Effects of Glomus intraradices and onion cultivar on Allium white rot development in organic soils in Ontario

Commercial products containing formulations of the vesicular–arbuscular mycorrhiza (AM) Glomus intraradices were assessed for their effectiveness in suppressing Allium white rot (WR) on onions (Allium cepa) in organic soils and compared with the fungicide Folicur 3.6F (430 g a.i./L tebuconazole) under field conditions. The trials were conducted during 2000 and 2001 in commercial onion fields in the Holland–Bradford Marsh in Ontario. The AM product MIKRO-VAM, which is used in transplanted onions, reduced the incidence of WR by almost 50%compared with the untreated control and was comparable with that of the fungicide treatment, Folicur 3.6F, applied according to label recommendations. This is one of the few studies to demonstrate season-long disease suppression with AM under field conditions, and it is the first to show that the AM products can be as effective as a fungicide treatment under commercial production practices. A consistent difference in incidence of WR was found between the cultivars ‘Hoopla’ and ‘Fortress’ onions. ‘Hoopla’ was more susceptible to WR than ‘Fortress’ in 10 of 13 field trials and all trials where WR incidence on ‘Hoopla’ was 4%. There was a significant negative correlation between disease incidence and AM root colonization, suggesting that AM colonization was an important factor in the reduction of WR observed in this study.