In fall 2012 and 2013, peanut (Arachis hypogaea L.) grown in commercial fields in Tift County, GA, showed pod rot symptoms. The disease was primarily damaging pods and kernels and symptoms included brown to black, water-soaked lesions on pods and blackened pegs with white fluffy mycelia. Ten random symptomatic pods collected from the field were plated on potato dextrose agar after surface sterilization with 0.5% NaOCl. The plates were incubated at 25°C for 5 days in the dark. Whitish fungus-like cultures with non-septate mycelium grew from all the pods. Single hyphal tip cultures were obtained on pimaricin-ampicillin-rifampicin-pentachloronitrobenzene (PARP) medium. Isolates on PARP agar plates had three different growth patterns: two groups of isolates produced sporangia and the third group produced oogonia. The isolates were identified as Pythium spp. based on growth pattern and sporangial and oogonial structures (2). DNA of one representative isolate from each group was extracted and the internal transcribed spacer (ITS1-5.8S-ITS2) regions of rDNA were amplified and sequenced with primers ITS1 and ITS4 (1). ITS sequences of the isolates shared 99 to 100% similarity with Pythium ultimum, P. vexans, and P. deliense isolates in GenBank (Accession Nos. KC689906, GU133594, and HQ643521, respectively). The isolates were identified as P. ultimum var. ultimum, P. vexans, and P. deliense based on molecular analysis and morphological characteristics. P. ultimum produced plenty of spherical sporangia, but no oogonia in the culture, P. deliense produced characteristic terminal oogonia and aplerotic oospores with oogonial stalks curved towards the antheridia, and P. vexans produced spherical sporangia and aplerotic oospores. ITS sequences of three isolates representing each of the three species were deposited in GenBank (KF500573, KF500574, and KF500572). Pathogenicity of one representative isolate from each group was tested on peanut under greenhouse conditions (30°C day and 20°C night). Nine 10-week-old peanut seedlings (cv. GA07W) grown in 20-cm pots (2:1 ratio of potting mix/sterilized field soil) were inoculated with the isolates separately by applying 5 ml of respective Pythium-infested beet seeds. Nine untreated plants were used as a control. Pods were washed off 1 month after inoculation for disease assessment. All plants inoculated with the isolates showed pod rot similar to those observed in the field. The three Pythium species were re-isolated from respective symptomatic pods and the identity was confirmed by morphological characteristics and molecular analysis. The untreated plants did not show typical pod rot symptoms and the Pythium species were not isolated from these plants. P. ultimum and P. vexans have been reported to be associated with peanut pod rot in the United States (3). To our knowledge, this is the first report of P. deliense causing peanut pod rot. Georgia is the top peanut producer in the United States and the occurrence of pod rot caused by the Pythium spp. needs to be taken into account in developing disease management programs in peanut production.
References: (1) M. A. Innis et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA, 1990. (2) A. J. Van der Plaats-Niterink. Stud. Mycol. 21:51, 1981. (3) T. A. Wheeler et al. Peanut Sci. 32:9, 2005.
