Resistance of Selected Malus Germplasm to Rosellinia Necatrix

Abstract

Isolates of Rosellinia necatrixfrom 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 U.S. 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. necatrixof Malus sieversiiseedlings 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. sieversiiseed lot, GMAL 3675 [Plant Introduction (PI) 6005464], showed resistance to the Korean isolate. Among 159 clones of Malusgermplasm in the apple core collection, 32 clones gave seedlings which had slow development of R. necatrixinfection 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. X domesticaE11-24 (PI 589571), M. X domesticaE31-10 (PI 590072), M. X domesticaRedspur Delicious (PI 589255), M. X domesticaSmith Jonathan (PI 589845), M. florentina“Skopje P2 (PI 589385), M. micromalus(PI 594096), M. prunifoliaNaga (PI 589930), and M. soulardii(PI 589391) showed necrotic symptoms of infection, indicating that the parent clones may have some resistance to R. necatrix.