Disease reaction of apple germplasm to white root rot (Dematophora necatrix)

Rooted suckers of 22 Malus species were screened for their reaction to white root rot (Dematophora necatrix) under artificially inoculated field conditions. Observation on the rate of disease development comprised of foliage chlorosis, wilting and defoliation was recorded in situ. The plants were uprooted from the sick plot 30 days after the inoculation and measurement for necrosis on stem bark, wood, vascular region and mycelial colonization on the infected parts were recorded. Results showed that none of the species exhibited immune reaction. However, some of the species like M. prunifolia var. ringoasami and M. purpurea could delay the expression of disease symptoms indicating the tolerance to disease. Malus baccata from Kashmir and Shillong restricted the disease development to some extent better than the other indigenous crab apples. These three Malus species were further evaluated at variable disease pressure. Marked differences with respect to disease progress curve and mortality rate was observed at lower disease pressure. Maximum tolerance was recorded in M. prunifolia var. ringoasami followed by M. purpurea at low disease pressure.