Pre-plant soil fumigation for control of violet root rot of carrots
Violet root rot (VRR), caused by Rhizoctonia crocorum, causes substantial economic losses and threatens the long-term viability of the carrot industry in the Ohakune region, New Zealand. Previous attempts at control have been largely unsuccessful. Preplanting soil fumigation trials were carried out in two heavily infested Ohakune carrot fields. In 2007/08, chloropicrin+dichloropropene (Tri-Form®60) and metam sodium (Fumasol™) failed to provide a commercially useful level of VRR control. Deficiencies in fumigant placement and distribution, and inadequate sealing of the soil for gas retention were identified as likely reasons for the failure. Modified application techniques, improved soil sealing and different combinations of fumigants in the 2008/09 season provided excellent control in some treatments. At Site A, VRR incidence was 0.6% and 66.7% in the chloropicrin+metam sodium and chloropicrin treatments, respectively, compared with 98.6% incidence in untreated control plots. At Site B, VRR incidence was 2.3% and 3.8% in the chloropicrin+dichloropropene and combined chloropicrin+dichloropropene+ metam sodium treatments, with and without plastic covering respectively, compared with 41.5% in untreated control plots.