First Report of Geotrichum candidum Causing Rubbery Rot on Potato in Arizona

In January 2023, stand loss due to seed decay (<3% incidence) was reported in a commercial seed potato field in Cochise County, Arizona. Several rotted 'Ciklamen' tubers with wilting shoots were submitted to the University of Arizona Extension Plant Pathology Laboratory for diagnosis. External symptoms included sunken black-colored lesions on affected tubers; white fungal-like growth was also present on the surface of the more severely affected tubers. Internal tissues were watery, rubbery-textured, and spongy with slight gray discoloration. Isolation from the necrotic tubers on PDA yielded yeast-like fungal colonies. Two derived fungal isolates were subcultured on PDA at room temperature for three days, and colonies had a white, dense, and creamy appearance. Hyphae were hyaline, septate, dichotomously branched, and broken into chains of cylindrical to oval-shaped, single-celled arthrospores ranging in size from 3.8-5.3 μm × 5.4-12.5 μm (n = 20). These morphological characteristics matched those of Geotrichum candidum (Carmichael, 1957). Other common potato pathogens, including Pythium, Phytophthora, and Fusarium, were not detected in the samples. For molecular identification, genomic DNA was extracted from the mycelia of two isolates using the DNeasy Plant Pro Kit (Qiagen Inc., Valencia, CA) according to the manufacturer's instructions. Partial DNA sequences of the internal transcribed spacer (ITS) region of rDNA and elongation factor 1-alpha (EF-1α) gene were amplified and sequenced using primers ITS1/ITS4 (White et al., 1990), EF1-728F/EF1-1567R (Carbone and Kohn 1999) and EF1-1018/EF1-1620R (Stielow et al., 2015). The resulting sequences showed no difference between the isolates and were deposited under the GenBank Accession Numbers OQ690039 and OQ981192. Based on BLASTn analysis, ITS sequence OQ690039 (369-bp) had a 99.46% match with Geotrichum candidum accessions MN861070 and KY103456. EF-1α sequence OQ981192 (843-bp) showed 100% identity with G. candidum accessions MT346368 of the G. candidum isolates responsible for sour rot of strawberries in Florida (Alonzo et al., 2020). A pathogenicity test was conducted twice on ten healthy 'Ciklamen' tubers, surface sterilized using 10% sodium hypochlorite solution, and rinsed copiously in sterilized water. A conidial suspension (106 conidia/ml) of the isolated fungus was sprayed onto the surface of the tubers. The control consisted of ten tubers that were mock-sprayed with sterilized water. All tubers were incubated in separate sterilized plastic bags in the dark at room temperature for four weeks and examined daily for symptom development. Symptoms of rubbery texture and fluid leaking when cut, and a distinctive smell, were produced on the inoculated tubers. In contrast, the controls did not show any symptoms. These symptoms were similar to those observed on naturally infected tubers. The fungus was re-isolated from symptomatic tissues of inoculated tubers and identified by PCR as G. candidum, thus fulfilling Koch's postulates. To our knowledge, this is the first report of G. candidum as a potato pathogen in Arizona. Rubbery rot of potatoes by G. candidum has been reported previously in Idaho (Duellman et al., 2020) and Michigan (Willbur et al., 2021). Arizona annually grows approximately 1,000 hectares of seed potatoes shipped to various states. Therefore, growers must obtain a correct diagnosis to avoid spreading the disease despite the low incidence of disease.