Colletotrichum is an important plant pathogenic fungi that causes anthracnose/-twister disease in onion. This disease was prevalent in the monsoon season from August to November months and the symptoms were observed in most of the fields. This study aimed to investigate the pathogenicity and cumulative effect, if any of Colletotrichum gloeosporioides and Fusarium acutatum. The pot experiment was laid out to identify the cause responsible for inciting anthracnose-twister disease, whether the Colletotrichum or Fusarium or both, or the interaction of pathogens and GA3. The results of the pathogenicity test confirmed that C. gloeosporioides and F. acutatum are both pathogenic. C. gloeosporioides caused twisting symptoms independently, while F.acutatum independently caused only neck elongation. The independent application of GA3 did not produce any symptoms, however, increased the plant height. The combined treatment of C. gloeosporioides and F. acutatum caused twisting, which enhanced upon interaction with GA3 application giving synergistic effect. The acervuli were found in lesions infected with C. gloeosporioides after 8 days of inoculation on the neck and leaf blades. Symptoms were not observed in untreated control plants. Koch's postulates were confirmed by reisolating the same pathogens from the infected plants.

Over the past decade, there have been accumulating reports from researchers, farmers, and field extension personnel on the increasing incidence and spread of onion basal rot in India. Onion basal rot disease is mainly caused by Fusarium spp. This study aimed to validate the information on the active prevalence of F. falciforme and F. acutatum causing Fusarium basal rot (FBR) in Maharashtra. A survey was conducted, and the infected plants/bulbs were collected from fields of 38 locations comprising five districts of Maharashtra, namely, Nashik, Aurangabad, Solapur, Ahmednagar, and Pune, in 2023. This disease was prevalent in high-moisture and high-oil-temperature conditions and the symptoms were observed in most of the fields, with the FBR incidence ranging from 17 to 41%. The available data of basal rot incidence from 1998 to 2022 were analyzed, based on which the prevalence of FBR was 11–50%. Tissue from the infected samples of onion bulbs was used for the isolation. The identification was performed based on colony morphology and microscopic features and confirmed through molecular markers using ITS and Tef-1α gene primers. Of the ten Fusarium isolates collected from selected locations, six species were confirmed as F. acutatum and four as F. falciforme. The pathogenicity tests performed with onion seedlings and bulbs under moist conditions proved that both F. acutatum and F. falciforme independently could cause basal rot disease symptoms but with different degrees of virulence. Koch’s postulates were confirmed by reisolating the same pathogens from the infected plants. Thus, the active prevalence of FBR was confirmed in Maharashtra and also, to the best of our knowledge, this is the first report of F. falciforme and F. acutatum causing basal rot of onion independently in Maharashtra, India.

Grapevine (Vitis vinifera L.), an economically important fruit crop. During field surveys in May 2022, an unusual decline in grapevine was observed in Narayan Tembhi and Wadner Bhairav villages of Nashik district of Maharashtra State in India. Four vineyards (n = ~3000 vines: 3.2 ha) showed 25-30% disease incidence with >70% disease severity (0-5 scale). Diseased vines showed wilting of cordons, and leaves, and "V"-shaped brown to black discoloration in xylem vessels. Ten diseased root samples (three replications) were collected, washed with running tap water for 10 min, cut into 5 × 5 mm pieces, dry blotted, surface sterilized, plated on a PDA medium amended with chloramphenicol (0.2 g/L) and incubated at 26 ± 2°C with a 12-h photoperiod. Hyphal-tip culture method was used to purify three isolates i.e., RI7, RI8, and RI9. Aerial mycelium, loose pinkish-white colonies with orange-white pigmentation. Morphological characteristics were consistent with Fusarium acutatum (Yilmaz et al. 2021). No other pathogens were recovered from roots. Microconidia (n = 100) were aseptate, single-celled (0 septa), oval to allantoid morphology with a size ranging from 5 - 12.5 x 2.5 - 5 μm. Macroconidia (n = 100) were falcate, 1 - 3 septate, with curved and tapering apical cells, with a size ranging from 16.25 - 35 x 2.9 - 4.5 μm. Chlamydospores (n = 50) were round, thick-walled, intercalary, observed in pairs, and single. Molecular characterization of three isolates was carried out based on inter-transcribed spacer (ITS1/ITS4: PQ560056, PQ560065) region, actin (ACT512F/ACT783R: PQ736334), β-tubulin (BT2A/BT2B: PQ736332, PQ736333), and second largest subunit of RNA polymerase II (fRPB2- 5F and fRPB2-7cR: PV232645, PV232646) genes (Carbone and Kohn 1999; Glass and Donaldson 1995; Liu et al., 1999; White et al. 1990). BLASTn sequence analysis revealed 97 - 100% similarity with type strains of Fusarium fujikuroi species complex (FFSC). Multilocus sequence typing (MLST) of three isolates in FUSARIUM-ID database (https://www.fusarium.org/) showed 98.04 to 99.75% identities to F. acutatum (Type strains: CBS 739.97, CBS 401.97, CBS 402.97, CBS 137545, CBS 131573, NRRL 13308). Multigene phylogenetic analyses of concatenated sequence datasets revealed that three isolates closely clustered with Fusarium acutatum based on neighbor-joining (NJ) method through MEGA11. Pathogenicity test (repeated twice) on grape seedlings revealed that about 90% of inoculated plantlets exhibited wilt symptoms (Akgul et al., 2024) except seedlings inoculated with PDA discs with no pathogen. Seedlings were then incubated at 25 ± 2°C at >85% RH under glasshouse. At 45 - 60 days post inoculation (dpi), seedlings exhibited leaf necrosis, shriveled, brown, discolored stems with weak and thin growth, rotten and decayed roots, compared to healthy control. Re-isolation of pathogen as F. acutatum from the roots of inoculated plants fulfilled Koch's postulates by achieving a 95 - 100% isolation rate. Previously, F. oxysporum and F. equiseti have been associated with vascular wilt disease of grapevine in Spain (Astudillo-Calderon et al. 2019). Recently, several Fusarium species have been known to cause various diseases in nurseries and vineyards (Akgul et al. 2024). This is the first evidence of Fusarium acutatum causing root rot and vascular wilt disease in grapevine in India and elsewhere.