First Record in Syria of Didymella fabae, the Teleomorph of Ascochyta fabae and Causal Organism of Faba Bean Blight

Abstract

Ascochyta blight, caused by Ascochyta fabae Speg., is a common and destructive disease of faba bean (Vicia faba L.) in the Middle East, Europe, Canada, New Zealand (4), and Australia. The main sources of inoculum are debris and seeds from which spores are air- and splashborne. The teleomorph of A. fabae has been reported previously only from England (2). The presence of the teleomorph supports the variability reported in the fungus populations from Canada (3) and Poland (1). Stems of faba bean plants, severely infected with A. fabae, were collected in July 1999 from Tel Hadya, Syria. The plants previously had been inoculated with a mixture of isolates of the pathogen, collected from the main faba bean-growing regions in Syria between 1996 and 1998, and kept under shade. The infested stems were used to inoculate the ICARDA Faba Bean Ascochyta Nursery planted on 29 November 1999. During late January 2000, symptoms appeared on the susceptible faba bean genotype. Stem pieces from debris used for inoculations were collected from the field and examined microscopically for the presence of ascomata. The maximum, minimum, and mean temperatures and rainfall at Tel Hadya during December 1999 were 16.5, 5.8, and 8.7°C and 22.4 mm, respectively. There were 16 nights when temperatures dropped below 0°C, and 10 nights when temperatures were between 0 and 5°C. Ascomata of A. fabae ranged from 76 to 209 μm wide (average 158 ± 3.9 μm) and 101 to 285 μm in length (average 178 ± 4.1 μm). Asci were 10 to 15 μm wide (average 14 ± 0.3 μm) and 51 to 96 μm long (average 63 ± 1.1 μm). Ascospores were 5 to 8 μm wide (average 7 ± 0.2 μm) and 15 to 20 μm in length (average 17 ± 0.3 μm). These measurements are comparable to those reported from England. Individual ascomata were dissected from stem tissue and fixed to the lids of petri dishes containing 2% water agar. After 24 h, the petri dishes were examined microscopically to locate ascospores on the surface of the medium. Germinating ascospores and developing colonies were transferred from water agar to faba bean dextrose agar. Colonies characteristic of A. fabae developed on the latter medium within 7 days of incubation at 20 ± 2°C. Pathogenicity tests of developing colonies were carried out on 3-week-old faba bean plants (Giza 4) using a spore suspension (2.5 × 105 spores per ml) of each of the isolates. Both inoculated seedlings and control seedlings inoculated with sterile water were covered with plastic bags for 48 h in a plastic house maintained at 18 ± 2°C. After removal of the plastic bags, seedlings were wetted four times per day by spraying with tap water to runoff. Inoculated plants showed characteristic symptoms of Ascochyta blight 15 days after inoculation. The fungus was reisolated from lesions that developed on leaflets of all inoculated seedlings, but not from any of the control seedlings. This is the first report of the occurrence of A. fabae, the sexual stage of Didymella fabae Jellis & Punithalingam in Syria, and indicates that the fungus could develop population variants. These findings have implications for breeding for resistance to Ascochyta blight. References: (1) A. Filipowicz. Faba Bean Abstr. 4:47, 1983. (2) G. J. Jellis and E. Punithalingam. Plant Pathol. 40:150, 1991. (3) P. D. Kharbanda and C. C. Bernier. Can. J. Plant. Pathol. 2:139, 1980. (4) K. Y. Rachid et al. Plant Dis. 75:852, 1991.