Fungi as Biocontrol Agents Against Weeds. I: Biocontrol of Weeds of Rice Crop

Abstract

Summary Currently, the weed science is going through a new era of technological change. Weed scientists are now facing new challenges, particularly in the light of emergence of weed resistant to herbicides and concerns and questions about the herbicide residues in food, soil, water and atmosphere. These have provided the impetus to evaluate alternative method of weed control and to deviate from path that relies on herbicides. Since last decade the scientists in the developed countries have made tremendous progress to evaluate various indigenous plant pathogenic fungi for their potential as biological agents and few my choherbicides developed so far have shown to be of great success against some problem weeds. The rice, an important cereal crop, is facing diverse cropping practices, which influence the intensity, diversity as well as competitive behaviors of weeds. Keeping in view the weed problem elaborate experiments were designed to develop an insight in the field of mycoherbicides, as alternative and environmentally begin method for controlling two of the, most notorious weed infesting rice fields. Marsilea minutia and paspalum paspaloides were selected for the biocontrol purpose by Drechsler hawaiiensis and Graphium sp. Respectively. Various assays carried out led to the following findings: • Elaborately designed surveys of Lahore and Gujranwala divisions revealed at total of 18 wees species infesting rice growing fields, showing wide spectrum of grass, sedges and broad leaf weeds. Survey analyses based on % abundance, % frequency of occurrence and density exhibited that the infection of weed species p. paspalodies (Mich) Scribner, Echinocholoa colonum (L) Link, E. glabrescens Munroex HK. F. Cyperus rotundus L and C difformis L. most abundant while, fimbristylis littoralis l. and alternantheris sesslis (L) DC were graded as more frequent. However, species viz, Heleocharis atropurpurea Kunth, Fmbristylis littoralis, scirpus roylei (Nees ex Wight) Parker ex du, Monochoria vaginalis Burn F.) k. persl ex kunth and Marsilea minuta L. infestations were found tobe very densely populating the rice fields. • In weds crop competition assays five weed species viz. C. difformis, E. colonum, O. paspalodies and M. minuta inter planted with rice test varieties Basmati 385 (385-B) and super basmati were found to exert 21-56% and 6-42% yields losses respectively in the aforesaid crop plants. Maximum yield losses were recorded by E. Colonum (56%) and Rotundas (42%). • Psychopathological surveys of weeds led to the selection of p. paspaloides and M. minuta as target species for biocontrol, hosting disease infection by Graphium sp. And D. hawaiiensis respectively. Both the candidate fungi through back inoculations o0nto host plant testified them as casual agents. • For the host specificity assays potential pathogens were insulated onto economically important crops viz maize, sunflower and rice. All the crops exhibited immunity to both pathogens indicating that selected pathogens were fairly host specific. • Optimization of nutritional and climate requirements of D. hawaiiensis on five media namely water agar (WA) maly extract agar( MEA) , Capek dox agar (CDA), potato dextrose agar (PDA) and Richards agar medium (RA) under varying temperature, PH and photoperiod confirmed the suitability of PDA and an exposure to 12 hrs. photoperiod at 25 C for the best growth as well as sporulation of the candidate agent. Similarly, PDA at 25 C under constant illumination was the best medium for the growth as well as sporulation of Graphium sp. • The assessment of control potential of both the candidate fungi as future mycoherbicides in laboratory and prevailing environmental conditions with the aqueous conidial concentration of 10 conidia/ml exhibited maximum potential of Graphium sp. And D> hawaiiensis to suppress their respective host weeds. Under controlled set of environmental conditions maximum control of both the target weeds was recorded in November. • On the basis of aforesaid efficiency assays of potential pathogens both in laboratory as well as prevailing environmental conditions, pathogens were further inoculated in various formulations under controlled as well as natural environmental conditions. Under laboratory trials, 10 conidia/ml concentration each of D. hawaiiensis in 20 % canola oil emulsion and Graphium sp.in 1:1% gelatin: carboxymethycellulose provided significant of M. minuta and p. paspaloides respectively at 35 C and humidity level of 36 hrs., • The two mycoherbicide formulations found most efficient against target weeds, were further evaluated for the efficacy by intercropping the weed with the test crop varieties in fields plots. The control of target weeds resulted in increased crop growth and economic yield. In test varieties of rice crop yield enhancement of 28.8% and 5.2%in 385-B and 18.6%and 4.2% against M. minuta and. Paspaloides, respectively in super basmati was acquired. • Mass production of the candidate agents on different substrates revealed the suitability of wheat straw amended with chickpea for maximum production of conidia. • The efficiency of mass-produced conidia was assessed through test trials. The conidia produced disease on host plants within one week of inoculations that confirmed 100% viability of the conidia of the fungi.

The results of all experiments conducted revealed a potential of mycoherbicides in the field condition when properly formulated. the great potential of other along with these pathogens can be further investigated by searching more pathogens, because the lack of availability of suitable indigenous pathogens on the weeds of rice crop was an obstacle in the 100%control of the weds.