Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/1/2004
Publication Date: 6/1/2005
Citation: Rosskopf, E.N., Yandoc, C., Stange, B., Lamb, E., Mitchell, D. 2005. First report of pithium root rot of rau ram (Polygonum odoratum). Plant Disease. 89:340, published on-line as DOI: 10.1094/PD-89-0340A. Interpretive Summary:
Technical Abstract: Polygonum odoratum [=Persicaria odorata], known as rau ram or sang hum, is native to Southeastern Asia and is a common herb in Vietnamese cuisine. It has been studied most extensively for its aromatic compound content. In Florida, rau ram commonly is grown hydroponically in greenhouses using large, cement beds with re-circulated water. The plants form dense mats from which new growth is trimmed for market. In January of 2002 a severe die-back was observed in one production house. Plants with less severe symptoms were yellowed and stunted. Roots of symptomatic plants were largely decayed with root symptoms beginning as a tip necrosis. The cortex of severely affected roots slipped off easily, leaving a stringy vascular system. Plating of symptomatic tissue from 20 randomly selected plant samples was performed using multiple general and selective media including potato dextrose agar, corn meal agar with pimaricin, ampicillin, rifampicin, and pentachloronitrobenzene (PARP) and PARP with hymexasol. All colonies produced were identified as Pythium helicoides Drechsler based on sporangial, oogonial, and antheridial characteristics. Isolates had proliferous, obovoid, papillate sporangia, and were homothallic with smooth-walled oogonia and thick- walled, aplerotic oospores. Multiple antheridial attachments per oogonium were common with the antheridium attached along its entire length. Pathogenicity tests were conducted with P. odoratum plants grown from commercial transplants. Two tests were performed. In the first, plants were maintained in 10-cm pots immersed in ¼ strength Hoagland's solution for the duration of the test. Plants were inoculated using five 7- x 70-mm sections of freshly growing culture per plant using 10-day-old cultures of P. helicoides grown on water agar. Chlorosis was observed at approximately two months after inoculation. Root necrosis was observed in inoculated plants approximately five months after inoculation. This test was performed in the greenhouse with temperatures ranging from 28-35 C. The second test was performed in growth chambers maintained at 40 C. Plants were maintained in 10-cm pots immersed in Hoagland's solution and were inoculated with four 6-mm plugs per plant. Symptoms were observed on inoculated plants at this temperature within one week of inoculation. No chlorosis or root decay was observed in non-inoculated, immersed plants. The pathogen was reisolated from inoculated, symptomatic tissue. This is the first report of root rot of P. odoratum caused by P. helicoides.