Technique for screening of apple and pear germplasm against white root rot (Dematophora necatrix).

A screening technique for the identification of resistant germplasm of apple (Malus spp.) and pear (Pyrus spp.) to white root rot (Dematophora necatrix Hartig) was standardized under field conditions. Rooted suckers were planted in the field during late dormancy stage (February). These were challenge inoculated when plants had established in the field and attained new growth (during the month of July) by introducing fungus inoculum multiplied on wheat grain in the rhizosphere (50 grains/ sucker). Soil moisture was maintained at the field capacity. Different species exhibited differential reaction to disease development parameters viz., mycelial colonization of the infected plant parts, rate of wilting, defoliation, necrosis on the bark, wood and vascular region. Necrosis on the leaves was observed as first symptom of disease expression which took 10 days after the inoculation in susceptible species whereas incubation period was extended in resistant species. Most of the population of different susceptible host species collapsed within 30 days of inoculation. The mortality rate varied with species. The technique evolved is easy, quick, reliable and capable of creating required disease pressure under field conditions for screening rootstocks against white root rot disease.