February 13, 2003
From: American Phytopathological Society, Plant Disease Notes [edited]

First Report of Pythium irregulare on lentils in the United States
T. C. Paulitz, F. Dugan, and W. Chen, USDA-ARS, Washington State University, Pullman 99163-6430; and N. J. Grunwald, USDA- ARS, 24106 N. Bunn Rd., Prosser, WA 99350. Plant Dis. 88:310,  2004; published online as D-2004-0114-01N, 2004. Accepted for publication 15 Dec 2003.

In late June and early July 2002, stunted, chlorotic, and partially defoliated lentils (_Lens culinaris_ Medik.) were observed throughout the lentil-growing areas of eastern Washington. These symptoms were investigated in 2 fields near Garfield, WA and one field near Genesee, ID. Cv. Mason was more affected than cv. Brewer. Roots were dry and brittle with black discoloration in some cases.

Isolates of _Fusarium oxysporum_ and _F. solani_ were obtained from washed roots plated on water agar, but they were nonpathogenic in greenhouse testing in pasteurized field soil and peat-based growing mixes.

On 21 Apr 2003, volunteer lentils growing in the same fields showed symptoms of root rot, and _Pythium_ oospores were observed in the roots. _Pythium_ spp. were isolated by using a selective medium (2).

Oospores were aplerotic, intercalary, 12.6 to 21 micrometers long x 11.2 to 18.2 micrometers wide, mostly smooth, and often formed in chains. Isolates resembled _P. paroecandrum_ Drechs. and _P. irregulare_ Buisman on the basis of morphological characteristics (3), but DNA sequences of the internal transcribed spacer region
were closer to _P. irregulare_ on the basis of a comparison with a worldwide database of _Pythium_ sequences (C. A. Levesque, personal communication). Isolates were deposited with the USDA- ARS Western Regional Plant Introduction Station, Pullman, WA.

Four hyphal-tip isolates were tested in the greenhouse with inoculum grown in autoclaved sandy loam amended with 1 percent ground rolled oats. Experiments were performed twice in Thatuna silt loam, first in pasteurized and then in nonpasteurized soil. Inoculum was added to the soil at 500 CFU/g, and seeds were planted on the same day. Each isolate was tested on cvs. Brewer and Mason, with 5 replicates per treatment. Plants were grown in 4
plastic tubes 20.5 cm long (2 plants per tube) for 1 month at 16 to 22 deg C and supplemented with 14 h light per day. _P. irregulare_ was reisolated from infected roots in both experiments.

Damping-off, stunting, chlorosis, and root rot were observed in the _Pythium_-inoculated treatments, which corresponded to symptoms observed in the field in 2002. In pasteurized soil, only one isolate reduced the whole, dry, plant weight of Brewer, but the other 3 isolates reduced the dry weight of Mason. All isolates reduced the root dry weight of Mason in natural soil, but only 2 isolates reduced the root dry weight of Brewer.

To our knowledge, _Pythium_ spp., but not _P. irregulare_, have been reported previously from lentils (1). _P. irregulare_ also causes root rot on winter wheat, which is rotated with lentils, and this pathogen likely causes yield reduction in both crops.

References:
(1) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989. (2) S. M. Mircetich and J. M. Kraft. Mycopathol. Mycol. Appl. 50:151, 1973. (3) A. J. van der Plaats-Niterink. Stud. Mycol. 21:1, 1981.

[_Pythium irregulare and _P. ultimum_ are common pathogens that affect many crops, causing damping-off diseases. Yield losses can be very high, especially in soils with heavy moisture-holding capacity. Prevention of diseases under field conditions is a priority, but that is easier said than done -- because once infestation occurs, oospores can remain viable for several years. Disease management depends upon planting of high-quality seeds and treatment of seed with approved fungicides. Infection of wheat by _P. irregulare_ in the Pacific Northwest USA may be more common than we know.
2 additional references: <http://www.gov.on.ca/OMAFRA/english/crops/pub811/3sdise.htm>
<http://pnwsteep.wsu.edu/tillagehandbook/chapter4/041187.htm> - Mod.DH]