First report of Geotrichum candidum causing rubbery rot of potato (Solanum tuberosum) in Michigan.

ABSTRACT

In April 2018, rotted 'Lamoka' tubers were received from a commercial storage facility (<1% incidence) in St. Joseph County, Michigan by the MSU Potato & Sugar Beet Pathology and Plant & Pest Diagnostics programs. Dense circular colonies of white fungal-like growth were observed on the surface of the tubers, and internal tissues were watery and spongy with gray to brown discoloration (Supplemental Figure 1). Tubers had a strong, sweet alcoholic odor. External and internal tuber tissues were surface disinfested in 0.825% sodium hypochlorite for 1 min, rinsed twice in sterile distilled water, blotted on sterile filter paper, and placed onto 1.5% (w/v) water agar (WA). After 3 days at 21-24°C and ambient light conditions, septate, branched mycelia and hyaline, cylindrical, single-celled conidia 5.2-8.9 µm x 3.6-5.2 µm (n=20 arthrospores) were observed singly or in chains (Supplemental Figure 2A&B). On potato dextrose agar (PDA), colonies were white, circular, and dense (Supplemental Figure 2C). These observations matched morphological descriptions of Geotrichum candidum (Carmichael 1957). No Pythium or Phytophthora spp. were detected. A mono-conidial isolate of the fungus was obtained and maintained on PDA. DNA was extracted from mycelia using a DNeasy plant mini kit (QIAGEN). Fragments of the internal transcribed spacer (ITS) and 18S ribosomal RNA gene regions were amplified using primers ITS1F/4 primers and NS3/8, respectively (White et al. 1990). Purified PCR products (QIAquick PCR purification kit, QIAGEN) were submitted for Sanger sequencing at the Genomics Research Technology Support Facility (East Lansing, MI). The ITS1F/4 and NS3/8 consensus sequences (OP142324 and OP153873) aligned with GenBank accessions of G. candidum KY103456.1 (100% identity) and JF262193.1 (99.75% identity), respectively. Healthy 'Lamoka' tubers were rinsed with tap water, surface disinfested in 0.825% sodium hypochlorite for 15 min, rinsed twice in sterile distilled water, and blotted dry on sterile paper towel. Ten tubers were inoculated by placing 10-mm diameter fully colonized agar plugs, excised from the margin of a 9-day-old PDA culture, onto the surface of each tuber (Duellman et al. 2021). Ten tubers were mock-inoculated using sterile PDA. Tubers were placed in a moist chamber and incubated in the dark at room temperature. After nine days, inoculated tubers exhibited white colony growth on tuber surfaces and an alcoholic scent was present. After 27 days, internal tissues were rubbery, but no discoloration was observed. No rubbery rot symptoms were observed on the control tubers. Samples were excised 1 cm laterally from and vertically beneath the inoculation site. Tissues were surface disinfested as described above and plated on 1.5% WA. After 9 days, a Geotrichum sp. identical to the original isolate was confirmed in 50% of samples from inoculated tubers. No Geotrichum sp. were detected from mock-inoculated tubers. Since 2018, G. candidum has been confirmed in commercial storages in three counties in the Lower Peninsula (incidences up to 1-2%). Geotrichum candidum was recently reported causing rubbery rot of potato in Idaho (Duellman et al. 2021); however, to our knowledge this is the first report of rubbery rot in Michigan. Despite increasing detection frequencies, incidences remain low and spread in storage appears limited. Seed decay leading to stand loss (incidence 1-3%) was observed after planting infected lots, which should be avoided or minimized.