The effect of linseed cropping on Verticillium dahliae soil infestation levels

摘要

Verticillium dahliae Kleb. attacks a very wide range of crops (1) but in the United Kingdom most concern has been about wilt in strawberry. The pathogen persists in the soil as microsclerotia, the bodies being formed in the decaying tissue of host crops from which they eventually are released into the soil. It is well known to UK strawberry growers that the presence of potatoes in a rotation is likely to increase the risk of there being levels of V. dahliae microsclerotia in the soil that could beeconomically damaging to strawberry plants. Recent studies have shown that the pathogen is also causing losses in the potato crop itself (2). During the period 1989-90, an investigation was carried out on the effect of field cropping history on V. dahliae soil infestation levels (3). Information from 65 sites included 15 years of cropping details in 44 cases, but no clear association could be found between any cropping history and soil infestation levels in excess of 0.4 colony forming units (c.f.u.) per g of soil. The pathogen was more readily detected in fields where vegetatively propagated crops, such as fruit or potatoes, had been grown and was less readily found in fields which had produced only crops grown from true seed such as cereals, grass, legumes and vegetables other than potatoes. Recently, linseed (Linum usitatissimum) has been reported as susceptible to V. dahliae in the UK and Germany (4). Microsclerotia develop abundantly in linseed stems and the potential for this crop to re-infest soil with the pathogen has caused concern to strawberry growers (5). The acreage of linseed increased from 0.4 percent of the total cropping area (excluding permanent pasture) in England in 1989 to 3.8 percent in 1993 (6), as a result of an E.U. subsidy,but has since declined. Since 1991 a soil dilution test has been available for the determination of V. dahliae infestation levels as a basis for giving advice to growers on the suitability of land for strawberry fruit and runner production (7,8). This test has a detection limit of 0.1 c.f.u./g soil. Levels of V. dahliae found in tested soils have rarely exceeded 100, and most occur in the range 0-20 c.f.u./g soil. Although many soil samples have been processed by the ADAS testing laboratory since 1991,no information on cropping history prior to sampling had been requested of growers.