Resistance of selected Malus germplasm to Rosellinia necatrix.
Isolates of Rosellinia necatrix from Korea, Japan, and the United States were used to inoculate 1-year-old seedlings from open pollinated McIntosh and 4-year-old M.9 apple rootstock plants to test the isolates pathogenicity. The Korean and US isolates were more virulent to both plant materials than the Japanese isolate. Three different methods, inserting infected sticks in pots, mixing infested soil into pots, and transplanting seedlings into infested soil, were evaluated for assaying the resistance to R. necatrix of Malus sieversii seedlings grown from seed collected in the wild in Kazakhstan. The second method gave more equal inoculum pressure and was selected for further tests. The M. sieversii seed lot, GMAL 3675 [Plant Introduction (PI) 6005464], showed resistance to the Korean isolate. Among 159 clones of Malus germplasm in the apple core collection, 32 clones gave seedlings which had slow development of R. necatrix infection or had no necrotic symptoms. In a second test of this germplasm with two-fold higher inoculum level, among the selected 32 clones, few seedlings of M. domestica E11-24 (PI 589571), M. domestica E31-10 (PI 590072), M. domestica Redspur Delicious (PI 589255), M. domestica Smith Jonathan (PI 589845), M. florentina Skopje P2 (PI 589385), M. micromalus (PI 594096), M. prunifolia Naga (PI 589930), and M. soulardii (PI 589391) showed necrotic symptoms of infection, indicating that the parent clones may have some resistance to R. necatrix.