Biological control of the weed Amaranthus retroflexus with fungal pathogens
Amaranthus retroflexus is a common annual weed world-wide and causes substantial yield reduction in many crops. In this research program three main objectives were fulfilled: firstly the effect of certain environmental parameters on germination and emergence of A. retroflexus collected from Iran were determined; secondly, potential pathogen strains as biological control agents for A. retroflexus were identified; and finally the activity of a candidate biological control agent against the target weed under various environmental conditions and with different formulation were evaluated. Results of several experiments showed that under controlled conditions A. retroflexus seeds are able to germinate at a wide range of temperatures, water potentials, burial depths of seeds and soil types. However, the degree of success of germination and establishment of this plant were influenced by temperature, water potential, planting depth, soil type and interactions between these factors. A. retroflexus growth as favoured by high temperature (25-35C), high water availability (0 to -1 bar), shallow burial (0.5-1 cm) and lighter soil types. The objective of the second section of this project was to find a potential pathogen as a biological control agent for A. retroflexus. Initially a culture collection of Amaranthus pathogens, collected throughout Europe and Iran, were screened. The final results in pathogenicity tests showed that A. altemata strains 423, 780 and 930 and Aposphaeria amaranthi showed the greatest pathogenicity against A. retroflexus. The fungi of A. alternata strain 423 and Aposphaeria strain were able to control 100% of A. retroflexus plants under certain environmental conditions. Ascochyta caulina and an unidentified fungal strain 5-1 (collected from Iran) caused less disease development. These results clearly indicated the potential of A. alternata and Aposphaeria amaranthi as mycoherbicides. A. alternata strain 423 was shown to be more pathogenic than strains 780 and 930. A spore concentration of 107 spores ml-1 without application of oil emulsion or 106 spores ml-1 with oil formulation was required for good disease development. A. alternata species demonstrates potential for controlling A. retroflexus only when 16 hours of high humidity/leaf wetness were provided after spore application. Also for maximum activity of A. alternata, it had to be applied at 2-4-leaf growth stages of the weed seedlings. The optimum dew temperatures for 100% mortality were between 20 and 25C. The post inoculum temperature for giving maximum disease development was between 20 and 30C. Finally, for maximum control of A. retroflexus by A. alternata there should be no delay in the occurrence of dew after inoculation. Formulation of A. alternata spores in the rape-seed oil emulsion significantly increased disease development and decreased plant vigour and dry weight of A. retroflexus. This formulation caused a reduction in minimum spore density required in the spore suspensions and the length of dew period required for disease development. However, formulations of A. alternata, need to be further improved to reduce the requirement for a long dew period. Application of granules at emergence stage gave better control of A. retroflexus plants than application at the 4-leaf stage or application simultaneously with planting. Because A. alternata granule applications needed to be applied at very high levels, it is unlikely to be economically liable. Both formulations of Alternaria (spore suspensions (liquid) and granule (solid)) caused no serious infection in sugar-beet, maize and wheat plants.