First Report of Postharvest Fruit Rot of Tomato (Lycopersicum esculentum Mill.) Caused by Penicillium olsonii in Pakistan

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A previously unnoticed fruit rot of tomato was observed during a survey in 2016 in the nursery of the University of Punjab, Lahore, Pakistan. Disease incidence was recorded at 9%. The symptoms were initially characterized as small water-soaked lesions on the epidermis that expanded and developed into watery rot. Isolations from symptomatic skin and tissue of the pericarp yielded species of Penicillium. A representative isolate of Penicillium was identified on the basis of morphological characteristics and by sequencing the internal transcribed spacer region of DNA and partial sequencing of the β-tubulin gene using ITS1 and ITS4 (White et al. 1990) and Bt2a and Bt2b (Glass and Donaldson 1995) primers, respectively. The associated fungus was identified as Penicillium olsonii. The colony of P. olsonii reached a diameter of 80 mm after 7 days of incubation at 25 ± 2°C on potato dextrose agar. The colony appeared white earlier in growth and then became dull green with conidiogenesis. The reverse side of the colony was pale yellow to cream yellow. The colony texture was velvety, occasionally with clear exudates and no pigmentation. The mycelium was white. Conidiophores were unbranched, smooth walled, and measured 650 to 800 × 4.5 to 6.5 µm. The phialides were flask shaped with a short neck and measured 8 to 10 × 2.5 to 3 µm (n = 50). The conidiophores had terminal terverticillate appressed penicillia. The conidia were subglobose to ellipsoidal with granular wall and measured 3.5 to 4 × 2.5 to 3.5 µm. The morphological characteristics were consistent with P. olsonii based on the description by Pitt (1979). The ITS1 region (GenBank accession no. LT900496) of 565 bp showed 100% homology with P. olsonii isolates (KY859385.1, JQ663620.1, DQ117963.1, and AY373925.1) from the NCBI database. The partial β-tubulin gene (accession no. LT900495) of 456 bp showed 100% identity with P. olsonii isolates (KR709179.1, AY674444.1, KU507286.1, and DQ645787.1) from GenBank. Pathogenicity of the fungus was demonstrated on tomato fruit according to Koch’s postulates. Healthy detached fruits of tomato were surface disinfested and stab inoculation was conducted using a needle rubbed over a 5-day-old culture of P. olsonii. A total of 20 control fruits were inoculated with a needle without inoculum and a total of 20 fruits were inoculated with P. olsonii and kept in a plastic bag at 25 ± 2°C for 20 days. Each fruit was inoculated at three points. Black spots appeared on inoculated tomato fruits after 10 days, which expanded to form larger gray-black lesions with evidence of mycelial growth in the center after 10 to 15 days of incubation. On the basis of mycological, molecular, and pathologic characteristics, the fungus reisolated from the fruits was identified as P. olsonii. P. olsonii has been reported previously to cause blue mold on tomato (Chatterton et al. 2012). This study appears to be the first report of postharvest fruit rot on tomato caused by P. olsonii in Pakistan. The occurrence of P. olsonii on other crop plants and its wide host range together suggest potential for this fungus to be a pathogen on a range of plant genera/species.