Verticillium longisporum (Karapapa, Bainsbridge and Heale)
Significance: In the summer of 2007, UK scientists identified Verticillium longisporum as the causal agent of a "wilt" disease of oilseed rape (Brassica napus) seen in some parts of England. Plant samples of the cultivar 'Castille' from Kent and the cultivar 'Barrel' from Herefordshire were found to be infected with the pathogen. These reports are the first confirmed records of V. longisporum occurring on oilseed rape in the UK and scientists there believe the fungus may be responsible for recent yield reductions observed in that crop.
Issues of Concern: Verticillium wilt of oilseed rape is caused by the soil-borne fungus Verticillium longisporum, a species that attacks oilseed rape and other Brassica species (Karapapa et al., 1997). Unlike other Verticillium diseases, V. longisporum in oilseed crops does not induce true wilting, but rather premature senescence and ripening which can lead to severe yield reductions, sometimes up to 50 percent. Infection occurs via soil-borne inoculum through the roots and then spreads within the plant to stems and leaves. As plants mature, the pathogen produces microsclerotia within the stems, which are then released when the crop is harvested and plants are ploughed back into the soil. Microsclerotia can remain viable in soil for many years and act as a reservoir to infect future crops. Other than deep ploughing and long crop rotations to non-susceptible species (e.g., cereals, grasses), there are few options available to growers to manage the disease. Host plant resistance is being researched in some countries but is not yet an option for producers. Over the last few decades Verticillium wilt has become a serious disease affecting oilseed rape production in several northern European countries (e.g., France, Germany, Sweden). Although not known to be seedborne, this recent report from the UK suggests the pathogen is able to spread, possibly with microsclerotia carried as contaminants in seed shipments or within soil attached to planting material of other species. Verticillium longisporum has not yet been reported from oilseed crops within North America (Kharbanda et al., 2001; Heale & Karapapa, 1999), although it has been found infecting cauliflower (Brassica oleracea var. botrytis) in coastal areas of California (Koike et al., 1994), as well as in horseradish (Armoracia rusticana) in Illinois (Babadoost et al., 2004). Care should be taken to ensure the fungus is not introduced as microsclertial contamination within seed shipments, or with the movement of soil on plants, machinery, etc.
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