Variability in pathogenicity among Phytophthora spp. isolated from citrus in Corsica

Gummosis, caused by several Phytophthora species, is one of the most serious constraints to citrus production in Corsica. The current Phytophthora population on citrus trees is mainly composed of P. citrophthora divided into two major groups, namely G1 and G2, with two additional minor groups. Following changes in cultural practices, favourable climatic conditions, and introduction of new hosts, repeated gummosis outbreaks have been observed. The pathogenicity of several Phytophthora isolates of each group collected in citrus groves was assessed for gummosis disease on 20 citrus scion and rootstock cultivars. All isolates tested proved virulent on citrus but extensive pathogenicity differences were observed among and within Phytophthora groups. The data indicate that G1 isolates of P. citrophthora are pathogenic to resistant rootstocks such as Poncirus trifoliata or Carrizo citrange, and moderately aggressive towards scions. In contrast, isolates of the group G2 tend to display a greater aggressiveness to a number of scion cultivars, but are not virulent to P. trifoliata and its hybrids. Implications of these data on disease management are discussed.