NS CRUMP¹, GJ ASH¹ and A NIKANDROW²

¹Farrer Centre for Conservation Farming, Charles Sturt University, PO Box 588, Wagga Wagga 2678, Australia; ² Agriculture and Veterinary Research Centre, Orange 2800, Australia

Background and objectives
Saffron thistle is a troublesome weed of both crops and pasture in Australia. The weed has been reported to cause a yield reduction in cereals of up to 70%. Saffron thistle seed also contaminates grain, attributing a financial penalty to the value of the grain. In pasture the spines of saffron thistle cause damage to the eyes, mouth and hooves of livestock, which make stock susceptible to diseases such as scabby mouth and pink eye. The thorny nature of the thistle reduces livestock access to palatable pasture. Chemical control, predominantly phenoxy herbicides, does not provide reliable or effective control. Furthermore, in pasture environments the herbicides damage desired Trifolium and Medicago species, reducing potential pasture production. Other options for the control of saffron thistle are limited.

In Australia, the major constraint to the application of classical biological control for saffron thistle is its genetically similar background to safflower (Carthamus tinctorius) [1]. However no consideration has been given to the use of inundative biological control. This paper outlines the suitability of a fungal pathogen for inundative biological control of saffron thistle.

Results and conclusions
Isolates of Phomopsis spp. were isolated from diseased plant material collected from several locations across New South Wales. Phomopsis spp. were also obtained from seed collected from apparently disease-free plants from Junee (47.5%), Mangoplah (10%), Warren (5%) and Barraba (0%). Pathogenicity tests conducted on 45 isolates of Phomopsis spp. have shown plants could be killed in 4-10 days. The most virulent isolates were selected for further investigations. Molecular studies including RAPDs, RFLPs and AFLPs were used to assess variation in the 45 isolates of Phomopsis sp. collected from saffron thistle. To identify the species of Phomopsis, the molecular profiles of the saffron thistle isolates were compared to known species of Phomopsis, including P. leptstromiformis, P. longicola, P. phaseolum, Diaporthe articii and D. phaseolum. Morphological characters such as spore morphology and vegetative growth compatibilities were recorded to consolidate the molecular study.

The host range of the Phomopsis sp. was tested by inoculating 29 species, from four different families. Six species within the family Asteraceae, Centaurea sp., Carthamus tinctorius, Lactuca sativa, Dorotheanthus sp. and Chrysanthemum morifolium were killed 3-5 days after inoculation. Severe lesions were recorded on Centaurea solstistialis, Senecio madagascariensis and Arctotheca calendula. In summary, no economically important plants which would be found in the same environment as saffron thistle were killed by Phomopsis sp. Therefore it appears feasible to continue investigations on the use of Phomopsis sp. as a mycoherbicide for saffron thistle. Furthermore, the susceptibility of other weed species promotes the advantage of using Phomopsis sp. as a biocontrol agent.

The most limiting factor of mycoherbicide development is the fungal pathogen's requirement for moisture to infect the plant. Studies on the dew period requirement of Phomopsis sp. on saffron thistle has shown that a period of 2 days' moisture is required to incite complete plant mortality. This dew period must not be delayed after application of the pathogen. Further studies are now focusing on additives which could be combined with the fungus to reduce the dew period requirement and increase plant mortality.

References
1. Wasphere AJ, 1984. In Bailey P, Swincer D, eds, Proceedings of the IV Australian Applied Entomological Research Conference, September 1984, pp. 324-332.