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

First report of pathogenic association between Fusarium graminearum and soybean

RN Pioli, L Mozzoni, EN Morandi, Facultad de Ciencias Agrarias, University Nacional Rosario, PO 14, 2123 Zavalla, Santa Fe, Argentina. Plant Dis 2004; 88: 220, published online as D-2003-1119-01N, 2004. Accepted for publication 27 Oct 2003.

Fusarium graminearum, a pathogen of wheat and corn, was reported recently as a saprophytic fungus colonizing soybean (_Glycine max_ L. _Merr._) fruits and seeds at R7 in Argentina (1).

To evaluate the capacity of _F. graminearum_ obtained from stem and seeds of symptomatic soybean plants that cause disease on soybean seedlings, isolates were obtained during the 2001 to 2002 growing season from: (i) the basal 1/3 of stems from field-grown soybean plants, collected at R5, with light brown external and internal discoloration and leaves with interveinal chlorosis; and (ii) soybean seeds with pink tegument.

The pathogen was isolated on potato glucose agar acidified with 0.2 per cent lactic acid (PGAA). Isolates were identified as _F. graminearum_ on the basis of growth rate and pigmentation of colonies on PGAA, lack of microconidia (2), and morphology and size of typical macroconidia in sporodochia developed on Spezieller Nahrstoffarmer Agar (3). Isolates of _F. graminearum_, CE135 and CE136 (from wheat) and CE137 (from corn) deposited in the Centro de Referencia en Micologia (CEREMIC), Fac. Farmacia y Bioquimica, UNR, Argentina, were used as references in identifying the soybean isolates.

Plants (14 day old) were inoculated separately with stem and seed isolates in the greenhouse at 26 ± 2 and 20 ± 2 deg C day/night temperature by inserting a piece of mycelium into a wound made with a scalpel in the hypocotyl. A completely randomized block design (RCB) was utilized with 4 replicate pots with 4 plants per pot. Plants wounded but without mycelium served as controls. This test was conducted twice (experiments 1 and 2).

Another test was completed by burying a thin layer of wheat caryopsis colonized by fungal mycelium of the stem isolate CE170 in the soil of pots. Plants in pots with soil without inoculum served as controls (4). The experiment was conducted twice (experiments 3 and 4) in an RCB with 5 replications, 4 plants per replication.

The progress of symptoms in experiments 1 and 2 were stem with light brown discoloration around the inoculation point that extended progressively along the stem, interveinal chlorosis or loss of turgence of unifoliate leaves, and interveinal chlorosis of trifoliate leaves followed by plant wilting and death. 21 days after inoculation, average percentages of dead plants were 42 and 21 per cent for stem and seed isolates, respectively.

For experiments 3 and 4, percentage of dead plants was 56, 45 days after emergence. These plants had roots with light brown, necrotic areas. Control plants remained healthy. The pathogen was reisolated from the stem (100 per cent) and root (57 per cent) tissues of symptomatic plants but not from similar tissues of control plants.

To our knowledge, this is the first report of a pathogenic relationship between _F. graminearum_ and soybean.

References:
(1) RN Pioli, et al. Fitopatologia 2000; 35(2): 111.
(2) PE Nelson, et al. Fusarium species: an illustrated manual for identification. University Park, PA: Pennsylvania State University Press, 1983.
(3) BA Summerell, et al. Plant Dis 2003; 87: 117.
(4) CE Windels. Fusarium. In: LL Singleton, JD Mihail, CM Rush, editors. Methods for research on soilborne phytopathogenic fungi. St Paul, MN: American Phytopathological Society, 1992: pp 115-28.

[Infection of wheat and barley by _Fusarium graminearum_ (Fg) is epidemic in USA and is increasingly becoming a threat to the world's food supply due to recent outbreaks in Asia, Canada, Europe, and South America. Crop
quality and yield have been adversely impacted.

A major effect of Fg infection in cereals is contamination of seeds with trichothecene and estrogenic mycotoxins, making them unsuitable for food or feed. In Brazil, soybean crop residues in fields under conservation tillage
have been found to be heavily infested with Fg. Although the fungus also reportedly grows on living soybean stems and seeds, many consider the fungus non-pathogenic to soybean.

Recent surveys of soybean seed grown in Brazil also revealed infection by Fg. Fg strains from Brazilian soybean seed consistently caused pod rot and root rot disease on all soybean varieties, under all conditions tested.
These same strains also caused FHB in wheat.

Farmers who use a soybean/wheat crop rotation should be aware of a potential build-up of strains that infect both wheat and soybean, reducing the usefulness of the rotation. Brazilian strains, which also produce a novel mycotoxin known as 3-acetylnivalenol, have not yet been found in the U.S. In Argentina, a range of wheat cultivars are known to be infected by Fg.

Additional references:
<http://www.nal.usda.gov/ttic/tektran/data/000013/85/0000138584.html> 
<http://www.fao.org/ag/AGP/AGPC/doc/field/Wheat/america/Argentina/agroeco_III.htm> - Mod.DH]