Root Regeneration and Tolerance of Citrus Rootstocks to Root Rot Caused byPhytophthora nicotianae

Abstract

Graham, J . H. 1995. Root regeneration and tolerance of citrus rootstocks to root rot caused by Phy1opl11hora nicotianae. Phytopathology 85: 111117. Citrus rootstock cultivars varying in tolerance to fibrous root rot were evaluated for their ability to regenerate roots in the presence of potentially damaging populations of Phytophthora nicotianae. In chlamydospore­ infested soils in the greenhouse, tolerance to root rot generally was exhibited as more rapid growth of undisturbed and pruned roots of the rootstocks trifoliate orange and Swingle citrumelo compared with Carrizo citrange, sour orange, Ridge Pineapple sweet orange, and Cleopatra man­ darin. The capacity for regeneration of Volkamer lemon roots in the presence of P. nicotianae varied with experiments and was associated with differences in greenhouse temperature conditions. In a field trial with damaging populations of P. nicotianae, growth rates of regenerating Phytophthora nicotianae Breda de Haan (syn. P. parasitica Dastur.) infects the root cortex and causes a decay of fibrous roots of all commercial citrus rootstocks in Florida (2, I0 , 11). Root rot can be especially severe in infested soils of citrus nurseries (2 1). In orchards, damage of fibrous roots causes tree decljne and yield losses (14,15, 17). With mature trees, the production of new fibrous roots apparently does not keep pace with root death, and the tree is unable to maintain adequate water and mineral uptake to sustain maximum fruit production (14). Rootstocks are referred to as tolerant rather than resistant because fibrous roots become infected under artificial inoculations (5,6,10) and in infested nursery and orchard soils (2). Tolerance was previously defined as the condition in which plants are infected but show little or no net root loss either because infected roots do not rot or because root mass density is maintained by root regeneration ( 10). Recent greenhouse evaluations with chlamydo­ spore inoculum in soil revealed that most commercial rootstocks in Florida were intolerant to root rot caused by P. nicotianae ( 10). In contrast, trifoliate orange (Poncirus trifoliata (L.) Raf.) and its hybrid, Swingle citrumelo (Citrus paradisi Macf. X P. trifoliata), were considered tolerant. Volkamer lemon (C. volkameriana Pasq.) was judged as intermediate in tolerance. In greenhouse inoculations, tolerance of trifoliate orange and Swingle citrurnelo appeared to be related to the capabiUty of these rootstocks to regenerate roots in the presence of P. nicotianae (10). In Florida rootstock trials, trifoliate orange and Swingle citru­ melo supported lower soil populations of P. nicotianae, whereas intolerant rootstocks had higher population densities in the soil of the root zone (2), here defined as rhizosphere populations.