Evaluation of ManKocide Alternatives for Bacterial Blight Control in Steckling-to-Seed Carrot Seed Crops

Series/Report Number: COARC2016

Abstract: Management of bacterial blight in carrot seed crops can be difficult and begins with the planting of healthy or treated seed. However, planting healthy or treated seed may not prevent the disease in biennial seed production fields because new plantings are often located adjacent to or in close proximity to the previous years’ plantings. The pathogen, Xanthomonas hortorum pv. carotae (Xhc), is readily disseminated by contaminated irrigation water, wind-blown rain, insects, soil or carrot refuse and newly emerged seedlings of the next biennial season can become infected from nearby fields of the previous biennial season that already harbor Xhc. The disease has even been observed in isolated plantings, suggesting long distance dissemination of the pathogen on aerosolized seed crop residues or introduction of the pathogen on seedborne inoculum. In addition to infested seed, infected carrot stecklings may be a source of inoculum for carrot seed producers. A previous study detected Xhc in 4 of 12 steckling crops that were sampled directly from shipping crates (du Toit et al. 2005). The use of disease-free stecklings is an important component of an integrated disease management program to reduce the impact of bacterial blight on harvested seed. However, there is a lack of effective control options for infected stecklings. Carrot seed producers would like to reduce Xhc populations on harvested seed in order to minimize the need for hot water treatment and lessen the impact of bacterial blight on subsequent root crops in California, Washington, and other carrot-producing states and countries. Copper-based bactericides such as ManKocide (mancozeb + copper hydroxide) are applied multiple times each season to manage bacterial blight and increase seed quality, and are currently a primary control measure for bacterial blight in carrot seed crops. However, copper-based bactericides are most effective when used as preventative treatments and have limited ability to reduce Xhc populations once the pathogen becomes established in a seed crop (du Toit and Derie 2008).

Chapter Number: 13