Wheat is the most important cereal corp. and weds are undesirable plants, wh* highly infest it and pour negative effect on its yield. Control of competing vegetati** can take many forms including removal by mechanical or manual means and chemical herbicides. These methods have distinct disadvantages such as cost-effectivence environmental and health hazards. This has necessitated a more intensive search alternative weed management strategies that are cost effective, efficacious, sustainb** environmentally safe and without any risk to health or life. The idea of suing *** pathogens for management of weeds is not new, however, the concept of *** development of a bioherbicide technology as a specialized filed of study is very young. The use of biological control strategy entails the enhancement of naturally occurri*** plant pathogens, thus quickening the decline of competing vegetation through *** manipulation of these pathogens.
In present study use of Alternaria alternate has been tested on two weed species of wheat crop and efforts are made to evolve commercial bioherbicides for their control in stepwise investigations as follow:
Extensive field surveys of Lahore and Gujranwala division were carried on from December 1999 to March, 2000. A total of 30 weeds species belonging to 2 genera and 15 families of angiosperms were collected among which anagall arvensis, Avena fatua, Phalaris minor, Rumes dentatus, Coronopus didymus, Medicago denticulate, Chenopodium album, Melilotus parviflora and Poa annu were found to be most abundant.
In order to analyze yield losses induced by the most abundant weeds, weed crop competition assay were designed. Consequently, six weed species viz P. minor, R. dentatus, C. didymus, M. denticulate, C. album and P. annua were grown along with two wheat varieties .i.e Inqalab 91 and Punjab 96. Maximum yield losses of 76% were caused by P. annua followed by C. didymus (75%) in case of Inqalab 91. In the case of other wheat variety, Punjab 96, maximum reduction (55%) was caused by R. dentatus followed by P. minor (28%), M. denticulate and C. album(23%). No gain or loss in yield was displayed by P. annua. The minimum losses of 10% were induced by C. didymus. Punjab 96 exhibited more resistance against weed competition.
Survey for pathogens, to be used as biological control agents, led to the host specific strain of Alternaria alternata.
Host specificity of A. alternate was determined through cross-inoculation against target weeds, as well as, few economically important crops like wheat, maize and sunflower.
Disease induction potential of selected pathogen strains was assessed under natural conditions from January to April, 2001. A alternata strain R( R. dentatus ) and strain C (C. album) was applied @ 107 conidia/ml and @ 109 conidia/ml to respective hosts. These plants were kept under observation up to 21 days after each application. Maximum 53% and 50% disease severity for R. dentatus and C. album respectively was recorded during the month of March.
Optimization of disease potential of selected pathogens against three growth stage of target weeds was carried out under controlled conditions, The weed species were exposed to different formulations, conidial densities, humidity durations and temperatures. The plant age, 2-3 and 4-5 leaf stage of R. dentatus and 5-10 and 10-15 leaf stage of C. album were found to be more susceptible when exposed to 20% canola oil emulsion containing 107 conidia/.ml of strain R. and 109 conidia/ml of strain C at 25-30oC and 100% humidity for 24 hours and displayed maximum mortality.
The efficacy of mycoherbicidal formulation was further investigated through small scale pot and large scale filed experiments. Pot experiments led to screening of most susceptible growth stages of R. dentatus and C. album for further trials in the field, with selected mycoherbicidal formulation. The assay further supported the hypothesis that 4-5 leaf stage of R. dentatus and 10-15 leaf stage of C. album were highly susceptible.
The mycoherbicidal formulations were retested against selected growth stages of target weeds in co-cultivation with wheat varieties viz., Inqalab 91 and Punjab under filed conditions. Findings parallel to pot experiments were reconfirmed with reference to the efficacy of mycoherbicidal potential. Successful field control of two target weeds i.e. R. dentatus and C. album, resulted in consequent increased in economic yield of Inqalab 91 and Punjab 96 by 30-47 % and 21-40% respectively.
Mass production assays of mycoherbicidal inoculum revealed, wheat straw + chickpea substrate most suitable for both the strains of A. alternata.
To substantiate the efficacy of mass produced mycoherbicides, viability of conidia in formulation was checked against target weeds through test trials. The same results were reproduced after 10 days of inoculation i.e. the mycoherbicides induced 100% mortality in target weeds.
To analyze the significance of findings in various experiments, the data was subjected to different statistical tests including t-test, Duncans multiple range test, Regression analysis and analysis of variance.
The results have been discussed with reference to the growth pattern and physiology of weeds,-crop-completion, effects of interaction of various environmental factors on pathogenicity and disease expression and the impact of these factor on weed biology.
The study concludes that the mycoherbicidal formulation of A. alternata has the potential to control target weeds and consequent provision to enhance economical yield. It has been suggested that manipulation of the pathogen through genetic engineering and biochemical characterization could further increase the efficacy of the pathogen