First Report of Damping-Off of Dent Corn Seedlings Caused by Pythium graminicola in Georgia

Dent corn (Zea mays L.) is an important crop produced in Georgia with a farm gate value of over $250 million annually (Wolfe and Stubbs 2016). An unknown disease was observed on corn seedlings at the V1 stage in two commercial corn fields (approximately 3 ha) in Tift County, Georgia, in April 2015. Stems of corn seedlings were brown, water-soaked, soft, decayed, and broke off easily at the soil line. Disease incidence ranged from 2 to 3%. Infected corn stems were surface sterilized with 70% alcohol and 0.5% NaOCl, each for 1 min, rinsed in sterile distilled water, and cultured on potato dextrose agar (PDA) at 25°C. A fungus-like organism was isolated from all infected stem samples, which produced whitish mycelium with no septum on PDA. The cultures were purified by subculturing hyphal tips on V8 juice agar. Spore production of six isolates was observed. Sporangia were filamentous or lobate, finger, petaloid or irregular, terminal or intercalary. Oogonia were globose, smooth-walled, terminal or intercalary. Oospores were globose, simple, smooth and colorless, and measured 14.5 to 19.0 µm in diameter. The isolates were identified as Pythium graminicola based on morphological characteristics of the oospores (van der Plaats-Niterink 1981). Genomic DNA was extracted from two isolates and the internal transcribed spacer regions of rDNA (ITS1-5.8S-ITS4) was amplified and sequenced with primers ITS1 and ITS4 (White et al. 1990). The sequences of the isolates were identical and MegaBLAST analysis determined 99% similarity to a P. graminicola isolate CBS 327.62 (accession no. AY598625) from Saccharum officinarum. The ITS sequence of a representative isolate Y1 was deposited in GenBank (KU569294). Pathogenicity of Y1 was evaluated by inoculating corn seedlings. Corn seedlings (cv. Providence) were grown in 10-cm diameter pots containing a commercial potting mix, one plant per pot. Inoculum was prepared by placing autoclaved wheat seeds on a V8 agar plate with a mycelial plug of the isolate Y1 at the center of the plate and incubating at 25°C for 10 days as reported previously (Li et al. 2017). Ten 7-day-old seedlings were inoculated by applying three infested wheat seeds around the stem of each plant under the soil surface. Ten seedlings each treated with three autoclaved wheat seeds were used as a control. The seedlings were placed in a greenhouse at temperatures of 24 to 32°C with a 12-h photoperiod and watered daily. All inoculated seedlings exhibited symptoms identical to those observed in the field 10 days after inoculation. No disease occurred on the control corn seedlings. P. graminicola was reisolated from diseased plants and the identity was confirmed based on morphological characteristics and ITS sequence analysis. To our knowledge, this is the first report of P. graminicola causing disease on corn in Georgia. Since corn is one of the most economically important field crops in Georgia, the occurrence of this new disease needs to be considered while developing and implementing disease management programs in corn production.