First Report of Agroathelia rolfsii Causing Southern Blight of Alocasia in Florida.

Abstract

Alocasia 'Calidora' (NCSU, 2025), known as upright elephant ear, is an ornamental plant grown for its large, glossy leaves and architectural value. In September 2023, diseased Alocasia 'Calidora' plants were submitted by a commercial grower from Highlands County, FL, to the Diagnostic Clinic at the University of Florida's Gulf Coast Research and Education Center. The grower reported about 10% disease incidence in a field of about 3 ha with an estimated 30,000 plants. The plants, initially propagated from tissue-cultured starters and established in the greenhouse, were transplanted to the field. About three months after planting, symptoms including stunted growth, yellowing leaves, and bulb rot were observed, which were accompanied by dense white mycelia and sclerotia. Symptomatic tissues excised from infected bulbs were disinfested in a 10% bleach solution for 90s, rinsed twice with sterile water, and plated on a general isolation medium (Amiri et al. 2018). Plates were kept at 25°C and a 12-h photoperiod. A fungus producing white mycelia with round sclerotia which resembled those of Agroathelia rolfsii (Redhead and Mullineux, 2023) was consistently isolated in 28 out of 30 isolations . Three isolates were obtained by hyphal-tip of isolations from different tissue pieces. For morphological characterization, the same medium and conditions described above were used, 6 plates per isolate, and experiments were repeated once. Colonies had rapid growth, with an average daily radial growth of 7.2 ± 0.28 mm, covering the 100-mm diameter plate within 5 days. Abundant sclerotia (most spherical) were formed, initially white maturing to tan/dark brown. Mature sclerotia measured 1.4 ± 0.1 mm in diameter (n = 50). Ten-day cultures produced 556 to 2,032 sclerotia/plate (aver. = 1,379 ± 105). Clamp connections were observed. The internal transcribed spacer region (ITS; primers ITS1/ITS4, White et al. 1990), large subunit ribosomal ribonucleic acid (LSU; primers LROR/LR5, Vilgalys and Hester, 1990), and the alpha subunit of the elongation factor (EF-1α; primers EF595F/EF1160R, Kauserud and Schumacher, 2001) were sequenced and deposited in GenBank [accession nos.: PQ436344 to PQ436346 (ITS), PQ428904 to PQ428906 (LSU), PQ441996 to PQ441998 (EF-1α)]. BLASTn searches revealed isolates were 100% identical to an isolate of A. rolfsii (AFTOL-ID 664 = CBS 745.84; Paul et al., 2017) in ITS (DQ484062, 596/596 nt), LSU (AY635773, 1431/1431 nt), and EF-1α (GU187681, 1068/1068 nt) sequences. One isolate was selected to carry out pathogenicity assays in the greenhouse (aver. temperature = 26℃; ranging from 22 ̶ 28℃). Alocasia 'Calidora' plants (5 per inoculated and non-inoculated control treatments) were transplanted into 1-liter pots with 100 sclerotia from 10-day cultures placed on the surface of the potting soil. Control plants were not inoculated. Plants were drip irrigated three times a day (10 seconds per cycle). The trial was repeated once. At 12 weeks, ~80% of inoculated plants were symptomatic, with symptoms resembling those in the field. The same fungus was re-isolated on a general isolation medium from inoculated diseased plants using the same methods mentioned above. The non-inoculated controls remained healthy. To our knowledge, this is the first report of A. rolfsii causing disease on Alocasia in Florida and elsewhere. A. rolfsii is the pathogen responsible for southern blight, also referred to as gray rot, and has been reported in over 260 plant genera (Farr et al. 2024). It also affects caladium (Caladium × hortulanum), an ornamental previously cultivated in the same field by the grower who submitted the Alocasia sample. Therefore, cross-contamination between the two crops is a possibility. Until now, A. rolfsii has been a minor concern for Alocasia production. However, monitoring this pathogen is crucial because its sclerotia can persist in the soil for many years. The trade of ornamentals and nursery plants regionally and internationally is common. Frequent movement of plants potentially can introduce this pathogen, which has the potential to become invasive in new areas.