Wilt and gummosis disease of subabul caused by Fusarium equiseti ‐ a first record from India

Subabul (Leucaena leucocephala) is a perennial, fast-growing, non-climbing tree originating from tropical America. The species is economically important for the paper and pulp industry supplying 35–40% wood required for these industries in India (Orwa et al., 2009; Global Invasive Species Database, 2023). In March 2022, more than 30% of three-year-old subabul trees exhibited wilting and oozing symptoms in many farmers’ plantations in Khammam, Telangana, India. The affected plants initially showed yellowing of the leaves and wilting symptoms (Figures 1a-b). Symptoms of gummosis were also observed subsequently (Figures 1c-e). Cross-sections of affected roots in nursery-grown trees revealed brownish discolouration and rotting. Diseased roots and bark sections were surface-sterilised in 75% ethanol for 15 seconds, followed by sodium hypochlorite (1%) for 45 seconds and rinsed three times in distilled water. After drying, samples were placed onto potato dextrose agar (PDA) plates. Six isolates (FS1-FS6) were selected from Fusarium-like colonies obtained from the diseased tissue and pure cultures were obtained by single spore isolation. All the isolates produced off-white colonies (Figure 2a) on PDA after five to seven days incubation at 28°C. Pinkish to reddish pigmentation appeared on the bottom side of seven to ten-day-old cultures (Figure 2b). Macroconidia, microconidia and chlamydospores (Figure 3) were produced on carnation leaf agar after five to seven days incubation at 28°C. Microconidia were septate, hyaline, elongated ovoid or reniform, with a mean size of 7.23 × 3.41 µm (n = 25). Macroconidia were hyaline, crescent or curved, slightly tapered at the apex with three-five septa and measured 32.1 × 5.3 µm (mean, n = 25). Chlamydospores were light brownish, globose or round with a mean diameter of 4.3 µm (n = 25) and were produced either singly, in clusters on conidium or intercalary. The rDNA-ITS and RPB2 genes of FS1, FS2 and FS3 were amplified with ITS1/ITS4 (White et al., 1990) and RPB2F/RPB2R primer pairs (O'Donnell et al., 2013) and sequenced (GenBank Accession Nos. OR272193-OR272195 and OR582636-OR582638 respectively). The sequences of FS1, FS2 and FS3 isolates were identical and the isolates grouped with other isolates of F. equiseti in phylogenetic trees based on the internal transcribed spacer region and RNA polymerase II subunit 2 gene (Figure 4). A pathogenicity test was done on healthy six-month-old subabul clones grown in sterile soil in pots (50 cm diameter, 35 cm height). Plants were inoculated separately with 100 mL of a spore suspension of isolates FS1, FS2 or FS3 (106 conidia/mL). Sterile distilled water was used for the controls (n = 3). Assays were repeated three times in a glasshouse kept at 28°C. Twelve days post-inoculation, clones treated with each of the Fusarium isolates exhibited yellowing and wilting symptoms similar to those observed in the field, but control pots remained asymptomatic (Figure 5). Fusarium equiseti was re-isolated from the inoculated plants. To our knowledge, this is the first report of Fusarium equiseti causing wilt and gummosis disease in subabul in India. Its rapid spread poses a threat to subabul plantations and warrants development of appropriate management strategies.