Root rots of paradise apple

The primary causal agent of root and crown rot in apple trees is the oomycete Phytophthora cactorum, with Phytophthora syringae, Phytophthora cambivora, and Phytophthora citricola also playing a pathogenic role. Infection by these pathogens leads to the degradation of the root system, where roots darken, soften, and lose connection with the central cylinder.

Visually, the disease manifests as general tree suppression, reduced growth, and discoloration of the foliage. A diagnostic symptom is the necrosis of cambial tissue in the root collar zone, where the damaged area turns brown or orange and is separated from healthy tissue by a distinct line of demarcation.

Resistance to Phytophthora rots in clonal apple rootstocks is a quantitative trait assessed phenotypically in breeding programs. Specialized resistance genes to this complex of pathogens have not been identified in rootstock breeding practice, and the selection of planting material is based on susceptibility ratings of specific genotypes.