Characterization of compatibility of relations between agents causing blackleg and oilseed rape varieties

Olufadekemi Fajemisin

Doctoral thesis

Clubroot of winter rape and cole vegetable

Abstract:

After the double zero breakthrough, which lowered the erucic acid and glucosinolates, oilseed rape is now one of the most important oilseeds in the world. It is useful for food, feed for animals, and as a biofuel. Unfortunately, the presence of a variety of pests and diseases has led to severe yield losses. One of the most important diseases of oilseed rape is phoma stem canker or blackleg disease. It is caused by a Leptosphaeria maculans/ Leptosphaeria biglobosa complex. The use of fungicides and genetic resistance have over the years mitigated these yield losses. However, these two control methods are usually not durable and after a few years lead to fungicide resistance and breakdown of genetic resistance. This study, therefore, focuses on improving our understanding of fungicide sensitivity, race structure as well as gene flow of L. maculans populations in the Czech Republic. During this study, L. maculans isolates were collected from 10 regions across the Czech Republic between 2014 and 2020. First, the fungicide sensitivity of L. maculans isolates to nine active ingredients (tebuconazole, tetraconazole, metconazole, prochloraz, propiconazole, prothioconazole, boscalid, dimoxystrobin, and trifloxystrobin), and six commercial fungicides (Caramba, Tilmor, Horizon 250 EW, Bumper Super, Pictor, and Efilor) were evaluated using two established methods, the mycelium growth plate, and microtitre plate method. These fungicidal substances represent three fungicide classes commonly used in the Czech Republic to control oilseed rape pathogens: (Demethylation inhibitors [DMI], Quinone outside inhibitors [QoI], and Succinate dehydrogenase inhibitors [SDHI]). In addition, the frequency of avirulence alleles and race structure of L. maculans isolates was determined. Also tested in this study were the mating type and genetic variability because understanding the composition of L. maculans populations is one of the first steps in maintaining the durability of control methods. The results of the fungicide sensitivity tests showed that the DMI fungicides were less effective at controlling L. maculans isolates than the other fungicides tested, suggesting that resistance to this group of fungicides may have begun. However, boscalid which belongs to the SDHI fungicide group is currently the most effective fungicide for controlling this pathogen. Cross and multiple sensitivity studies also showed that these fungicides are more effective when two fungicides belonging to different fungicide classes are used as mixtures. For the study on the frequency of avirulence alleles and race structure of L. maculans isolates, it was determined that the major resistance gene Rlm7 is the most effective resistance gene present in oilseed rape cultivars in the Czech Republic. However, there is still the danger that after several years, L. maculans isolates will become virulent to this resistance gene. Therefore, rotating the oilseed rape cultivars with different resistance genes or introgressing the Rlm7 gene into a new breeding line might be the best way of preserving this major resistance gene. Interestingly, the third study on mating types and genetic variability showed that in the Czech Republic, there is currently only one population structure, which provided considerable insight into how genes flow among the L. maculans populations.

Abstract:

After the double zero breakthrough, which lowered the erucic acid and glucosinolates, oilseed rape is now one of the most important oilseeds in the world. It is useful for food, feed for animals, and as a biofuel. Unfortunately, the presence of a variety of pests and diseases has led to severe yield losses. One of the most important diseases of oilseed rape is phoma stem canker or blackleg disease. It is caused by a Leptosphaeria maculans/ Leptosphaeria biglobosa complex. The use of fungicides and genetic resistance have over the years mitigated these yield losses. However, these two control methods are usually not durable and after a few years lead to fungicide resistance and breakdown of genetic resistance. This study, therefore, focuses on improving our understanding of fungicide sensitivity, race structure as well as gene flow of L. maculans populations in the Czech Republic. During this study, L. maculans isolates were collected from 10 regions across the Czech Republic between 2014 and 2020. First, the fungicide sensitivity of L. maculans isolates to nine active ingredients (tebuconazole, tetraconazole, metconazole, prochloraz, propiconazole, prothioconazole, boscalid, dimoxystrobin, and trifloxystrobin), and six commercial fungicides (Caramba, Tilmor, Horizon 250 EW, Bumper Super, Pictor, and Efilor) were evaluated using two established methods, the mycelium growth plate, and microtitre plate method. These fungicidal substances represent three fungicide classes commonly used in the Czech Republic to control oilseed rape pathogens: (Demethylation inhibitors [DMI], Quinone outside inhibitors [QoI], and Succinate dehydrogenase inhibitors [SDHI]). In addition, the frequency of avirulence alleles and race structure of L. maculans isolates was determined. Also tested in this study were the mating type and genetic variability because understanding the composition of L. maculans populations is one of the first steps in maintaining the durability of control methods. The results of the fungicide sensitivity tests showed that the DMI fungicides were less effective at controlling L. maculans isolates than the other fungicides tested, suggesting that resistance to this group of fungicides may have begun. However, boscalid which belongs to the SDHI fungicide group is currently the most effective fungicide for controlling this pathogen. Cross and multiple sensitivity studies also showed that these fungicides are more effective when two fungicides belonging to different fungicide classes are used as mixtures. For the study on the frequency of avirulence alleles and race structure of L. maculans isolates, it was determined iii that the major resistance gene Rlm7 is the most effective resistance gene present in oilseed rape cultivars in the Czech Republic. However, there is still the danger that after several years, L. maculans isolates will become virulent to this resistance gene. Therefore, rotating the oilseed rape cultivars with different resistance genes or introgressing the Rlm7 gene into a new breeding line might be the best way of preserving this major resistance gene. Interestingly, the third study on mating types and genetic variability showed that in the Czech Republic, there is currently only one population structure, which provided considerable insight into how genes flow among the L. maculans populations.

Keywords

Phoma stem canker, Fungicide sensitivity, Avirulence genes, Mating types, Population structure

Keywords

Phoma stem canker, Fungicide sensitivity, Avirulence genes, Mating types, Population structure

Language used: Czech

Date on which the thesis was submitted / produced: 23. 8. 2022

Thesis defence

• Supervisor: prof. Ing. Pavel Ryšánek, CSc.

Czech University of Life Sciences Prague

Faculty of Agrobiology, Food and Natural Resources

Doctoral programme / field:
Agricultural Specialization / Agricultural and Forestry Phytopathology and Plant Protection