A root inoculation technique to screen potato seedlings for resistance toPseudomonas solanacearum

A seedling inoculation technique for screening potato progenies for resistance toPseudomonas solanacearum was devised, and the effect of certain treatment variables on efficiency of the technique was determined. Concentration of inoculum and age of seedlings were critical factors; with very young seedlings (16 days old) or low concentration of inoculum (10⁵ cells/ml for isolate K-60) many seedlings escaped infection. With older seedlings (25 days old) and high concentrations of inoculum (10⁹ cells/ml), very few or none of the seedlings survived. Inocula containing mixtures of the most virulent isolates, such as S-207, S-213, and S-123, even at relatively low populations (ca. 10⁵ cells/ml), killed most of the susceptible seedlings. In the standard procedure, true seed was sown in rows 3.2 cm apart in 50 x 35 cm flats and seedlings were grown for 20 days in a growth room at 22 C and a 14-hr photoperiod. For inoculation, 5 liters of a cell suspension of two pathogenic isolates ofP. solanacearum (S-207 and S-213, or S-207 and S-123) were added to each flat, and roots were cut immediately with a knife run between and across the rows of seedlings. Inoculated seedlings were incubated in a growth room at 28 C for 2 weeks, and survivors then were moved to the greenhouse for propagation. Stem inoculation of 20 clones derived from plants that survived the standard procedures showed that 16 of them had clear-cut resistance to the bacterium. Tests with 39 hybrid families from crosses of resistant clones with susceptible Mexican and Colombian cultivars showed a relatively high survival rate in progeny from crosses involving three specific resistant parents.