Bispyribac-sodium, fenoxaprop-p-ethyl and cyhalofop-butyl + penoxsulam were highly effective against R. solani when applied at all the three doses i.e., higher, recommended and lower doses, and were at par. At recommended doses of application the per cent inhibition zone ranged from 47 to 54 for all herbicides except carfentrazone-ethyl which recorded an inhibition of 17 per cent and chlorimuron-ethyl + metsulfuron-methyl, which did not inhibit the fungus. [41] As per experimental results, indicated that the herbicide mixtures bispyribac-sodium + metamifop and penoxsulam + cyhalofop-butyl had immense suppressive effect on the growth of R. solani. Cyhalofop-butyl showed 100 per cent inhibition of R. solani at 24 h incubation, followed by 89 per cent inhibition at 18 h incubation. However it was ineffective in inhibiting at 5 min (56.66%), 30 min (0%) and 6 h (26.67%) incubation [48]. A similar study reported retardation in the growth of the pathogen by the herbicides pendimethalin, anilofos, paraquat, butachlor, isoproturon, alachlor and 2,4-D at 25, 50, 100 or 500 ppm. Paraquat inhibited fungal growth by 99.5 and 78.6 per cent when applied at 500 and 25 ppm, respectively. Alachlor at 500 ppm reduced fungal growth by 92.2 per cent [40].
Increased zone of inhibition of P. oryzae with higher doses of cyhalofop-butyl (65.45%), carfentrazone-ethyl (64.50%) and cyhalofop-butyl + penoxsulam (64.05%) was recorded. At recommended concentrations, the inhibition ranged from 47 to 62 per cent, except in chlorimuron-ethyl + metsulfuron-methyl which did not inhibit fungal growth.Among the different fungicides, highest per cent inhibition of mycelial growth of fungus was recorded in tebuconazole + trifloxystrobin (98.40, 99.90 and 99.90%) and minimum zone of inhibition observed in tricyclazole, with 55.83, 63.62 and 70.52% at 50, 100 and 150 ppm concentration respectively [25]. [30] Recorded highest mycelial inhibition (100%) at 1000 ppm and least inhibition in chlorothalonil (73.82%). Similarly, propiconozole, hexaconozole and carbendazim inhibited to the maximum mycelial growth (100%) and tebuconazole accounted for 100 per cent mycelial growth inhibition at 100 and 200ppm whereas least mycelial growth inhibition was in case of thifluzamide (14.69%) at 50ppm.
Xanthomonas oryzae pv. oryzae, causing rice bacterial blight, was found to be unaffected by the herbicides tested (Table 2). [7] Showed that the colony width growths of the pathogen recorded after 48 hours of incubation were in the order Blitox-50 + streptocycline (70.25%), Ergon 44.3 SC (66.4%), Nativo 75WG (49.91%) and Mirador (35.33%).
The inhibition zone of T. viride with higher herbicide doses was 54.85 per cent with bispyribac-sodium and 52.84 per cent with cyhalofop-butyl + penoxsulam. At recommended dose it was 45.8 per cent with cyhalofop-butyl and 49.5 per cent with cyhalofop-butyl + penoxsulam. Unlike its effect on plant pathogens, chlorimuron-ethyl + metsulfuron-methyl had a high inhibition zone of 44 per cent, indicating that its application was detrimental to the bioagent T. viride. [44] Found that the mycelial growth of the strains T.17 and T.75 was affected by the concentrations of control (0D), recommended dose (1D), and 10 times higher dose of fenoxaprop-p-ethyl derivatives and 2,4-D amine salt. Bispyribac-sodium had no effect on any of the strains tested. [51] Discovered that the herbicides imazethapyr and pendimethalin were compatible with soil borne pathogens, allowing them to be used safely alongside antagonist, whereas quizalofop was found to be highly inhibitory to both pathogen and antagonist. [6] Claim that T. viride was sensitive to pendimethalin, which caused mycelial growth of 84.66 mm at a concentration of 0.2%. According to [55] the weedicides diuron and atrazine raised tolerance in the test bioagent from 500 to 2000 g/ml. Bispyribac-sodium + metamifop, on the other hand, was found to be harmless and safe to antagonistic fungi, T. viride at doses of 60 to 90 g/ha, which were classified as Class I. (growth inhibition of 8.15 to 22.95%). Toxicity levels in the Class II toxicity category (higher dosages of 100 and 110 g/ha) inhibited development by 31.48 and 37.04 per cent, respectively, and were mildly detrimental to the T. viride [41].
An inhibition zone of around 25 per cent was observed only at higher doses of chlorimuron-ethyl + metsulfuron-methyl and carfentrazone-ethyl (21.08 and 22.33%, respectively). [43] Studied the compatibility of bacterial bioagent with herbicides and found that anilofos, showing mean inhibition zone of 0.8 mm, could be considered as compatible with P. fluorescens isolate 83, whereas butachlor and pendimethalin showed compatibility with P. fluorescens at lower concentrations (500 and 250 ppm) as well as at higher concentrations (2000 and 1000 ppm) and mean inhibition zone was exhibited only at lower concentrations. This observation was in agreement with the findings of [54] that P. fluorescens (PF 43) was highly compatible with 2,4-D sodium salt, metsulfuron methyl + chlorimuron ethyl, cyhalofop-butyl, pyrazosulfuron ethyl, pretilachlor, penoxsulam and bispyribac-sodium. [41] Found that that bispyribac-sodium + metamifop at different tested concentrations viz., 100, 120, 140, 160, 180, 200 and 220 µL/L (corresponding to the field doses of 50, 60, 70, 80, 90, 100 and 120 g/ha) did not exert any inhibition on the growth of P. fluorescens. [17], documenting experimental results pertaining to compatibility of P. fluorescens with herbicides, found that the strain P. fluorescens was compatible with all the tested herbicides viz., quizalofop ethyl, pyrithiobac sodium, oxyflourfen, cyhalofop-butyl, glyphosate + ammonium sulphate, pendimethalin, 2,4-D sodium salt, imazethapyr, atrazine and glyphosate at all the three concentrations (100, 500 and 1000 ppm) tested.
Most of the commonly used herbicides at recommended doses had inhibitory effects on growth of plant pathogens R. solani and P. oryzae. However, beneficial bioagent T. viride was also inhibited. Herbicides used in combination could have an additive effect and result in enhanced disease control compared to their individual application. The judicious selection and use of herbicides is warranted to obtain disease control in addition to weed control. The possibility of their utilization in integrated disease management needs to be investigated.
In vitro evaluation of herbicides revealed that bispyribac-sodium and cyhalofop-butyl + penoxsulam had greatest inhibitory effect on pathogenic microorganisms Rhizoctonia solani and Pyricularia oryzae, but showed no effect on Xanthomonas oryzae pv. oryzae, and were less harmful to biocontrol agents Trichoderma viride and Pseudomonas fluorescens at recommended doses.