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First Report of Pythium Root Rot on Aeroponically Grown Sweetpotato Caused by Pythium myriotylum in Louisiana

Clark, C. A., Doyle, V. P., Villordon, A. Q., Gregorie, J. C.
Plant disease 2019 v.103 no.2 pp. 378
Ipomoea batatas, Pythium aphanidermatum, Pythium myriotylum, Pythium spinosum, Pythium ultimum, aeroponics, agar, ampicillin, appressoria, autumn, chlorosis, cytochrome-c oxidase, dipping, fibrous roots, hyphae, internal transcribed spacers, leaves, mycelium, necrosis, oogonia, oospores, plant propagation, quintozene, rifampicin, root rot, roots, sequence analysis, sodium hypochlorite, soil, sporangia, sweet potatoes, vines, weather, zoospores, Louisiana
Increased demand for virus-tested sweetpotatoes (Ipomoea batatas L.) has led to the use of aeroponic propagation. In fall, 2017, Bayou Belle and Beauregard sweetpotatoes growing in aeroponics in Franklin Parish, Louisiana, became stunted with mild interveinal chlorosis after warm weather, which was associated with pale to dark brown soft rot of the secondary roots. All aeroponic plants became affected, but plants subsequently regenerated secondary roots. Affected roots were sampled randomly from two tubs, surface disinfested in 0.525% NaOCl for 1 min, rinsed with water, and placed on water agar. Fast-growing coenocytic hyphae were transferred to PARP, containing pimaricin + ampicillin + rifampicin + pentachloronitrobenzene (Jeffers and Martin 1986). All seven of the investigated hyphal tipped isolates had filamentous, inflated, lobate nonproliferating sporangia in complexes ranging from 17 to 114 (mean = 43) µm long and 9 to 21 (mean = 15) µm wide, and terminal oogonia and oospores with curved stalks. Oogonia were 24 to 36 (mean = 28) µm in diameter; oospores were not colored, aplerotic, 16 to 26 (mean = 20) µm in diameter with walls 1 to 2 µm thick. Diclinous and monoclinous antheridia were paragynous, club-shaped wrapped around part of the oogonium, two to six per oogonium, and some antheridial stalks coiled around the oogonial stalk. Appressoria were present in finger-shaped clusters. Based on these morphological criteria, the isolates were identified as Pythium myriotylum Dreschler (Le et al. 2017; Lévesque and De Cock 2004). This was confirmed by sequence comparison of four isolates to accessions from P. myriotylum voucher isolate CBS254.70. The internal transcribed spacer (ITS) (amplified with ITS1/ITS4 primers) sequences of all the isolates had 99% similarity to accession AY598678, whereas the cytochrome oxidase subunit 1 (COX1; amplified with Cox1-Levlo primers) sequences all had 100% similarity to accession HQ708745. The ITS sequences of isolates SPOo-1, SPOo-2, SPOo-3, and SPOo-4 were deposited in GenBank with accession numbers MH255945, MH255946, MH255947, and MH255948, and for COX1 the accessions were MH306196, MH306197, MH306198, and MH306199. Greenhouse-grown vine cuttings of Bayou Belle were established in sanitized aeroponics tubs for 10 days. Six plants were inoculated separately with each of seven isolates by dipping the root systems in a suspension of zoospores and mycelia, prepared by incubating 1-week-old V8 agar cultures for 3 days in 10 ml of sterile soil extract. Sterile distilled water was used for the controls. The secondary roots of inoculated plants developed necrosis at 4 days postinoculation (DPI), and by 7 to 10 DPI, rotting was extensive and most pronounced in roots growing in the bottom of the tubs. At 7 to 10 DPI, lower leaves exhibited mild interveinal chlorosis. At 23 DPI, new secondary roots emerged, with few symptoms evident on the primary roots, and vines had largely recovered from the interveinal yellowing. No symptoms were observed on noninoculated controls. Reisolations from inoculated plants, but not the noninoculated plants, consistently resulted in oomycete growth morphologically resembling the primary isolations, fulfilling Koch’s postulates. Pythium ultimum, P. scleroteichum, P. spinosum, and P. aphanidermatum have been associated with mottle necrosis of storage roots and/or root rot of fibrous roots of sweetpotato in the field (Clark et al. 2013). This is the first report of P. myriotylum on sweetpotato, and of root rot of sweetpotato grown in aeroponics. The disease may reduce clean plant propagation.