Effect of Plasmodiophora brassicae resting spore concentration and crop rotation on growth of clubroot-resistant crops

Effect of Plasmodiophora brassicae resting spore concentration and crop rotation on growth of clubroot-resistant crops Jill Allison Dalton Advisors: University of Guelph Dr. Mary Ruth McDonald Dr. Bruce D. Gossen Clubroot caused by Plasmodiophora brassicae Woronin is an important threat to production of canola (Brassica napus L.) and Brassica vegetables in Canada and worldwide. To help understand the role of crop rotation in integrated clubroot management, this research examined the pattern of decline in resting spores and the influence of spore concentration on growth and development of clubroot-resistant crops. It was found that a portion of the resting spore population is long-lived, but most (>99%) spores survive for only 1–2 years. Higher concentrations of resting spores resulted in reduced plant growth and delayed development in resistant canola and napa cabbage. However, the growth response was inconsistent across studies and repetitions, and may be influenced by other factors such as soil type and crop species. For canola growers, a ≥ 2-year break from canola, in combination with a clubroot-resistant cultivar, is recommended as a clubroot management strategy wherever clubroot is found. iii ACKNOWLEDGEMENTS I would like to express my sincere gratitude to my co-advisors Dr. Mary Ruth McDonald and Dr. Bruce D. Gossen for providing me with this opportunity and for their guidance, patience, insight and support throughout this process. Thank you for helping me to strengthen my skills in scientific analysis, writing and public speaking. I would also like to recognize the advice and encouragement of committee members Dr. Katerina Serlemitsos Jordan and Dr. Sean Westerveld. Thank you all for your time and expertise. I appreciate the financial support provided by the Canola Council of Canada, Agriculture and Agri-Food Canada through the Canola Science Cluster of Growing Forward 2, and the University of Guelph Department of Plant Agriculture. Thank you to Dr. Abhi Deora, Dr. Fadi Al-Daoud, Travis Cranmer and many other lab colleagues for technical advice, support and encouragement. I am very grateful to Kevin Vander Kooi, Laura Riches, Shawn Janse, Misko Mitrovic, Dennis Van Dyk and summer staff at the Muck Crops Research Station for all of your help with the 2014 and 2015 field trials. I would also like to acknowledge Godfrey Chu, Ken Bassendowski, Denis Pageau and others at AAFC and University of Guelph for providing materials and support for the crop rotation studies. I am very grateful to my parents for encouraging me to continue learning, to always do my best and to believe in myself. I would like to say a special thank you to my sister for teaching me to think critically. I appreciate the tremendous amount of support I have received from my friends and family, especially Jack & Wilda Mardlin, Elizabeth Fraser, Gabe Sawhney, Ashleigh Dalton, Laura Dalton, Martin Dalton and, finally, John Jeffrey Mardlin, for truly going above and beyond a reasonable amount of support. iv TABLE OF CONTENTS CHAPTER ONE – LITERATURE REVIEW . 1 1.1 Brassica crops and their relatives . 1 1.1.1 Canola and other oilseed Brassica crops . 1 1.1.2 Brassica vegetables . 2 1.1.3 History . 4 1.1.3 Economic importance of Brassica crops in Canada . 4 1.1.4 Pests and diseases in Brassica crops . 5 1.2 Clubroot of Brassica . 7 1.2.1 Host range and history of clubroot . 7 1.2.2 Causal agent and taxonomy . 8 1.2.3 Disease cycle . 9 1.2.4 Pathotypes . 12 1.2. 5 Incompatible Interactions . 13 1.3 Factors influencing infection and development . 14 1.3.1 Soil moisture . 15 1.3.2 Temperature . 15 1.3.3 Soil pH . 16 1.3. 4 Resting spore concentration . 17 1.4 Disease management . 17 1.4. 1 Cultural control . 17 1.4. 2 Synthetic fungicides and surfactants . 19 1.4. 3 Biological control and biofungicides . 21 1.4. 4 Host plant resistance . 23 1.4. 5 Metabolic cost of resistance . 25 1.5 Summary and objectives . 26 CHAPTER TWO -­‐ EFFECT OF RESTING SPORE CONCENTRATION ON GROWTH OF CLUBROOT-­‐RESISTANT BRASSICA CROPS . 28 2.1 Introduction . 28 2.2 Materials and Methods . 33 2.2.1 Controlled environment study – canola . 33 2.2.2 Large pot studies – outdoors . 35 2.2.3 Controlled environment study of canola, cabbage and napa cabbage . 36 2.2.4 Field trials . 37 2.2. 5 Controlled environment pH study . 39 2.2. 6 Statistical analysis . 40 2.3 Results . 41 2.3.1 Controlled environment study . 41 2.3.2 Large pot studies – outdoors . 44 2.3.3 Controlled environment study – canola, napa cabbage and cabbage . 45 2.3. 4 Field trials . 46 2.3. 5 pH study . 52 2.4 Discussion . 54 v CHAPTER THREE -­‐ DECLINE IN RESTING SPORES AND EFFECT OF CROP ROTATION FOLLOWING A SUSCEPTIBLE CROP . 62 3.1 Introduction . 62 3.2 Materials and Methods . 67 3.2.1 Decline in resting spores over time following susceptible canola . 67 3.2.2 Effect of resistant canola on the concentration of resting spores in soil . 70 3.2.3 Controlled environment study – crop rotation and spore concentration . 71 3.2.4 Statistical analysis . 73 3.3 Results .