Mechanisms of resistance in Brassica carinata, B. napus and B. juncea to Pseudocercosporella capsellae

Abstract

Studies were undertaken to compare susceptible and resistant host responses to Pseudocercosporella capsellae in cotyledons of Brassica carinata, B. juncea and B. napus in order to define the mechanisms of resistance in these three species. On both resistant and susceptible hosts, hyphal penetration was always through stomatal openings and without infection pegs or appressoria. On resistant B. carinata ATC94129P, up to 72% of spores disintegrated and, generally, germination (<22%) and germ tube lengths (<25 lm) were comparatively low. Resistant B. napus Hyola 42 had the lowest germination (8%) and susceptible B. carinata UWA#012 had the highest (51%). On resistant B. carinata ATC94129P, germ tube extension was impeded across 24-60 h post-inoculation (hpi) and percentage stomatal penetration lower (4%) at 60 hpi compared with susceptible B. carinata UWA#012 (26%). Stomatal densities (stomata/ 14 757 lm 2 ) on resistant B. juncea Dune (2Á12) and B. napus Hyola 42 (1Á62) were lower than for susceptible B. juncea Vardan (2Á40) and B. napus Trilogy (2Á03). Resistant B. carinata ATC94129P had greater stomatal density (1Á89) than susceptible B. carinata UWA#012 (1Á58). Overall, B. juncea had greater stomatal density (2Á26) compared with B. napus (1Á83) and B. carinata (1Á74). In resistant B. carinata ATC94129P, P. capsellae induced 28% stomata to close, while in susceptible B. carinata UWA#012 no such closure was induced. Epicuticular wax crystalloids were present only on resistant B. carinata ATC94129P and probably also contribute towards resistance.

Unique infection structures produced by Pseudocercosporella capsellae on oilseed crops Brassica carinata, B. juncea and B. napus in Western Australia

“…This re‐emergence was observed 2 weeks post‐inoculation (Lartey et al ., ). However, the brown structures and ropy strand extrusions in the current study appeared 24 hpi when stomatal penetration of the pathogen was as low as 1% (Gunasinghe et al ., ).…”

“…The observed protoplast extrusion may explain the comparatively low germination rates historically recorded for this pathogen (Gunasinghe et al ., ). Extrusion of protoplasts was also observed from suspensions of hyphal fragments in water.…”

“…(Gunasinghe et al ., ). Thirdly, other recent studies have shown that sensitivity of different Brassicaceae host tissues to cercosporin was analogous with their susceptibility to the pathogen (Gunasinghe et al ., ). A number of plant pathogenic fungi produce phytotoxins that facilitate initial pathogen invasion and disease development (Boenisch & Schäfer, ).…”

“…Furthermore, several other pathogens produce thinner than normal hyphae, known as ‘runner hyphae’, during the infection process (De Wit, ). However, in the current study it was not possible to establish a role for such fine thread‐like structures as observed under SEM, despite the fact that P. capsellae penetrates host tissue through stomata without producing any physical infection structures (Gunasinghe et al ., ), which are recognized as integral components in the majority of plant pathogenic fungi (Emmett & Parbery, ).…”

Infection process of Phoma koolunga on stem and leaf tissue of resistant and susceptible field pea (Pisum sativum)

“…The SEM sample preparation protocol described by Gunasinghe et al (2016) was used. Immediately after harvesting inoculated tissues, samples were fixed in 2.5% glutaraldehyde by cutting into approximately 5 9 5 mm pieces while immersed in the glutaraldehyde solution.…”