J U L Y / A U G U S T 2 0 1 9 26 POTATO GROWER Verticillium Wilt Resistance In NDSU Potato Genotypes As A Sustainable Management Strategy by Upasana Ghosh, Robert Sabba, Neil C. Gudmestad, and Susie Thompson The potato early dying complex is perhaps the most economically damaging problem for the potato industry in the US, considering yield loss and the cost of control (Gudmestad 2019). Potato early dying refers to the early senescence and death of the potato crop, and may be caused by several factors. Verticillium wilt (VW) is the major component of the early dying com- plex. It is caused by Verticillium dahliae Kleb. or Verticillium albo- atrum Reinke & Berthold. Both fun- gal species invade the host xylem tissue, causing vascular wilt by dis- rupting water transport. Wilt symp- toms do not usually appear until the latter part of the growing season, during the rapid tuber bulking peri- od, although fungal infection may have occurred early in the season (Johnson and Dung 2010). The first symptom is interveinal leaf chloro- sis on individual or groups of plants, scattered among healthy plants. Wilting and necrosis of the stem fol- lows and progress acropetally from the stem base. Eventually, the entire plant becomes necrotic, senescing prematurely, although it may remain upright. V. dahliae persists in the soil as microscleorotia for 2 to 3 years, without the presence of a suscepti- ble host (Johnson and Dung 2010). V. dahliae has an extensive host range and widespread distribution in temperate climates; the degree of host adaptation and the level of fun- gal infection differs significantly, depending upon host species and cultivar within a species (Chen 1994). V. albo-atrum has a more lim- ited host range; it does not form microsclerotia, and survives for a relatively shorter period in the soil as melanized hyphae (Chen 1994). Potato tuber yield loss due to VW commonly ranges from 10 to 15%, and may reach up to 30 to 50% (Johnson et al. 1986; Rowe and Powelson 2002). Synergistic interac- tions of V. dahliae and/or other pathogenic fungi, including V. albo- atrum and Colletotrichum coccodes, and the root-lesion nematode (Pratylenchus penetrans), have been observed to dramatically increase potato early dying incidence and yield reduction (Rowe and Powelson 2002). However, potato yield reduc- tion is often not correlated with aboveground symptoms (Johnson and Dung 2010). Soil fumigation with metam sodium (VAPAMĀ®) and producing VW resistant cultivars are the primary means of disease management; however, VAPAMĀ® is a highly toxic, costly, and restricted-use biocide (Pasche et al. 2013). The economic threshold for V. dahliae inoculum for susceptible cultivars is 8 to10 Verticillium propagules per gram of soil (vppg) (Nicot and Rouse 1987), although it is very common to have soil levels of more than 100 vppg in the northern plains potato growing region (Gudmestad 2019). The pri- marily soil-borne fungi can also be transmitted via certified seed tubers, but tuber-borne inoculum has negli- gible effect on VW symptoms or potato yields (Johnson and Dung 2010). Pasche et al. (2013) developed a method of quantifying V. dahliae colonization in potato stems using RT-PCR (real time reverse transcript polymerase chain reaction) tech- niques. Quantifying the fungal colo- nization in potato plant stems using traditional plating is labor intensive and time consuming. A strong cor- relation (r=0.98, P