First report of blue mold caused by Penicillium polonicum on apple in the United States

Apples (Malus domestica, Rosaceae) are one of the most widely grown and economically valuable fruits worldwide. In Hood River County, Oregon in 1991 decayed apples exhibiting blue mold signs and symptoms were collected and spores from the causal agent of the disease were isolated. The decayed area of the infected apples was brown colored with soft, decayed tissue, which had bluish-green colored spores on the fruit surface. The whole genome of this isolate was sequenced (GenBank number: JYNM00000000) and it was originally identified as Penicillium solitum strain RS1 (Yu et al. 2016). Subsequent genome-wide species-level investigations showed higher homology to P. polonicum. Therefore, we taxonomically and phylogenetically reevaluated the fungus in question. Colonies were analyzed growing on potato dextrose agar (PDA), Czapek yeast autolysate agar (CYA) and malt extract agar (MEA) at 25°C. Colonies on PDA were blue-green and growth was moderately deep and raised at the center with low margins. Colonies on CYA were blue-green. The range of the colony diameter after 7 days at 25ºC was 24-27 mm on CYA and 20-25 mm on MEA. Colony reverse color on CYA was yellow-brown and on MEA was cream. Conidiophores were terverticillate. Stipes were septate with smooth walls and measured 62-250 × 3-5 µm, x̄ = 111.1 × 3.8 µm with 1-4 branches per stipe. Branches measured 8-25 × 2-6 µm, x̄ = 4.9 × 16.3 µm with 2-4 metulae per branch. Metulae measured 7-14 × 2-5 µm, x̄ = 10.1 × 3.6 µm with 1-3 phialides per metulae. Phialides were flask shaped and measured 5-11 × 2-5 µm, x̄ = 7.5 × 3.4 µm. Conidia were globose to subglobose, borne in columns measuring 2.2-5.4 × 2.1-5.3 µm, x̄ = 3.6 × 3.4 µm. Morphologically, the fungal strain RS1 matched the description of Penicillium polonicum K. Zaleski from Bashir et al. (2017), Duduk et al. (2014) and Frisvad and Sampson (2004) with some minor differences. The ITS, TUB and RpB2 sequences of the strain RS1 were extracted from GenBank accession number JYNM00000000. The sequences were then submitted for nucleotide BLAST (NCBI) analysis in GenBank and evaluated. The ITS sequence aligned 100% with the type specimen (CBS 222.28) of P. polonicum (GenBank number: NR_103687). The TUB sequence aligned over 99% with P. polonicum (GenBank numbers: MK450898, MK450935, MK450899). The RpB2 sequences aligned 99.9% or higher with multiple P. polonicum specimens deposited in CBS and CMV (GenBank numbers: MK450847, MK450846, JN985414 and JN985415). Koch's postulates were conducted. Ten apples were wounded with the point of a 16-penny nail, and 10ul of a conidial suspension adjusted to 106 conidia-distilled water/tween solution was added to the wound. Ten separate apples served as a control that were wounded and 10ul of sterile Tween treated water was used to simulate inoculation. None of the control apples developed signs or symptoms of the disease. The inoculated apples all developed typical blue mold symptoms. The fungus was reisolated from the fruit and deemed to be morphologically identical to those of the original RS1, P. polonicum isolate. To the best of our knowledge this is the first report of blue mold caused by P. polonicum in the USA on apples (Farr and Rossman 2021). This information is important for the apple industry for which blue mold is a major problem.