Fitness Cost Associated with Loss of the AvrLm4 Avirulence Function in Leptosphaeria maculans (Phoma Stem Canker of Oilseed Rape)

• Published in: European journal of plant pathology Vol. 114; no. 1; pp. 77 - 89
• Main Authors: Huang, Y.J, Li, Z.Q, Evans, N, Rouxel, T, Fitt, B.D.L, Balesdent, M.H
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Nature B.V 2006
• Subjects: Ascospores; avirulence alleles; Bacteria; Brassica napus; Canker; Conidia; Crop science; Cultivars; defense mechanisms; disease resistance; Field tests; Fitness; fungal diseases of plants; genes; genetic resistance; genetic variation; Growing season; host-pathogen relationships; Immunization; In vitro methods and tests; Incubation; Inoculation; inoculation methods; Inoculum; isogenic lines; Leaves; Leptosphaeria maculans; Lesions; microbial genetics; Mycelia; Oilseeds; Phoma; plant pathogenic fungi; Pycnidia; Rapeseed; resistance-breaking strains; Stem canker; stem cankers; Stems; strain differences; strains; vigor; virulence; Europe
• ISSN: 0929-1873; 1573-8469
• DOI: 10.1007/s10658-005-2643-4

Near-isogenic isolates of Leptosphaeria maculans differing at the AvrLm4 avirulence locus (AvrLm4 or avrLm4) were produced in vitro. Methods for inoculation of leaves of oilseed rape with ascospores or conidia were compared. The 'ascospore shower' inoculation was the most efficient method for use when inoculum is limited (e.g. ascospores produced in vitro). It was used in controlled environments to compare fitness of AvrLm4 and avrLm4 isolates at 5, 10, 15, 20 or 25 °C on leaves of oilseed rape cultivars Eurol and Darmor lacking the resistance gene Rlm4, which corresponds to AvrLm4. At all temperatures tested, AvrLm4 ascospores produced more lesions than avrLm4 ascospores. The diameters of lesions produced by AvrLm4 ascospores were greater than those of lesions produced by avrLm4 ascospores. At 15-20 °C, more lesions initiated by AvrLm4 ascospores produced pycnidia than did lesions initiated by avrLm4 ascospores. However, there were no differences between AvrLm4 and avrLm4 isolates in incubation period (from inoculation to appearance of lesions) or rate of mycelial growth in leaves from lesions towards the stems. In field experiments with winter oilseed rape cultivars lacking Rlm4, the frequency of AvrLm4 isolates increased from 5.7% at the phoma leaf lesion stage (autumn) to 20.5% at the stem canker stage (summer) during 2002/2003 and from 7.9 to 11.5% during 2003/2004 growing seasons. Results of controlled environment and field experiments indicate that avrLm4 isolates have a fitness cost compared to AvrLm4 isolates.