Anther culture as a probable source of resistance to tobacco black shank caused by Phytophthora parasitica var 'nicotianae'

Anther-derived doubled haploid (ADH) tobacco lines possessing a high level of resistance to tobacco black shank, Phytophthora parasitica (Dast.) var 'nicotianae' (b. de Haan) Tucker (Ppn), have been identified from a cross of two tobacco cultivars susceptible to this disease. The objective of this study was to investigate the origin of black shank resistance in ADH lines developed from two susceptible parental cultivars: 'Ovens 62' and 'Ky 15'. In addition to the ADH lines, sexually-derived F(2:8) lines were produced using the single seed descent (SSD) method. Seventy-five ADH lines and 75 SSD lines along with two black shank resistant and three susceptible controls (including parental cultivars) were evaluated under field conditions for resistance to Ppn in 1989 and 1990. Lines were assigned at random to five sets and were planted in a randomized complete block design with three replications/set. A disease index was computed from weekly stand counts of the number of surviving plants per plot for each tobacco line. The 1989 experiment revealed resistance to Ppn in 8 ADH lines, numbers 29, 31, 32, 40, 68, 69, 74, 76, and 1 SSD line, number 32. The 8 ADH lines continued to show resistance in 1990, but SSD line number 32 exhibited a susceptible reaction. There were no other resistant SSD lines. A second field experiment was conducted in 1990 using the putative resistant ADH lines and SSD line nr. 32, which expressed resistance to black shank in 1989. In addition, 12 randomly selected lines from the original 150 ADH and SSD lines were evaluated along with the same five controls as in the previous experiment to further substantiate the resistance of the ADH lines. Lines were planted in a randomized complete block design with eight replications. ADH lines continued to express resistance. SSD line nr. 32 did not show any resistance nor did any other line derived through the single seed descent procedure. These results support the hypothesis that anther culture can generate a high level of black shank resistance. The basis for this resistance is postulated to be due to gene amplification of factors that regulate host response to the black shank fungus.