Genetic tools and strategies for citrus breeding aiming at resistant rootstocks to gummosis disease

Phytophthora nicotianae Breda de Haan (syn. Phytophthora parasitica Dastur), causal agent of citrus gummosis disease, has caused great damage to citrus orchards throughout the world. While chemical and horticultural measures do not guarantee the preventive or curative control of citrus gummosis, the use of resistant rootstocks is the most reliable management strategy against the disease. Aiming at the development of citrus rootstocks resistant to gummosis and to better elucidate the Phytophthora-citrus pathosystem, citrus breeding programs have been ongoing worldwide, mostly employing directed crosses. These studies have succeeded in identifying differences in symptom development between resistant and susceptible rootstocks, as well as in the progeny of their crosses. In addition, differentially expressed genes were assessed, which ultimately should lead to the identification of markers involved in resistance to P. nicotianae. In this review we summarize the current knowledge of the molecular basis of citrus gummosis and the main strategies employed to obtain genetically resistant rootstocks.