Evaluation location: Wisconsin, United States
Leaves of each plant were mechanically inoculated at six points with a Paasche model H airbrush (Chicago) at 40 psi using carborundum power as an abrasive. The inoculum was made by extracting sap from leaves of infected tobacco plants (3 g of leaf per 100 ml of 0.1 M potassium phosphate buffer, pH 7.0). Plants were inoculated at the seven to eight-leaf stage (4 to 6 weeks after transplanting). Four weeks after inoculation, plants were visually evaluated for virus symptoms (leaf chlorosis, lower leaf necrosis, and leaf drop). Uninoculated leaves from asymptomatic plants were tested for the presence of PVY using a doubleantibody sandwich enzyme-linked immunosorbent assay (ELISA). Absorption at 405 nm was measured with an ELISA reader (Bio-TEK, ELx800). Absorbance values of uninoculated control samples were 0.00 to 0.01. PVY is not transmitted through true potato seed; therefore, all uninoculated seedlings were virus-free. ELISA absorbance values were 0.0 to 3.0. Plants were considered resistant if they exhibited scores <0.1 in the initial screen and after reinoculation. Plants were considered susceptible if they exhibited symptoms or if any leaf samples had ELISA scores ?0.1. Data from the ELISA tests were censored because 3.0 was the maximum ELISA absorbance value recorded. Because of this scoring system, a binary data set was analyzed. Plants with absorbance values consistently <0.1 were given a value of 0 (resistant) and all others were given a value of 1 (susceptible).