First report of rubbery rot of potato caused by Geotrichum candidum in the United States.

Rubbery rot of potato caused by Geotrichum candidum Link is characterized by symptoms of damp, flaccid tubers that feel rubbery when squeezed (Humpreys-Jones 1969), similar in consistency to potato diseases such as pink rot (caused by Phytophthora erythroseptica) and Pythium leak (caused by species of Pythium). In November 2019, several symptomatic tubers of potato variety 'Ciklamen' that had been held in storage since harvest and originated from an over-head irrigated, sandy-loam production field in Bingham county, Idaho were submitted to the University of Idaho for diagnosis. Shipping-point inspection records indicated 4-9% of tubers were affected. External symptoms included irregularly shaped, randomly located sunken black-colored lesions on more severely affected rubbery-textured tubers. When cut, internal affected tissue developed a greyish appearance after several minutes. Lens-shaped cavities were apparent in two of the tubers, indicating an advanced infection. A sour-milk smell accompanied the sample. To isolate the pathogen, pieces of tuber tissue approximately 5 mm in diameter were collected from the margins of symptomatic areas and surface-sanitized in sodium hypochlorite (2%) for two minutes, rinsed twice in sterile water and plated onto tap water agar amended with penicillin G (0.2 g/liter) and streptomycin sulfate (0.8 g/liter). After three days at 21°C, colonies having distinct creamy white mycelia, a sweet, juniper-like odor, and hyaline hyphae were consistently associated with diseased tissue. Cylindrical to oval-shaped arthroconidia ranged in size from 6.6-11.0 × 3.2-5.9 µm (mean = 8.4 × 4.7, n=21), within dimensions as reported by Carmichael (1957). No other pathogens including species of Pythium and Phytophthora were recovered from the sample. Pure cultures were obtained by transferring hyphal tips to potato dextrose agar plates. Species identity was confirmed via rDNA ITS sequencing using primers ITS5/4 (White et al., 1990). DNA extraction and PCR conditions were as previously described (Woodhall et al., 2013). Resulting sequences (NCBI accession numbers MT893312 and MT893315) shared 99.4% identity with G. candidum Accession KY103453.1 on GenBank. To confirm pathogenicity, ten tubers (cv. Ciklamen) were inoculated by placing a 10mm2 plug of fully colonized PDA of G. candidum on the tuber surface, and ten tubers were mock inoculated with sterile PDA plugs. After 27 days at 21C in a dew chamber, tubers were examined for symptoms. Eight of the 10 inoculated tubers exhibited a rubbery texture and fluid leaking from tubers when cut, with two tubers also exhibiting a grey internal discoloration and the distinctive smell. Control tubers did not exhibit any symptoms. Isolations were attempted from four symptomatic tubers and G. candidum was successfully recovered from three tubers. The disease has been reported sporadically in the United Kingdom (Humphreys-Jones 1969) and Korea (Kim et al., 2011) and the pathogen occurs worldwide (Carmichael 1957). Though the fungus causes a tomato rot in the United States (US) (Pritchard and Porte, 1923; Bourret et al., 2013), and potatoes with rubbery rot originating from Australia were intercepted at a US port (Farr et al., 2020), the disease has not to our knowledge been documented on potato grown in the US. Because symptoms may be confused with pink rot and Pythium leak, it is critical for producers to obtain a correct diagnosis to facilitate appropriate management strategies.

Rhizoctonia solani AG 2-2 IIIB causing root rot of onion in Idaho.

In September of 2018, onion plants (Allium cepa cv. Joaquin) grown in one field in southwest Idaho were observed to have roots with brown discoloration over 10-20% of the total root surface area. Approximately 10% of plants over a 1 ha area were affected and these plants were about visually 50% smaller than the typical bulb size present in the field. To determine the causal agent, 3 mm pieces of symptomatic roots from four plants were placed in sodium hypochlorite (2%) for one minute, followed by two rinses in sterile water and plated on to water agar medium amended with penicillin G (0.2 g/liter) and streptomycin sulfate (0.8 g/liter). After 3 days at 21°C, fungal colonies with septate hyphae with right-angled branching resembling Rhizoctonia solani were observed in over half of the 16 isolations attempted. Species identity was confirmed through rDNA ITS sequencing, as described previously (Woodhall et al., 2013), with DNA obtained from a single representative hyphal tip culture grown on Potato Dextrose Agar (PDA) which was designated isolate ON3. The resulting sequence (MT672318), was 100% identical (678/678bp) to a sequence previously identified as R. solani AG 2-2 IIIB on GenBank (FJ492137). Pathogenicity of the culture was determined by inoculating ten 20-day-old plants (cv. Joaquin) grown in premium potting compost (Scotts) with a single, fully colonized 10 mm2 plug taken from a 2-week-old PDA culture of isolate ON3. A further nine plants were inoculated with sterile PDA plugs as controls. Plants were grown in the greenhouse at 21C in a 16-hour light regime. After 24 days, each plant was assessed for root rot disease as described previously (Misawa et al. 2017). Root rot was observed on nine of the inoculated plants. Mean diseased root area was 32% of the total root surface, with a minimum of 5% and a maximum of 100% diseased root area and a standard deviation equal to 39.6. No root browning was observed on any of the control plants. Isolations were attempted from nine symptomatic plants and R. solani was successfully isolated from seven plant samples onto water agar. Sequencing was used to confirm identity as AG2-2IIIB. To our knowledge, this is the first report of R. solani AG 2-2 IIIB affecting onions in Idaho. Previous work in the Pacific Northwest recovered R. solani AG2-1, 3, 4 and 8 and also BNR AG A from stunted onions (Patzek et al., 2013). In Japan, Misawa et al. (2017) found AG 2-2 IIIB to be pathogenic to Welsh onion (Allium fistulosum). In Idaho, R. solani AG 2-2 IIIB has was previously reported causing disease in sugar beets (Strausbaugh et al. 2011) and potatoes (Woodhall et al. 2012). Growers should consider crop rotation strategies or soil treatments if R. solani AG2-2IIIB is causing disease in their crops.