First Report of diaporthe fruit rot and stem canker of hydroponic cucumber caused by Diaporthe sp. in the United States

Authors: Moodispaw, Margaret; ROTONDO, FRANCESCA - The Ohio State University; MCKEMY, JOHN - Animal And Plant Health Inspection Service (APHIS); Testen, Anna

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/8/2023
Publication Date: 8/14/2023
Citation: Moodispaw, M.R., Rotondo, F., Mckemy, J., Testen, A.L. 2023. First Report of diaporthe fruit rot and stem canker of hydroponic cucumber caused by Diaporthe sp. in the United States. Plant Disease. 107(11). Article 3638. https://doi.org/10.1094/PDIS-06-23-1214-PDN.
DOI: https://doi.org/10.1094/PDIS-06-23-1214-PDN

Interpretive Summary: A new plant disease was identified in hydroponically grown cucumber plants in Ohio, and this disease could have detrimental impacts on controlled environment agriculture cucumber production. The unknown pathogen caused long, tan lesions on stems, rotted the tissues that transmit water and nutrients, and caused plants to rapidly collapse and die. Additionally, the pathogen rapidly rotted cucumber fruits . The scientific team worked to identify the fungus killing the cucumbers and found that it was an unknown species of Diaporthe, a common plant pathogen. The scientific team can now work to study this new pathogen further and develop management strategies for this disease to reduce its impacts on indoor cucumber production.

Technical Abstract: Multiple Diaporthe spp. cause fruit rots, stem lesions, or root rots on Cucumis spp.: D. cucurbitae, D. melonis, D. longicolla, D. pterocarpi, D. sclerotioides, D. sojae, and D. ueckerae (Udayanga et al. 2015, 2012; Broge et al. 2020; Fukada et al. 2018). From May-August 2021, cucumbers (Cucumis sativus) ‘Katrina’ and ‘Alcazar’ were grown in a 24-unit, Bato bucket system in Wooster, Ohio. At maturity, plants collapsed rapidly without foliar chlorosis (25% of ‘Katrina’ and 17% of ‘Alcazar’), but stem lesions were observed. Lesions were 7.5-15 cm in length, tan to golden-brown with black pycnidia and located 5-15 cm above the crown. Stems shredded easily, and vascular discoloration was present around the lesion. Two identical fungal strains were isolated on ½ acidified potato dextrose agar (APDA) following surface disinfestation with 0.6% sodium hypochlorite and sterile water rinse. Fungal cultures were floccose, white to tan mycelia with pycnidia forming on older cultures. Oblong, elliptical, aseptate alpha conidia with oil globules were observed in both strains with mean dimensions: 8.0 µm (5.2-9.8 µm) by 3.1 µm (2.5-3.8 µm) on ½ APDA and 9.8 µm (6.6-12.4 µm) by 3.0 µm (1.9-5.3 µm) on petioles. Beta conidia were rarely observed. On prune extract agar, conidial dimensions were similar and beta conidia more frequent. DNA from each strain was amplified with ITS (ITS4/ITS5), TUB2 (Bt1a/Bt1b), CAL (CAL228F/737R), and TEF1 (EF1 728f/EF1 986R) primers (Glass and Donaldson 1995; Carbone and Kohn 1999; White et al. 1990) and sequenced bidirectionally (GenBank: OP265712-13, OP288460-65). Based on a maximum likelihood phylogenetic tree of concatenated genes, this Diaporthe sp. has not been reported on Cucumis spp. For Koch’s postulates, one-month old cucumbers ‘Katrina,’ grown in rockwool cubes (5 plants per isolate) and potting mix (6 plants per isolate), were inoculated with a one-week-old culture of either fungal strain. The second true leaf was cut and a pipette tip containing an inoculated plug of ½ APDA was placed on the remaining 2.5 cm of petiole (Mathew et al. 2018). Non-inoculated ½ APDA was used as a control. Plants were tarped for 24 hours to increase humidity and pipette tips removed after one week. After two weeks, petioles were shrunken, tan to golden brown with pycnidia. Fungi were reisolated from petioles as described above. Based on morphology, Diaporthe was recovered from petioles of all but one inoculated plant. After 3-4 weeks, stem lesions matching those described above were observed on inoculated plants, and plants collapsed. One plant each inoculated with either strain did not collapse. Fungi were isolated from stem sections as describe above. Based on morphology, Diaporthe was isolated from every inoculated plant but never from controls. For fruit rot, three beit alpha cucumbers were rinsed with tap water, dried, a 5 mm plug was removed from the fruit and replaced with a 5 mm plug of one-week-old fungus on ½ APDA. After 3 days, fruits were water soaked and soft. No symptoms were observed on control fruits. For root rot, two plates of one-week-old cultures were macerated in 500 mL of sterile distilled water and mixed with 1500 mL of vermiculite. Two seeds of cucumber ‘Katrina’ was planted into three reps of each isolate and control. All control seeds germinated, but all inoculated seeds experienced pre- or post-emergence damping off. Due to its aggressiveness and novelty, this Diaporthe sp. is a potential new constraint to hydroponic cucumber production.