Main content area

First Report of Cucurbita Fruit Rot Caused by Choanephora cucurbitarum in Slovenia

Žerjav, M., Schroers, H.-J.
Plant disease 2019 v.103 no.4 pp. 760
Brassica, Capsicum, Choanephora cucurbitarum, Cucurbita moschata, Cucurbita pepo subsp. pepo, DNA primers, Phaseolus, Solanum, blight, corolla, culture media, fruits, fungal growth, home gardens, hosts, humidity, internal transcribed spacers, lighting, monitoring, multigene family, mycelium, pathogens, plant rots, polymerase chain reaction, ribosomal DNA, ribosomal RNA, sequence analysis, sporangia, spores, tropics, zucchini, China, Korean Peninsula, Maine, Romania, Slovenia, Western European region
Blossom blight, abortion, and soft rot of fruits were observed on Cucurbita pepo L. var. cylindrica Paris, C. pepo L. subsp. pepo var. styriaca Greb., and C. moschata Duchesne in home gardens and commercial fields at six geographically separated locations in Slovenia in August and September 2017. Symptoms appeared after warm and humid weather. Yield losses appeared to be negligible. Fungal growth was first observed on flower petals, and fruit rot was associated with superficially developing long sporangiophores producing brown and eventually black spore masses. Sporangiophores with monosporous sporangiola forming typically broadly ellipsoidal, longitudinally coarsely striate, brown spores measuring (12.5 to) 14 to 15.5 (to 19) × (8.5 to) 9.5 to 10 (to 12) μm (n = 60) were seen in all six samples. Sporangiophores with sporangia and typically ellipsoidal, longitudinally finely striate, brown and appendiculate bipolar spores measuring (11.5 to) 16.5 to 20 (to 24) × (7 to) 9 to 10.5 (to 13.5) μm (n = 60) were seen in two samples. These morphological characters allowed identification of the pathogen as Choanephora cucurbitarum (Berk. & Ravenel) Thaxt. (Kirk 1984). Germinating single spores produced fast-spreading colonies on potato dextrose agar (PDA, BD-Difco) with abundant white mycelium and bright yellow reverse. DNA was extracted by using the NucleoSpin Plant II kit (Machery-Nagel). The internal transcribed spacer (ITS) regions and a fragment of the 28S rDNA of the ribosomal RNA gene cluster were amplified by using the KAPA2G Robust Hotstart polymerase chain reaction (PCR) kit and therein specified protocols (Kapabiosystems). Primers ITS1f and NL4 or ITS1 and ITS4 were used for PCR and sequencing. ITS sequence of strain DSM 107258 (KIS 17-0734) from Slovenia (NCBI GenBank accession no. MH061335) and sequences of C. cucurbitarum strains CBS 150.51 (unknown origin, JN206232), CBS 674.93 (China, JN206233), and KA47637 and KA47639 (Korea, KJ461159 and KJ461161) (Park et al. 2014) were identical. Partial 28S rDNA sequences of the same strain (MH061335) were identical to the sequence of CBS 674.93 (JN939195). Surface-sterilized zucchini fruits were wounded with a scalpel and inoculated with mycelium taken from the margin of a 3-day-old PDA culture. After 4 days of incubation in the dark at 25°C and high humidity, water-soaked lesions appeared around inoculation points. After 2 days of additional incubation under natural illumination, lesions were overgrown with mycelium and a few mature sporangiophores; after 10 days fruits were entirely rotten. C. cucurbitarum was reisolated from the progressing lesions. No such symptoms were observed on wounded noninoculated fruits. According to CABI (2018), Maine (U.S.A) is currently the most northern collection place of C. cucurbitarum, which has not been detected in Central and Western Europe, but Puşcaşu (1984) described it from C. pepo in Romania. Specimens listed in Kirk (1984) and other studies (CABI 2018) are from Cucurbita, Capsicum, Brassica, Phaseolus, Solanum, and so on. Monitoring the distribution and spread of this pathogen in Europe is advisable because of its polyphagous nature. Comparison of C. cucurbitarum isolates from squashes in temperate and tropical regions and from diverse hosts is required.