Morpho-cultural variability, pathogenicity and molecular identification of Phytophthora cajani isolates causing blight disease of pigeonpea in India

Phytophthora blight (PB) caused by Phytophthora cajani (Pc), is one of the most devastating diseases of pigeonpea. This study investigated the morphological, cultural, and pathogenic variability of 30 Pc isolates collected from diverse geographical regions of India. Additionally, pigeonpea genotypes were screened for resistance against Pc over five consecutive years (2017–2022) to identify stable resistant sources for future genetic improvement. Based on colony colour and texture, the Pc isolates were categorized into five groups: cottony-white fluffy (18), cottony-white mat-type (3), creamy-white fluffy (2), creamy-white mat-type (6), and light pink fluffy (1). Three isolates (Pc1, Pc24, Pc26) exhibited fast sporulation, with radial growth exceeding 80 mm. Regarding sporangial morphology, the isolates were classified into 3 groups, i.e., papillate (14 isolates), semi-papillate (11 isolates), and non-papillate (5 isolates). Among these, isolate Pc17 exhibited the largest sporangia (22.6 μm × 14.2 μm). Of the 30 isolates screened in vivo, 16 were classified as highly pathogenic, causing >50 % disease mortality, with six isolates (Pc12, Pc14, Pc16, Pc18, Pc21, and Pc24) being highly aggressive causing >90 % disease mortality. The pathogenicity of all Pc isolates was confirmed and highly pathogenic isolates were further characterized using ITS primers. Among the 30 pigeonpea genotypes tested, 13 exhibited moderate resistance over five years. Genotypes IPAC-3 and IPAPB-7-2-1-7 showed the lowest mean disease mortality (9–10 %), while IPAC-2 demonstrated the most stable performance, with a mean disease mortality of 16.2 %. Our study highlights the significant differences in morphological, cultural, and pathogenic features of Pc isolates. The stable resistant genotypes identified here provide promising avenues for developing durable resistance against PB in pigeonpea.