Verticillium longisporum on oilseed rape (Brassica napus L.)

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Oilseed rape (Brassica napus) is the second most important source of vegetable plant oil. Products from oilseed rape are nowadays used as edible oil for humans, as meal for animal feeding and as biodiesel for industry. With the growing demands, the global oilseed rape production is rapidly increasing since the last 40 years. Due to the intensive production of oilseed rape, several biotic and abiotic stresses threaten the growth and productivity of this crop. Among the economically important diseases, Verticillium longisporum, a soil-borne vascular pathogen causing ‘Verticillium stem striping’ on oilseed rape, has become a potential threat in major oilseed rape production areas. This Brassicaceae host-specific pathogen leads to abnormal foliar chlorosis, reduction of plant height under greenhouse conditions, and unilateral stem striping in the field. It has been estimated to cause 10% to 50% of yield losses in the field. No registered fungicide is available on the market to control this pathogen. In spite of intensive research on V. longisporum in the last decades, some basic knowledge on epidemiology, seed transmission and resistance mechanisms are still lacking. Therefore, the present study has focused on exploring the time course of infection and disease development in the field including the potential of seed transmission. Besides, the role of salicylic acid (SA) in resistance of oilseed rape to V. longisporum was investigated. Furthermore, tests to identify oilseed rape genotypes and accessions with enhanced resistance to V. longisporum were conducted. The present study revealed that the colonization patterns of V. longisporum are different in winter and spring oilseed rape. Besides, soil temperature had a positive correlation to the root colonization of the fungus. Seeds obtained from diseased plants under greenhouse condition had an incidence of V. longisporum infestation ranging from 0.08% to 13.3% in in vitro tests, among which the highest level of seed transmission incidence was recorded for the susceptible cultivar Falcon. However, none of seeds harvested from diseased field-grown winter oilseed rape have been affected by V. longisporum. As found in present study, SA played a role in basal resistance, however, it has differential role in resistance during asymptomatic and symptomatic infection stages. A faster increase of SA was observed in the resistant cultivar at 7 dpi, which allowed poorer growth of V. longisporum in the hypocotyl, indicating that elevated SA is important for disease defense in the asymptomatic phase of colonization. At later stages of infection at 14 and 21 dpi (symptomatic stages), the increased SA levels were similar in the susceptible and resistant cultivar. However, a remarkable increase in the levels of several phenolic acids was observed in resistance cultivar but not in susceptible cultivar. These results indicated that SA and phenolic acid synthesis are important in cultivar-related resistance during asymptomatic and symptomatic stages of infection, respectively. last but not least, the present study has also found large variation of responses to infection with V. longisporum occurred among the screened accessions of B. napus.