Hema Kasinathan Thesis May 1 2012.pdf - Atrium - University of ...

supports the observations of previous studies (Strelkov et al. 2006, Cao et al. 2009) thatpathotype 3 is more aggressive than pathotype 6.In Experiment 2 with pak choy, both Prestop and Serenade reduced clubrootincidence or severity in all the growth media employed, except Serenade was noteffective in soil-less mix. The response patterns of Prestop and Serenade were similar tothat of saturated trial of Experiment 1 with canola. Inclusion of canola in Experiment 2 toobserve clubroot level across the experiments showed that there was no difference inclubroot level between the studies.Clubroot levels were consistently higher on Shanghai pak choy than on canola,which supports previous reports that pak choy is more susceptible to clubroot than canola(Cao et al. 2009). Roots of pak choy are more branched and dense than canola roots,which are long with relatively few side branches. It is possible that the densely branchedand wide spreading roots of pak choy provided more opportunities for zoospores near thesoil surface to make contact with roots and initiate infection (Ayers 1944).The main objective of experiment 3 was to determine if pore size or total porespace affected the development of clubroot or the efficacy of the biofungicides. In aprevious study, the large pore size of muck soil, which allows it to hold more water perunit volume than mineral soil, was associated with higher levels of clubroot than on asilty loam soil (Dobson et al. 1982). In this study, the plants were watered with a finenozzle to allow the growth media to settle naturally without undue compaction. Clubrootlevels were lowest in soil-less mix, even though the pore volume of soil-less mix wasgreater than that of the muck soil and mineral soil. Although soil-less mix has a larger101