Can desiccant application improve carrot seed quality?

Abstract

Though not a common practice, the desiccant diquat is applied to some carrot seed crops in Canterbury to facilitate seed harvest. In a laboratory experiment, diquat had reduced both mycelial growth and spore germination of the seed-borne carrot pathogen Alternaria radicina, the extent being application rate-dependent. In three field trials over two seasons, diquat was applied to carrot seed crops when 80-90% of the secondary umbels had turned brown (approximately four days before swathing) at a rate of 200, 400 or 600 ml active ingredient (ai)/ha. Seeds from five pre-tagged primary and secondary umbels per plot were hand harvested, cleaned and assessed for the presence of A. radicina and for germination. In control plots, seed-borne A. radicina ranged from 18 to 49%, differing with season and trial site, but seeds from secondary umbels always had higher infection. For all sites, diquat application significantly reduced the pathogen in seeds from both umbel positions, with infection decreasing as application rate increased. However, at 600 ml ai/ha, diquat negatively affected germination by killing the seeds. At the two lower rates, germination did not differ from the control, but the desiccant increased the number of dead seeds, whereas for the control the pathogen increased abnormal seedlings. While diquat does have fungicidal properties, further work will be required to determine whether its use can improve carrot seed quality.