First Report of Fusarium petroliphilum Causing Fruit Rot of Spaghetti Squash in California.

Abstract

Spaghetti squash (Cucurbita pepo L. subsp. pepo) is a yellow-skinned squash that forms translucent spaghetti-like strands when cooked. California leads the nation in total squash production, the majority of which is grown in the San Joaquin Valley. In October of 2019, severe fruit rot of C. pepo L. subsp. Pepo (C. pepo) was observed in fruit harvested from seven cultivated fields in San Joaquin County, California. Infected fields incurred up to 30% postharvest losses. At harvest, fruit appeared healthy. After ten days in a shaded storage shed, scattered buff to tan ringed lesions extending into the flesh of infected fruits were observed. Lesions had visible sporodochia at the center that were variable in size and continued to expand in storage. Tissue (∼1 mm3) from the lesion margins of symptomatic fruit (n=8) was surface sterilized in 75 % ethanol for 1 min then 0.6% sodium hypochlorite for a minute, and aseptically transferred to half strength acidified potato dextrose agar (0.5 APDA) and incubated at 22-25 °C. Fungal colonies which grew from the pieces were light yellow, with mycelium that was flat and mucoid. Sporodochial conidia were falcate and robust with 3 to 5 septa and measured from 44.2 to 51.6 × 4.6 to 5.9 μm (average 46.3 × 5.2 μm). Aerial conidia were profuse, borne on short monophialides, ovoid to reniform, and measured 5.1 to 12.6 μm × 3.2 to 5.6 μm (average 4.2 × 6.1 μm). DNA extracted from two isolates, was amplified with primers ITS1/ITS4, and EF1-728F/EF1-986R using PCR, to obtain sequences from the internal transcribed spacer (ITS) (White 1990), and elongation factor 1α (EF1α) (Carbone et al. 1999) genetic regions. Sequences from both isolates were identical. Sequences from isolate MVAP50001827, GenBank nos. MZ081401 (ITS) and MZ102267 (EF-1α) matched 100% to sequences of representative isolates of Fusarium petroliphilum (Q.T. Chen & X.H. Fu; Short et al., 2013, MB 802539) from Cucurbita species, MF535516 (ITS) and MF580776 (EF-1α) respectively (González, V. et al. 2018). To fulfill Koch's postulates, conidia were harvested from a culture of isolate MVAP50001827 and grown for 7 days on 0.5 APDA at room temperature (22-25 °C). A 3-cc syringe with a 25-gauge needle was used to wound and inject 200 μl of 1 × 106 conidia ml-1 into three equally spaced points 1 mm deep into the rind of C. pepo fruit (n=4). C. pepo fruit (n=4) serving as negative controls were treated similarly with 200 μl of sterile deionized water. Fruit was incubated in a growth chamber at 27 °C under 12-h diurnal cycle lighting conditions. Ten days post inoculation, lesions densely covered with white sporodochia had expanded to 7 cm diameter and 5 cm deep on average (average fruit size 31×11 cm). Twenty days post inoculation, severe fruit rot was observed. F. petroliphilum did not grow from the controls, and was successfully reisolated from the symptomatic inoculated fruits, completing Koch's postulates. Seeds inside the inoculated fruits were completely colonized and covered in conidia. Twenty-five seeds from the source seed lot was tested for F. petroliphilum by surface sterilizing and plating onto 0.5 APDA. No F. petroliphilum grew from tested seed. Postharvest fungal diseases can affect profitability of winter squash, which is often held in storage, and sold when market prices are optimal. To our knowledge, this is the first report of Fusarium petroliphilum infecting spaghetti squash (Cucurbita pepo L. subsp. pepo) in California.