Systemic growth of Leptosphaeria maculans from cotyledons to hypocotyls in oilseed rape: influence of number of infection sites, competitive growth and host polygenic resistance

Abstract

The influence of competitive effects between two isolates, of the number of infection sites on cotyledons and of host polygenic resistance on the systemic growth of Leptosphaeria maculans , the cause of phoma stem canker in oilseed rape ( Brassica napus ), were investigated. Controlled-condition experiments were conducted with two oilseed rape doubled haploid lines, one susceptible and the other with a high level of polygenic resistance, inoculated via wounded cotyledons with conidial suspensions obtained from two isolates. Expression of cankers in plants was enhanced by exposing inoculated plants to low temperature (6 ° C) followed by warm temperature (20 ° C). The fungus was detected by PCR amplifications of three minisatellite markers in all stems with visible canker symptoms and also in the stems of 14 of the 59 plants without visible cankers on the hypocotyls. Disease severity increased with the number of infection sites on cotyledons: in one of the three replicate experiments, the mean external necrosis length on the hypocotyl ranged from 6·47 to 35·3 mm for one and eight infections sites on cotyledons, respectively. The probability of an isolate reaching the hypocotyl from inoculated cotyledons decreased with increasing competing inoculum load on cotyledons: for instance, for isolate A290v it decreased from 1 when inoculated alone to 0·28 when coinoculated with six drops of competing isolate P27d. Polygenic resistance significantly reduced disease incidence and severity. For instance, in one of the three replicate experiments, disease incidence ranged from more than 74% in susceptible plants to 16% in resistant ones, while mean external necrosis length was up to 35·3 and 6·5 mm on susceptible and on resistant plants, respectively. This study offers new possibilities for assessing levels of polygenic resistance to stem canker in B. napus and studying the aggressiveness of L. maculans isolates.

Assessing Quantitative Resistance against Leptosphaeria maculans (Phoma Stem Canker) in Brassica napus (Oilseed Rape) in Young Plants

“…This is consistent with results of previous work [6]. The development of more severe necrosis in hypocotyls of A30 than in those of C119 [28] suggests that quantitative resistance against L. maculans may also impede the growth of the pathogen from the cotyledon to the hypocotyl. There is a need to test a wide range of cultivars and DH lines under different conditions to investigate the relationship between quantitative resistance against L. maculans in leaf petiole and stem tissues identified in controlled environment experiments and quantitative ‘field’ resistance identified by end of cropping season disease assessments in field conditions.…”

“…Although canker symptoms can be produced on hypocotyls after cotyledon inoculation [28], production of canker symptoms requires 4 weeks at 6°C followed by 6 weeks at 20°C and hypocotyl infection is not representative of what happens in field conditions in the UK. Furthermore, the structure of hypocotyl is different from that of stem [41].…”

The full life cycle of Leptosphaeria maculans completed on inoculated oilseed rape incubated under controlled conditions

“…Quantitative resistance reduces disease severity, measured by canker length (Delourme et al, 2006;Travadon et al, 2009) and mycelial systemic growth (Huang et al, 2009). Competition effects during systemic growth have been detected (Travadon et al, 2009), as the canker severity in coinfected plants differed from the severity achieved by the same isolates inoculated alone. So far, fitness of isolates has been compared among isolates grown on media or as disease symptom size on young plants (Huang et al, 2006).…”

“…Unlike the situation in Canada and Australia (Barbetti, 1976;Hall et al, 1996), secondary cycles of infection by means of conidia produced on leaf spots have not been documented in Europe. Cankers develop due to the systemic growth of fungal hyphae from leaf spots to the leaf petiole through xylem vessels, and subsequently to the stem base (Hammond et al, 1985;Travadon et al, 2009). The fungus can survive as hyphae in crop stubble, forming two kinds of fruiting bodies: pycnidia and pseudothecia.…”

“…On stubble collected from fields, the incubation process started outside has been completed under controlled conditions, enabling studies on the effect of climate on maturation Toscano-Underwood et al, 2003;Naseri et al, 2009). In addition, infection, the systemic pathway and cankers can be obtained under controlled conditions (Huang et al, 2009;Travadon et al, 2009). However, achieving the full life cycle of the fungus, from plant infection to sexual reproduction of the next generation of ascospores, has not yet been achieved under climate-controlled conditions.…”