exhibited a half-life ( t 1 / 2 ) of 14, 20 and 27 days in soil with no history of nematicide use, whereas the t 1 / 2 was 6, 13 and 9 days in soil pre-treated seven times with nematicides and 28, 28 and 32 days in unsterilised soil, respectively. In cross adaptation studies, carbosulfan exhibited a t 1 / 2 of 7-9 days in soil pre-treated seven times with cadusafos and triazophos. The t 1 / 2 of cadusafos (22 days) was not affected in carbosulfan-treated soil but was affected (13 days) in triazophos-treated soil. Triazophos had a t 1 / 2 of 20 days in carbosulfan-treated soil and a t 1 / 2 of 8 days in cadusafos-treated soil. These results indicate that carbosulfan, cadusafos and triazophos can be applied effectively to the same field at least four times without decrease in efficacy due to accelerated biotransformation. Rotation of nematicide from different groups can be used in long-term nematode management strategies to avoid accelerated degradation and/or cross adaptation.