Etiology of Septoria Leaf Spot of Pistachio in Southern Spain.

Septoria leaf spot (SLS) is the most prevalent disease of pistachio (Pistacia vera L.) in Spain. To elucidate its etiology, 22 samples of pistachio leaves showing SLS symptoms were collected mainly from 1993 to 2018 across southern Spain. Affected leaves from terebinth (P. terebinthus) were also collected for comparative purposes. Six Septoria-like isolates were recovered from pistachio leaves. They were identified as Septoria pistaciarum by sequencing ITS, RPB2 and LSU genes. The phenotypic characteristics of conidia and colonies were evaluated, confirming the identity of S. pistaciarum. Conidia were solitary, hyaline, and straight to curved. Large differences in length were observed between conidia from leaf samples, with those from terebinth being slightly larger than those from pistachio. Colonies showed slow mycelial growth on PDA. The effect of temperature on conidial germination and mycelial growth was evaluated in vitro on PDA. For both characters, the optimum temperature was approximately 19-20°C. Eight culture media were tested, with oatmeal agar (OA) and Spezieller Nahrstoffarmer agar (SNA) showing the highest mycelial growth and pistachio leaf agar (PLA) showing the highest sporulation. A specific culture medium integrating lyophilized-powdered pistachio leaves into diluted PDA improved sporulation in comparison with PLA. Pathogenicity tests were conducted by inoculating detached and in planta pistachio and terebinth leaflets with conidial suspensions. Typical symptoms of SLS and cirri of S. pistaciarum developed at 10 and 21 days after inoculation, respectively, in both hosts. This is the first report of S. pistaciarum causing SLS in pistachio and terebinth in Spain.