Efficacy assessment of major resistance genes against blackleg of oilseed rape in Germany

Phoma lingam is one of the most important diseases on oilseed rape worldwide. The sexual stage is divided into two different species: Leptosphaeria maculans and Leptosphaeria biglobosa, whereby L. maculans is considered to be the more aggressive type at the stem base of oilseed rape. Furthermore L. maculans build strains (pathotypes), which differ highly in there virulence on different oilseed rape cultivars.
The spatial abundance of those pathotypes is of high interest for farmers and breeders for a durable production of oilseed rape. In two different projects we analyze the spatial distribution of Phoma pathotypes. Together with farmers and breeders we install plots of oilseed rape with a highly resistant and susceptible cultivar, regarding Phoma resistance. The highly susceptible cultivar carries no resistance genes and serves therefore as a 'capture-organism' for all Phoma isolates being present in the observed region. During the season cotyledons and leaves of oilseed rape with typical Phoma lesions (pycnidia on chlorotic spots, Fig 1) are collected. Pure isolates are gained after incubation of the infected tissue on moist filter paper and subsequent subculturing on agar media. These isolates are tested on a set of oilseed rape cultivars carrying different monogenic resistance genes to analyze the spectrum of Phoma strains in different geographic regions (Fig 2). On the other hand the highly resistant cultivar carries the effective RLm7 resistance gene; hereby resistance breaking isolates of Phoma are recognized. Resistance genes in oilseed rape play an important role in the control of Phoma.
Less is known about the temperature stability of those resistance mechanisms. In the presence of a climate change with predicted higher amount of rainfall in spring and hotter mean temperature we will analyze the temperature dependency of the known resistance genes in oilseed rape regarding Phoma susceptibility. Additionally we determine the susceptibility of a set of german winter oilseed rape cultivars with a specific hypocotyl inoculation test in the greenhouse (Fig 3). Furthermore little is known on broad leaf infections of Phoma on oilseed rape and the resistance mechanisms behind it, which we want to elucidate in experiments under controlled conditions. The main cause for Phoma infections in autumn are sexually produced airborne ascospores released from pseudothecia on plant residues. Therefore the airborne inoculum in oilseed rape producing areas is of high interest. With Burkhard spore traps we determine the amount of airborne ascospores of Phoma in different regions. Thereby it is of main interest to distinguish between the aggressive (L. maculans) and non-aggressive (L. biglobosa) type of Phoma. By the means of species-specific quantitative PCR we can distinguish between the two species and quantify the amount of spore DNA of each species in the different geographical regions.