April 12, 2006
From: ProMED-mail<promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes, May 2006 [edited] <http://www.apsnet.org/pd/searchnotes/2006/PD-90-0686B.asp>

First report of seedling blight caused by Sclerotium rolfsii on wheat in Oklahoma
V. Choppakatla and R. M. Hunger, Department of Entomology and Plant Pathology, Oklahoma State University, Stillwater 74078; and H. A. Melouk, USDA-ARS, Stillwater, OK. Plant Dis. 90:686, 2006; published on-line as DOI:
10.1094/PD-90-0686B. Accepted for publication 16 Feb 2006.

Wheat (_Triticum aestivum_) is an important crop in Oklahoma and throughout the Central Plains of the United States. The soilborne fungus, _Sclerotium rolfsii_, is a major pathogen on peanut (_Arachis hypogaea_) but is not known to cause major damage on wheat. During September of 1998, damping-off and rotting of young wheat
seedlings were observed in breeder plots in Payne County, OK. The occurrence of symptoms was sporadic with an estimated stand reduction of 10 to 15 percent. Symptomatic plants were collected from the field and brought to the laboratory. Sclerotia-like bodies from the symptomatic plants were surface disinfested in aqueous one percent
NaOCl for 2 min and allowed to germinate at 25 ± 2 C on sterile filter paper moistened with a one percent aqueous solution of methanol. Aerial mycelia from germinating sclerotia were transferred to potato dextrose agar amended with 100 ppm of streptomycin (SPDA) to obtain pure cultures. Pure cultures had coarse, white mycelium distinctive of _S. rolfsii_ and produced very small (0.05 to 0.1 mm), abundant, round, brown sclerotia on the surface of the medium after 15 days of incubation. Pathogenicity was tested on 3 hard red winter wheat cultivars
commonly grown in Oklahoma (Jagger, 2137, and 2174). Four plants of each cultivar were inoculated at the 2-leaf stage (Feekes' scale stage 1) by placing a 0.5-cm agar disk removed from a 3-day-old culture onto a 1-cm diameter filter paper that was then pressed to the base of the shoot. Noninoculated plants were used as a control. After inoculation, pots were covered with polyethylene sheets to maintain 95 to 100 percent relative humidity and incubated at 25 ± 2 C in the greenhouse. Lesions were initially superficial, yellowish, and water soaked. Lesions expanded and resulted in damping-off of  seedlings. Noninoculated plants were free of disease and remained healthy. No significant difference (P < 0.05) in disease severity was observed among the cultivars. To fulfill Koch's postulates, the fungus was reisolated onto SPDA where it had the same characteristics as the
initial culture. To our knowledge, this is the 1st report of _S. rolfsii_ on wheat in Oklahoma. Even though _S. rolfsii_ is not expected to pose a significant risk to wheat production, infection of wheat may enhance survival of _S. rolfsii_ and facilitate infection and losses in a following peanut crop. This is especially important in certain areas of Oklahoma where a wheat-peanut rotation is occasionally practiced.

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ProMED-mail
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[The crop plant wheat (_Triticum aestivum_) develops symptoms of the disease seedling blight when infected by the fungus _Sclerotium rolfsii_, which is a major pathogen on peanut (_Arachis hypogaea_), where it also causes seedling blight. This report of _S. rolfsii_ on wheat in a single plot of experimental wheat is unusual, and the
authors point out the possible threat to peanut crops from seedling blight were they to be planted on land that had grown infected wheat in a previous cropping period. There are no previous postings in ProMED-mail on this disease in either wheat or peanut. A different seedling blight of wheat can be caused by the fungal pathogen
_Fusarium graminearum_.

_Sclerotium rolfsii_, is an omnivorous, soilborne fungal pathogen that causes disease on a wide range of agricultural and horticultural crops. Although no worldwide compilation of host genera has been published, over 270 host genera have been reported in the United States alone. Susceptible agricultural hosts include sweet
potato (_Ipomea batatas_), pumpkin (_Cucurbita pepo_), corn (_Zea mays_), wheat (_Triticum vulgare_) and peanut (_Arachis hypogea_). From a global perspective peanut crops sustain higher losses than any other agricultural crop. In 1959, the United States Department of Agriculture estimated losses from USD 10 million to 20 million associated with _S. rolfsii_ in the southern peanut-growing region, with yield losses ranging from 1-60 percent in fields in the NC coastal plains region. The fungus develops protective structures known as sclerotia, and these contain viable hyphae that can serve as primary inoculum for disease development. The sclerotia also serve as the principal overwintering structures and primary inoculum for disease. Persisting near the soil surface,
sclerotia may exist free in the soil or in association with plant debris. Those buried deep in the soil may survive for a year or less, whereas those at the surface remain viable and may germinate in response to alcohols and  other volatiles released from decomposing plant material. Thus, deep plowing serves as a cultural control tactic by burying sclerotia deep in the soil. Since _S. rolfsii_ does not produce spores, dissemination depends on movement of infested soil and infected plant material. Use of contaminated equipment and machinery may spread sclerotia to uninfested fields. Cool, moist weather is required for southern blight, which usually appears during fall or late summer. There is a biosecurity issue associated with _S. rolfsii_, and it appears on lists of potential bioterrorism agents. Its broad host range, rapid action and general recalcitrance to fungicides make it an attractive candidate for such lists.

Map: Oklahoma, USA
<http://www.merriam-webster.com/cgi-bin/nytmaps.pl?oklahoma>

Pictures:
Sclerotia of _Sclerotium rolfsii_:
<http://www.ces.ncsu.edu/depts/pp/notes/Peanut/gallery/srolfsc2.jpg>

Links:
<http://www.cals.ncsu.edu/course/pp728/Sclerotium/Srolfsii.html>
<http://www.cbwinfo.com/Biological/PlantPath/SR.html>
- Mod.JAD]