First Report of White Mold Caused by Sclerotinia sclerotiorum on Strobilus of Zamia furfuracea in Mexico

Zamia furfuracea, native to Mexico, has evolutionary, biological, ecological and economic significance due to its high ornamental and commercial value (Mayett et al. 2014). It is the world's second most traded cycad and is endangered in Mexico (NOM-059-SEMARNAT-2010). In November 2024, at UPAEP University's Research Center in Horticulture and Native Plants (18°55'52.9"N, 98°23'56.0"W), 35% of 10-year-old Z. furfuracea plants under shade netting showed strobilus rot and seed detachment. Symptoms included necrosis and soft rot of the strobilus, megasporophylls and seed including sarcotesta and sclerotesta, with abundant white-to-gray mycelium and irregular black sclerotia. The seeds and megasporophylls detached prematurely from the strobili. The fungus was isolated from 30 symptomatic strobili. Sclerotia were collected, disinfected with 3% NaOCl, rinsed with sterile distilled water (SDW), and plated on Petri dishes with potato dextrose agar (PDA) using sterile tweezers. Also, mycelium fragments were transferred into Petri plates with PDA using a sterile dissection needle. Plates were incubated at 25°C in darkness for 9 days. One isolate per diseased strobili was obtained using the hyphal tip method (15 from sclerotia and 15 from mycelium). After 9 days, colonies exhibited rapidly growing, dense, cottony white aerial mycelium, forming irregular sclerotia measuring 3.3 ± 0.57 mm (mean ± SD, n = 100). Each Petri dish produced 18.8 ± 10.8 (mean ± SD, n = 30) sclerotia after 13 days; initially white, they gradually turned black. Based on these morphological characteristics, the isolates were tentatively identified as Sclerotinia sclerotiorum (Saharan & Mehta, 2008). Two representative isolates were selected for molecular identification, and genomic DNA was extracted using the CTAB protocol. The ITS region and G3PDH gene were amplified and sequenced (Staats et al. 2005; White et al. 1990). The sequences of isolates SsZf4 and SsZf5 were deposited in GenBank (ITS, PV034383 and PV034522; G3PDH, PV035701 and PV035702). BLAST analysis of the partial ITS (463 bp) and G3PDH (912 bp) sequences of both isolates showed 100% identity to S. sclerotiorum isolates (GenBank: MG249966 and MZ388475) (Hu et al. 2018; Zhang et al. 2022). Pathogenicity was confirmed by inoculating the strobili of 10 healthy plants introducing one sclerotium at a depth of 1 cm of isolate SsZf4 with sterile tweezers. As a control treatment, SDW was injected using a sterile syringe to a depth of 1 cm into 5 strobili. Plants were maintained under shade netting at 25°C and 90% relative humidity. After 20 days, all inoculated strobili displayed symptoms consistent with the initial samples, whereas control plants remained asymptomatic. The fungus was re-isolated from inoculated strobili and characterized both molecularly and morphologically based on mycelial color, growth pattern, and sclerotia formation, identifying it as S. sclerotiorum, fulfilling Koch's postulates. Pathogenicity tests were conducted 3 times. S. sclerotiorum has been reported to cause white mold in Echeveria gigantea (Terrones et al. 2024a) and Hymenocallis glauca (Terrones et al. 2024b) in Atlixco, Puebla, Mexico. To our knowledge, this is the first report of S. sclerotiorum causing white mold in the strobili of Z. furfuracea in Mexico. Information on diseases affecting this plant is limited; thus, this research is crucial for designing management strategies and preventing its spread to other production areas.