Phytophthora nicotianae and P. cryptogea causing gummosis of citrus crops in Tunisia

Surveys were conducted in 2013/2014 across seven Tunisian citrus production areas for the identification of pathogens associated with citrus gummosis. Samples were analyzed using PARP-BH selective media. Collected isolates were identified based on morphology and DNA sequences of the internal transcribed spacer (ITS) region analysis. Ten isolates of P. nicotianae and five isolates of P. cryptogea were obtained. In vitro inoculation of five citrus varieties (Clementine Hernandina, Mandarins Tangerine, Blood Orange Maltese, Valencia Late and Thomson Navel) showed that P. nicotianae were pathogenic to cuttings twigs and P. cryptogea were pathogenic to leaves. In the case of P. nicotianae, the most susceptible variety was C. hernandina, while the most tolerant variety was M. tangerine. For P. cryptogea the most susceptible variety was C. Hernandina, while the most tolerant variety was V. late. In greenhouse assays, inoculation of trunks of C. Hernandina with P. nicotianae and P. cryptogea inocula induced the appearance of necrotic area and gum-exudation when inoculated with P. nicotianae. However, inoculation of rootstocks Sour orange with the same isolates induced root necrosis and leaf wilt. This is the first report of P. cryptogea causing gummosis in citrus orchards in Tunisia.