Screening of Herbicidal Potential of Crude Extract of Isolated Fungal Strains against some Noxious Weeds of India: A Preliminary Evaluation

Weeds are one the most serious cause of economic losses in agricultural production. The exploitation of synthetic herbicides for weed control has been increasing; however, their heavy application in crop fields has resulted in environmental and medical problems. Exploitation of secondary metabolites or biorationals of both pathogenic and non-pathogenic fungi in weed management have attracted the attention of scientists in recent years. Several comprehensive reviews have been published on biological, technological and economical feasibility of microbial products as herbicides [1,2]. Potential herbicidal properties of host specific toxin, Maculosin produced by Alternaria alternata, against spotted knapweed [3] and Cornexistin from Paceilomyces variotii [4] against several monocotyledonous and dicotyledonous weeds are well known and used as alternative means in developing eco-friendly herbicides.

Natural herbicides, probably obtained from microorganisms are eco-friendly, biodegradable and less toxic are now considered as one of the best alternative to existing weed management strategies [5,6] Fungi are well recognized for their ability to produce diverse biologically active metabolites including herbicides [7]. Therefore, screening for fungal products with herbicidal activity has been one of the most interesting features in weed management research. The main objective of this study was to determine herbicidal potential of cell free culture filtrates obtained from 21 days old fermented broth of nine different isolates of fungi against seven prominent weeds viz., Parthenium hysterophorus, Lantana camara, Xanthium strumarium, Cassia tora, Hyptis suaveolens, Sida acuta and Antigonon leptopus recovered earlier from various places of Madhya Pradesh, India. Activities of the CFCF were measured under control laboratory conditions.

The test strains were obtained from Fungal Germplasm Collection Centre, Department of Biological Sciences, R. D. University, Jabalpur. These were isolated earlier from infected/infested parts of weeds collected from various places of M.P. The cultures were maintained by subculturing on PDA (Potato- 200gms; Agar- 18gms; Dextrose-20gms; D.W.-1000ml) slants at 28±1°C in tightly capped culture tubes.

250ml Erlenmeyer’s flasks containing 200ml, modified richards broth were seeded with 5mm disc of inoculum separated from 7 days old cultures grown on PDA medium. Inoculated flasks were incubated at 28±1°C in a BOD incubator (Remi, India) and the Cell Free Culture Filtrate (CFCF) was extracted after 21 days [8].

The metabolized broth was passed through a pre-weighed what man filter paper no.1 under aseptic conditions and was centrifuged at 400xg for 15-20 min. The pellet was thrown and the supernatant was again filtered in vacuo by microfiltration using sterile micro filters, 0.45µm pore size, Minisart (Sartorius, Gottingen, Germany) making it cell free [9]. Thus cell free culture filtrate was obtained.

The potential metabolites of recovered isolates were assessed through following bioassays:

Shoot cut bioassay: 30-35 days old seedlings were grown in pots containing soil: Sand: Peat (1:1:1) inside a plant growth chamber (Yorco, India). These were taken and an inclined cut was made at the tip in sterilized water. They were then dipped in different dilutions i.e., 25%, 50%, 75% and 100% of the CFCF in test vials. These were then incubated under daylight or artificial illumination (3.5 x 10⁴ erg/cm²/s). The vials were sealed with cotton buds and the effects of different days old fermented broths and different dilutions of the toxic metabolites were observed on the shoots of the test weed after 48 h at room temperature [10].

Seedling bioassay: Seedlings of the target weeds were raised in pots containing soil: Sand: Peat (1:1:1). 15-20 days old seedlings were treated with different days old CFCF (7, 14, 21, 28 days) and also with different dilutions (25%, 50%, 75% & 100%) of the phytotoxic metabolites and were then incubated. Phytotoxicity of CFCF of various strains was determined by employing shoot cut and seedling bioassay tests in accordance to Thapar et al., [11]. Phytotoxicity was determined following the visual method suggested by Abbas et al. [12], and Buckle and Sanders on a rating scale of 0-5.

Data recorded in tables 1&2 showed that toxicity of CFCF varied significantly amongst various fungal strains. CFCF obtained from Fusarium oxysporum FGCCW#43 showed strongest herbicidal potential (100%) against Parthenium hysterophorus, Lantana camara, Xanthium strumarium, Cassia tora, Hyptis suaveolens, Sida acuta and Antigonon leptopus while CFCF from F.moniliforme FGCCW#16 and F.roseum FGCCW#55 ranked second and third respectively. CFCF of these strains showed broad-spectrum herbicidal property. Production of herbicidal metabolites viz., fumonisins, Fusaric acid, moniliformin, trichothecenes by Fusarium spp. have also been recorded by many workers [12,13]. Similarly, CFCF obtained from fermented broth of Phoma sp FGCCW #54 also exhibited broad spectrum herbicidal activity and was responsible for 100% damage to P.hysterophorus. CFCF of Sclerotium rolfsii FGCCW#08 caused significant damage (>50%) to P. hysterophorus but failed to exhibit herbicidal property against the other target weeds. CFCF of Alternaria alternata FGCCW#32; Colletotrichum dematium FGCCW# 09; Curvularia lunata FGCCW#25 and Myrothecium roridum FGCCW# 03 did not cause any appreciable damage to the target weeds. Variation in phytotoxicity due to toxin has also been recorded by other workers [10,12,14-17]. Quereshi et al. [18], recorded significant herbicidal activity in CFCF obtained from 21 days old fermented broth of Phoma sp FGCCW#54 against Parthenium. Variations in phytotoxicity of CFCF of S.rolfsii isolates against P.hysterophorus have also been reported by Pandey et al., [19]. A.alternata FGCCW#32 and C.lunata FGCCW#25 failed to cause considerable damage against target weeds, however in contrast, to this, Saxena and Pandey [7] and Thapar et al. [11], have reported appreciable phytotoxicity of secondary metabolites from Alternaria isolate LC#110 and LC#104 against Lantana camara and C.lunata Ph#38 against Parthenium respectively.

 

Tested strains		Phytotoxicity (%)*
	Parthenium hysterophorus	Lantana camara	Hyptis suaveolens	Xanthium strumarium	Cassia tora	Sida actua	Antigonon leptopus
Alternaria alternata FGCCW#32	17±3	5±4	4±2	6±4	9±3	15±3	6±4
Colletotrichum dematium FGCCW#09	51±2	18±1	0	0	0	0	0
Curvularia lunata FGCC#25	35±1	30±1	38±2	30±1	25±1	21±1	30±1
Fusarium oxysporum FGCC#43	100	100	100	100	100	100	100
Fusarium moniliforme FGCCW#16	98±2	98±1	98±4	98±3	98±2	98±1	98±3
Fusarium roseum FGCCW#43	96±1	96±3	96±3	96±4	96±3	96±2	96±4
Myrothecium roridum FGCCW#55	0	9±1	10±	0	12±2	0	0
Phoma herbarum FGCCW#54	100	65±2	61±3	69±2	68±1	58±4	69±2
Sclerotium rolfsii FGCCW#08	60±4	35±3	30±1	0	19±2	4±1	0

The above findings clearly indicate that among nine strains screened, CFCF of Fusarium spp. and Phoma sp. FGCCW#54 have significant broad-spectrum herbicidal potential against the test weeds viz Parthenium hysterophorus, Lantana camara, Xanthium strumarium, Cassia tora, Hyptis suaveolens, Sida acuta and Antigonon leptopus. Thus, it can be concluded that these strains could be used for production of herbicidal compounds. However, further investigations are needed to determine mode, mechanism and site of action and develop suitable formulations and production technologies before its large-scale application.