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Incidence and impact of Verticillium dahliae in dirt associated with certified potato seed lots.
J. K. DUNG (1), P. B. Hamm (2), J. E. Eggers (2), D. A. Johnson (1). (1) Department of Plant Pathology, Washington State University, Pullman, WA, U.S.A.; (2) Department of Botany and Plant Pathology, Hermiston Agricultural Research & Extension Center, Oregon State University, Hermiston, OR, U.S.A.

<i>Verticillium dahliae</i> (<i>Vd</i>), causal agent of Verticillium wilt of potato, can be found in the vascular system of infected seed tubers and has been found in dirt associated with certified potato seed. The purpose of this research was to quantify <i>Vd</i> in dirt associated with certified seed tubers and determine if this inoculum is related to the development of Verticillium wilt in the subsequent crop. <i>Verticillium dahliae</i> was recovered from 68% of dirt collected from the surfaces of seed tubers between 2009 and 2011. Over 82% of dirt samples collected from trucks and bags used to transport seed tubers contained <i>Vd</i>. Most dirt from seed lots exhibited <i>Vd</i> levels between 5 and 50 CFU/g but <i>Vd</i> levels between 50 and 500 CFU/g were observed all three years. Over 93% of isolates recovered belonged to vegetative compatibility group 4A. Levels of <i>Vd</i> in stem sap increased with increasing levels in dirt from seed tubers but only when levels of the pathogen in field plot soils were low (<i>P</i><0.01). Postharvest levels of <i>Vd</i> increased in all field plots compared to preplant levels and greater postharvest levels of <i>Vd</i> in the field soils were related to increased levels of <i>Vd</i> in dirt on seed tubers (<i>P</i>=0.04). The transport of infested soil with seed tubers provides a means to introduce <i>Vd</i> into fields not previously cropped to potato or have received preplant management practices such as soil fumigation and, once established, increase after the cropping of susceptible hosts.<p><p>Keywords: Fungus, Root-Tuber Crops, Potato

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