Effects of nitrogen fertility and crop rotation on onion growth and yield, thrips densities, Iris yellow spot virus and soil properties

• Onion fertility and crop rotation were manipulated.
• Increased nitrogen raised thrips populations and N leaching potential.
• Rotation with corn decreased thrips densities in one of two years.
• Soil microbial activity decreased with high nitrogen rates.
• Reduced nitrogen rate only slightly decreased bulb yield.

Onion thrips and Iris yellow spot virus (IYSV) are two primary yield reducing factors in onion production worldwide. Current management practices rely on heavy use of insecticides and fertilizers, threatening the sustainability of onion systems. Little is known about how cultural practices such as reduced fertility, soil biostimulants, and crop rotation affect onion yield, thrips densities, soil properties, and IYSV incidence. In a replicated field experiment, reduced nitrogen (N) (134 kg N ha−1, one-third the standard grower rate), slightly decreased yield and onion size. Adult thrips populations were 23 to 31% lower in the reduced as compared to standard N (402 kg N ha−1) and biostimulant treatment, respectively. Growing onions following a one year cycle in corn rather than wheat reduced onion thrips in one of two years. The addition of a biostimulant had no effect on soil properties, but may have slightly increased yield, attracted adult thrips, and increased thrips populations. IYSV incidence was not influenced by fertilizer rate or crop rotation. Soil microbial biomass and readily mineralizable carbon were greater following wheat, while soil nitrate (NO3−) accumulation was greater in standard N treatments. Soil microbial activity, as measured by dehydrogenase enzyme potential, may have been adversely affected by high N rates. Results suggest that reduced N, without biostimulant, sustained onion yields, decreased onion thrips densities and potential for IYSV incidence, created a more favorable soil environment for microbial activity, and reduced the risk of NO3− leaching