Molecular characterization of curvularia leaf blight disease of olive and determination of pathogenicity by a novel disease rating scale

During the last decade, olive (Olea europaea) production has significantly increased in Pakistan due to oil producing ability and medicine importance. Arbequina, Arbosana, BARI Zaitoon-1 and BARI Zaitoon-2 has recently introduced in Sukhan, Uthal Balochistan. During 2023-24, symptoms of Curvularia leaf blight disease (CLBD) first appeared, with a 97% disease incidence. Subsequently, the infection progressed, eventually covering the entire leaf and olive plant. For the first time, a novel disease rating scale for CLBD of olive was accomplished. White fungal colonies (n=89) were recorded which turned into brownish black. Virulence category of isolates were characterized as moderately (43), slightly (37) and highly (9) virulent with 37.5% disease severity. Under microscope, straight to slightly curved conidia with 3 to 4 septation were recorded in highly virulent isolates and range (n=3) of conidial and conidiophores length and width was observed as 23±2.1 µm to 30±2.4 µm and 10.4±0.8 µm to 12.3 µm ±1.2 µm and 99.6±20.4 µm to 130.6±25.5 µm and 4.2±0.5 µm to 4.8±0.7 µm, respectively. The internal transcribed spacer (ITS) regions were amplified through polymerase chain reaction (PCR). Sequences from highly virulent isolates (n=9) were deposited to NCBI with GenBank Accession numbers PP565050 to PP565058. The nucleotide sequences of local isolates were compared with sequences from all available isolates of different Curvularia species (n=12) known to cause blight disease across several host (n=26) species in Pakistan. The obtained ITS sequences as a causal agent of CLBD of olive were exhibiting 98 to 100% and 99 to 100% genetic similarity with previously reported isolates of C. lunata (n=5) and C. geniculate (n= 4). According to our knowledge, it is first comprehensive study of CLBD of olive caused by C. lunata and C. geniculate in Balochistan as well as in Pakistan. This emerging disease has serious threat to olive production in the province as well as in the country and precaution measures are required to manage this distractive disease.