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Poster: Biology & Disease Mgmt: Genetics of Resistance


Cavity spot of carrots: progress in screening for resistance
M. MCDONALD (1), M. McDonald (2) (1) University of Guelph, Canada; (2) University of Guelph, Canada

Cavity spot of carrot is caused by several Pythium species, especially Pythium violae and P. sulcatum. Field trials were conducted from 2013 to 2015 to screen 60 carrot lines from the USDA breeding program at the University of Wisconsin lines for differences in susceptibility to cavity spot. Cultivars Atomic Red, Cellobunch and Envy were included as commercial checks. Carrots were seeded into ‘muck’ soil (pH 5.7- 6.5, organic matter 65- 78%) where the cavity spot pathogens occur naturally. Carrots were direct seeded (≈75 seeds/m) in early June, onto raised beds, 66 cm apart and 6 m in length. Fifty carrots per replicate were harvested in late October, placed in cold storage for 6 – 8 weeks and washed and assessed for cavity spot incidence and disease severity (DSI). Cavity spot incidence and severity were very high in 2013 (97% and 69 DSI) and 2014 (96%, 54 DSI) and lower in 2015 (50%, 26 DSI). A wide range of susceptibility to cavity spot was found each year. Purple lines 6139B, 6244B, 7262B and 3497B were highly resistant (1- 8%, 1-7 DSI). Two orange carrot lines, in the trials in 2014 and 2015, had consistently low disease. Line 1137B had 8% cavity spot in both years, with DSI of 7 in 2014 and 4 in 2015. Line 5367 B had 14 and 21 % cavity spot in 2014 and 2015, respectively, with a DSI of 8 and 7. Field screening has identified breeding lines with promising levels of resistance to cavity spot.