Control of Soilborne Pathogenic Fungi in Fields of Sweet Onion

Area of Science:

• Agricultural Science
• Plant Pathology
• Soil Science

Background:

• Soilborne fungal pathogens significantly impact sweet onion production.
• Phoma terrestris, causing pink root, is a primary pathogen in continuously cropped onion fields.
• Rhizoctonia solani AG-4 and Pythium irregulare are also highly virulent to onion seedlings.

Purpose of the Study:

• To evaluate the efficacy of various soil fumigants in controlling key soilborne fungal pathogens affecting sweet onions.
• To assess the impact of fumigation treatments on soil pathogen populations and subsequent onion yield.
• To identify the most effective fumigants for managing diseases like pink root in commercial onion cultivation.

Main Methods:

• Four field experiments were conducted over two years using sweet onion.
• Soil populations of fungi were assayed on selective media.
• Pathogenicity tests identified virulent fungal species.
• Fumigation treatments included methyl bromide (MBR), chloropicrin (CP), MBR + CP, metam sodium, 1,3-dichloropropene (1,3-D), and 1,3-D + CP.
• Onion yield and bulb decay were measured at harvest and after curing.

Main Results:

• Methyl bromide + chloropicrin (MBR + CP) and chloropicrin (CP) effectively reduced populations of Phoma terrestris, Pythium spp., Fusarium spp., and Rhizoctonia solani AG-4.
• Metam-sodium and 1,3-D + CP showed moderate efficacy, while MBR and 1,3-D were ineffective.
• No significant differences in bulb decay were observed among treatments.
• Increased marketable bulb yield was correlated with the control of soilborne pathogenic fungi.

Conclusions:

• Soil fumigation, particularly with MBR + CP or CP, can effectively manage key soilborne pathogens in sweet onion production.
• Effective pathogen control through fumigation leads to increased marketable onion yield.
• Phoma terrestris is a critical pathogen to manage in fields with continuous onion cropping.