First Report of Alternaria Leaf Spot Caused by Alternaria cucumerina of Muskmelon in New York

Muskmelon (Cucumis melo var. reticulatus) is a popular high-value warm-season annual fruit. In July 2024, leaf spots were observed on muskmelon (cv. Athena) at Geneva, Ontario Co., New York (NY; 42º52’N, 77º13’W). Initial symptoms were small, ovate to circular chlorotic leaf spots, which expanded into circular tan to dark brown sunken lesions with dry center rings. Fruit size on affected plants was reduced. Similar symptoms were observed in at least 10 muskmelon fields across central NY in 2024 and 2025. Disease incidence was ~45% with severities up to 100%. Isolations were made from 20 symptomatic leaves after surface sterilization in 10% bleach for 60 s, immersion in 70% ethanol for 10 s, and three rinses with distilled water. Small sections from lesion margins were plated onto water agar (WA) + ampicillin (0.02% w/v) and incubated at 21°C with a 12-h photoperiod. Emerging colonies were hyphal-tipped onto potato dextrose agar (PDA) and carrot potato agar (CPA) and incubated at 21°C. Morphological characteristics of three representative isolates (ACKD01, 04, and 06) were evaluated on CPA. After 12 days, colonies developed concentric rings with defined margins. Average daily growth rates were 6.52 ± 0.25, 4.72 ± 0.25, and 7.50 ± 0.27 mm/day on CPA, PDA, and WA, respectively. Conidia (n = 270) were narrow-ovoid to ellipsoidal, multicellular, and measured 31.7 to 110.2 long × 9.4 to 34.4 µm wide, with filamentous beaks measuring 4.8 to 421 long × 1.7 to 9.7 µm wide, frequently exceeding one-third of the body length with a pointed apical terminus. Longitudinal and transverse septa ranged from 0 to 10 and 3 to 11, respectively. Genomic DNA was extracted from each isolate, and PCR amplified the internal transcribed spacer (ITS) region (ITS1/ITS4; White et al. 1990) and histone 3 (HIS3; H3-1a and H3-1b; Landschoot et al. 2017) and RNA polymerase II subunit (rpb2) genes (RPB2-5F2 and fRPB2-7cR; Liu et al. 1999). Sequences were deposited in GenBank with accessions PX720579 to PX720581 (ITS-rDNA); PX802974, PX802977, PX802978 (HIS3); and PX802973, PX802975, PX802976 (rpb2). Sequences were 100% similar to references of A. cucumerina: ITS rDNA (MK684059), HIS3 (MK483964), and rpb2 (MK684154). For pathogenicity testing, 5-week-old muskmelon plants were spray-inoculated with conidial suspensions (8.2 × 105/ml) from CPA plates. Conidia were harvested by flooding plates with water and filtering through double-layered cheesecloth. Conidial suspensions (+ 0.1% v/v polysorbate-20) were sprayed onto five plants per isolate until run-off (6 to 7 ml/plant). Noninoculated plants (n = 5) received the same volume of sterile water + 0.1% v/v polysorbate-20. All plants were enclosed in clear polyethylene bags for 48 h. After bag removal, plants were maintained at 25 ± 2°C with a 12 h photoperiod and daily misting for approximately 8 h. The experiment was conducted twice. After 10 days, all inoculated plants developed symptoms consistent with field symptoms, while noninoculated plants remained symptomless. The pathogen was reisolated from 90% of symptomatic tissues, and recovered isolates exhibited similar morphology and conidial dimensions consistent with original isolates, fulfilling Koch’s postulates. Alternaria leaf spot (ALS) of cucurbits caused by A. cucumerina has been reported in NC (Peterson and Campbell 2002), SC (Daley et al. 2017), and IN (Egel and Harmon 2001). This is the first report of ALS on muskmelon in NY and adds to the complex management of muskmelon production.