Impact of Verticillium longisporum on Yield and Morphology of Winter Oilseed Rape (Brassica napus) in Relation to Systemic Spread in the Plant

The impact of Verticillium longisporum on single plant and whole plot yield of oilseed rape (OSR) was studied in field experiments with natural infection or artificial soil infestation. Disease incidence (DI) and disease severity (DS) correlated with the amount of inoculum provided at four different levels by addition of infested rapeseed straw to the soil. DI and DS were higher in 2003, a year with higher average soil and air temperatures, than in 2004, at similar levels of added inoculum. Maximum DI and DS levels achieved with artificial soil infestation were 54.3% and 0.57 (rating scale from 0 to 2), respectively, which was insufficient to induce significant yield reduction in whole plots. In contrast, a significant decrease in yield was recorded on single naturally infected plants removed from the field at growth stage (GS) 83 to 85. Yield losses of single plants accounted for 20 to more than 80% at DS levels above 5 (scale 0 to 9). The systemic spread of V. longisporum was significantly delayed in plants in the field with substantial colonization of the shoots not occurring before maturity stages. However, fungal systemic spread into the shoots was faster in 2003, consistent with the visual disease assessment in the field. Conversely, the pathogen systemically spread after 28 days in plants (susceptible cultivar 'Falcon') in the greenhouse. Root dip inoculation with conidia suspension induced earlier and more severe disease symptoms than microsclerotia added to the soil. In contrast to the field, intense stunting was recorded on inoculated plants in the greenhouse. The susceptible cultivar, 'Falcon', showed higher DS, stronger reduction in root and shoot lengths and a faster fungal spread in the plant tissue than the moderate susceptible cultivar, 'Talent'. Climatic conditions appear to be responsible for the strong delay in fungal invasion of plants in the field. This may prevent the plants from stunting and mitigate the overall yield effects of the disease. This study indicates a significant yield damage potential of V. longisporum in areas with DI above 60% and conditions accelerating the systemic pathogen spread in the plants.