R SLADIC¹, S SAVOCCHIA¹, P KOLESIK², SR RAO³, J DEMAJNIK¹, ES SCOTT¹, MD RAMSEY⁴ and PJ MURPHY¹

¹Department of Crop Protection, and ²Department of Horticulture, Viticulture and Oenology, University of Adelaide, Waite Campus, Glen Osmond, South Australia 5064, Australia; ³Department of Biotechnology, CGO Complex, Lodhi Road, New Delhi 110003, India; ⁴South Australian Research and Development Institute (SARDI), GPO Box 397, Adelaide 5001, South Australia, Australia

Background and objectives
Ascochyta blight caused by the fungal pathogen Ascochyta fabae is a serious disease of faba beans in southern Australia. We are investigating the early stages of infection with the eventual aim of developing a novel means of control. A. fabae is pathogenic on faba beans, whereas A. lentis is pathogenic on lentils but does not infect faba beans. Our approach is to compare early infection by A. fabae and A. lentis, as well as infection on resistant and susceptible host varieties.

Results and conclusions
A detached leaf assay modified from that of Evans et al. [1] has been used to study infection. With this assay, considerably more disease developed on leaves from the susceptible faba bean variety, Icarus, than on leaves from a resistant variety, Ascot. This result confirms that the detached leaf assay is a reflection of whole-plant response to infection.

To study the early stages of infection, we compared infection of faba bean leaves by A. fabae with that by A. lentis. Examination by light microscopy and laser confocal microscopy indicates that when A. lentis is applied to leaves of faba bean, there is hyphal growth and development of appressoria but no direct penetration via infection pegs or through stomata [2]. However, when A. lentis spores are applied in conjunction with spore germination fluid from A. fabae, infection occurs. This infection develops at a slower rate than when faba bean leaves are infected with A. fabae, and pycnidia do not develop. The induced infection appears not to be related to the resistance mechanism operating in Ascot, since the infection factor (IF) induces infection by A. lentis equally well on Icarus and Ascot. The infection factor in spore germination fluid is currently being characterized.

References
1. Evans KJ, Whisson DL, Scott ES, 1996. Mycological Research 100, 675-680.
2. Savocchia S, 1996. PhD thesis, University of Adelaide.