The onion (Allium cepa L.) is a lucrative and high-value vegetable–spice crop in India, but it is sensitive to several of diseases caused by fungi, bacteria, viruses, and nematodes, of which a fungal disease, anthracnose, caused by Colletotrichum spp., is a major issue for both onion producers and researchers since it severely affects the bulb production. Twister disease is currently one of the most common problems in onion production, particularly in humid regions, and it reduces productivity while also lowering the value and profitability. Twister disease is visualised by white or pale-yellow water-soaked oval depressed lesions on leaf blades, which are the first symptoms. Lesions expand as the disease advances, and numerous black-coloured, slightly elevated structures/fruiting bodies appear in the middle area, arranged in concentric rings. Curling, twisting, chlorosis of the leaves, and aberrant extension of the neck or pseudo-stem occurs, followed by rotting of the bulb. In an unmanaged crop, an excess gibberellin production by Colletotrichum gloeosporioides and Gibberella moniliformis is suspected to induce twisting and aberrant neck elongation, which will ruin onion productivity. It is difficult and environmentally unfriendly to control these infections. Since, to the best of our knowledge, this is the first review on onion anthracnose, we tried to consolidate information. This review updates our knowledge of the pathogen, including the disease cycle, infection pathways, and disease management techniques. As a result, growers will be benefit from the application of cultural, biological, and chemical measures and the use of resistant varieties.

The use of beneficial microbes is hitherto known and constantly increasing in agriculture due to their positive impact on crop growth and yield, and their minimal negative impact on the environment. The objective of this study was to evaluate the impact of eight Trichoderma strains of diverse origin on crop growth and yield of onion under field conditions. The identity of the strains used in the current study was confirmed by ITS and Tef1 gene sequencing. Field experiments were conducted in the Rabi season for 2 years (2020–21, and 2022–23) to evaluate the effect of the application of eight different Trichoderma strains that were applied individually and separately as eight different treatments (T1–T8) in experimental plots. In the plant growth promotion assay conducted in vitro, all strains showed the ability to produce IAA (indole-3-acetic acid), with levels ranging from 23.52 μg/mL (T6) to 45.54 μg/mL (T3). Our results revealed that Trichoderma treated experimental plots displayed better growth indices (plant height, pseudostem diameter), RWC (Relative water content), leaf chlorophyll content, and yield-attributing features like biomass (bulb and root dry mass), bulb diameter, and harvested bulb yield compared to the untreated control plants. In terms of yield, the T2 strain exhibited the highest bulb yield consistently for both the years (2020–21 and 2022–23) followed by T3 being statistically at par with T5. Among all the evaluated Trichoderma strains, the strain T2 (OGRDT2) and T3 (GRDT1), taxonomically identified as Trichoderma longibrachiatum, registered bulb yield of 32.24 t/ha and 30.76 t/ha, respectively while T5 (GRDT3), identified as Trichoderma asperellum, registered 30.55 t/ha average yield for 2 years compared to 24.08 t/ha average yield recorded for untreated control plants with an increase of 34, 28 and 27%, respectively. Based on our findings, it is concluded that the T. longibrachiatum strains OGRDT2 (T2) and GRDT1 (T3), T. asperellum strain GRDT3 (T5) are the best inducers of the onion crop growth and yield in the Rabi season and would be explored further for its commercial application in onion farming.

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.

In India, the productivity of onions during the kharif season (June-October) is quite low compared to the Rabi season (December-April). The yield losses in kharif onion are primarily attributed to the increased incidence of anthracnose-twister disease, which is often exacerbated by concurrent monsoon rains and high relative humidity during the kharif. To address this issue, a field study was conducted over three consecutive years (2021–2023) to assess the performance of eleven different Trichoderma strains in managing anthracnose-twister disease and improving plant growth and bulb yield in kharif onion. The strains were initially selected based on their ability to inhibit the growth of the anthracnose pathogen Colletotrichum gloeosporioides (in vitro confrontation assay) and HCN production; amylase, and protease production. The selected strains were also screened in vitro for PGPA traits (IAA and siderophore production; ability to grow in N₂ free media, PO42-, Zn, and K solubilization). The antagonistic activity of Trichoderma strains against C. gloeosporioides ranged from 18.8% to 70.0% with T. longibrachiatum (OGRDT2) recording the highest inhibition (70%). Application of GRDT2 and OGRDT2 reduced the PDI to 36.00 ± 0.98 and 37.06 ± 1.38, respectively, compared to 62.77 ± 0.14 in the untreated control. The T. longibrachiatum (OGRDT2) and T. harzianum (GRDT2) treated plots registered an increased plant height (54.97 ± 0.80 and 53.29 ± 0.79 cm) and pseudostem diameter (13.96 ± 0.18 and 13.76 ± 0.20mm) compared to the control (42.68 ± 1.90 and 10.55 ± 0.04 mm), respectively, under field conditions. Additionally, these treatments supported higher biomass, as reflected in the increased normalized dry weights of the shoot (10.82 ± 0.89% and 10.39 ± 1.76 %), root (12.27 ± 0.11 % and 11.86 ± 2.67 %), and bulb (12.90 ± 0.65 % and 12.21 ± 0.81 %). The chlorophyll contents were again higher (4.1 ± 1.44 and 4.1 ± 0.83 mg/ml) in OGRDT2 and GRDT2 treated plants than 3.3 ± 0.80 mg/ml in the untreated control. The GRDT2 and OGRDT2 treated plots supported higher bulb yield of 20.66 ± 1.42 t/ha and 19.21 ± 0.87 t/ha compared to control (16.52 ± 0.69 t/ha). Our study demonstrated that T. harzianum (GRDT2) and T. longibrachiatum (OGRDT2) effectively reduced anthracnose severity and therefore can be explored for management of Anthracnose-twister disease of onion.