Identification of quantitative trait loci (QTL) for resistance to Fusarium crown rot (Fusarium pseudograminearum) in multiple assay environments in the Pacific Northwestern US.

Fusarium crown rot (FCR), caused by Fusarium pseudograminearum and F. culmorum, reduces wheat (Triticum aestivum L.) yields in the Pacific Northwest (PNW) of the US by as much as 35%. Resistance to FCR has not yet been discovered in currently grown PNW wheat cultivars. Several significant quantitative trait loci (QTL) for FCR resistance have been documented on chromosomes 1A, 1D, 2B, 3B, and 4B in resistant Australian cultivars. Our objective was to identify QTL and tightly linked SSR markers for FCR resistance in the partially resistant Australian spring wheat cultivar Sunco using PNW isolates of F. pseudograminerarum in greenhouse and field based screening nurseries. A second objective was to compare heritabilities of FCR resistance in multiple types of disease assaying environments (seedling, terrace, and field) using multiple disease rating methods. Two recombinant inbred line (RIL) mapping populations were derived from crosses between Sunco and PNW spring wheat cultivars Macon and Otis. The Sunco/Macon population comprised 219 F(6):F(7) lines and the Sunco/Otis population comprised 151 F(5):F(6) lines. Plants were inoculated with a single PNW F. pseudograminearum isolate (006-13) in growth room (seedling), outdoor terrace (adult) and field (adult) assays conducted from 2008 through 2010. Crown and lower stem tissues of seedling and adult plants were rated for disease severity on several different scales, but mainly on a numeric scale from 0 to 10 where 0 = no discoloration and 10 = severe disease. Significant QTL were identified on chromosomes 2B, 3B, 4B, 4D, and 7A with LOD scores ranging from 3 to 22. The most significant and consistent QTL across screening environments was located on chromosome 3BL, inherited from the PNW cultivars Macon and Otis, with maximum LOD scores of 22 and 9 explaining 36 and 23% of the variation, respectively for the Sunco/Macon and Sunco/Otis populations. The SSR markers Xgwm247 and Xgwm299 flank these QTL and are being validated for use in marker-assisted selection for FCR resistance. This is the first report of QTL associated with FCR resistance in the US.

A Fluidizing Column for Extracting Cysts of Heterodera avenae from Soil.

The cereal cyst nematode Heterodera avenae can be extracted from soil using several different floatation or elutriation methods. Automated methods are prohibitively expensive for use in small labs and, for optimal efficiency, floatation methods require that the soil be air dried for an extended period. A method which suspends soil particles in a water column above a fluidizing plate was reported as being most efficient with wet and dry soils. Use of the fluidizing column for extracting H. avenae has not been reported in the United States and materials to construct the column using contemporary components have not been described. Objectives of this research were to construct a column with components available in the United States, and to compare numbers of cysts and eggs plus juveniles (from cysts) extracted by the column and three other floatation methods: Fenwick can, flask, and Cobb sieving. From a soil containing recently produced (more dense) cysts, the column extracted at least 18% more cysts and 23% more eggs plus juveniles than the Fenwick and flask methods. The fluidizing column was found to be useful for small laboratories because it is inexpensive ($253 for two columns), easily and quickly constructed by nonprofessional labor, and produces adequately repeatable results.

Effect of Foliarly Applied Spirotetramat on Reproduction of Heterodera avenae on Wheat Roots.

The cereal cyst nematode, Heterodera avenae, has the potential to reduce yields of cereal crops in the Pacific Northwest. Spirotetramat (Movento) is a foliar-applied insecticide with ambimobile translocation that reduces fecundity of sucking insects which feed on roots as well as foliage. Spirotetramat (88 g/ha) was applied to foliage during 2010 in two wheat fields infested by H. avenae near St. Anthony, ID and Palouse, WA. In Idaho, two applications at 2-week intervals during late spring to plants already exhibiting swollen white females reduced the postharvest density of H. avenae eggs plus juveniles by 35% (P = 0.03) compared to the nontreated control. In Washington, a single application before white females became apparent reduced the nematode density by 78% (P = 0.01). Grain yields and test weights were not significantly affected by application of spirotetramat at either location. In addition, symptomatic plants from the Idaho field were transplanted into greenhouse pots and treated with spirotetramat. One application (110 g/ha) reduced numbers of eggs plus juveniles/plant by 78% (P = 0.02). Spirotetramat effectively reduced H. avenae populations and warrants further evaluation as a substitute for crop rotations or long fallow periods that reduce nematode population densities in infested fields.

First Record of the Cyst Nematode Heterodera filipjevi on Wheat in Oregon.

Plant and soil samples from an irrigated winter wheat (Triticum aestivum) field near Imbler (Union County), OR were evaluated for root diseases during April 2007. The field exhibited patches with as much as 90% plant mortality. Previous crops were winter wheat (2004), chickpea (Cicer arietinum, 2005), and spring wheat (cv. Jefferson, 2006). Stubble was baled and removed, and the field was cultivated before replanting to winter wheat cv. Chukar in October. Patches of stunted seedlings (three- to five-leaf stage) appeared in March 2007. Stunted seedlings exhibited chlorotic or necrotic lower leaves, healthy younger leaves, few or no tillers, rotting of lower culms and crowns, and light brown roots with little or no branching. Signs and symptoms of fungal pathogens (Pythium spp., Gaeumannomyces graminis var. tritici, Rhizoctonia solani AG-8, and Typhula incarnata) were present on affected plants. Most small grain fields in Union County are infested with Heterodera avenae (4) but none of the roots, on either healthy or stunted plants, exhibited the bushy branching pattern typical of sites where H. avenae females penetrate and encyst. Extraction of motile nematodes (Whitehead tray method) from soil revealed high populations of Pratylenchus neglectus (6,560/kg of soil), Tylenchorhynchus spp. (2,369/kg of soil), and a species initially thought to be H. avenae (3,098 juveniles/kg of soil). Cysts were also extracted. During PCR-restriction fragment length polymorphism identification (1) of H. avenae collected in Oregon, Washington, and Idaho, four restriction enzymes applied to amplified DNA of cysts from the Imbler field consistently revealed a pattern identical to that of a H. filipjevi DNA standard and distinct from patterns of H. avenae, H. schachtii, and H. latipons. DNA standards were obtained from R. Rivoal, INRA, Rennes, France. Morphological evidence confirmed that the specimens were H. filipjevi, a member of the 'H. avenae Group' of cereal cyst nematodes (2,3). Measurements of second-stage juveniles (n = 15) included length of body (range = 530 to 570 µm, mean = 549, st. dev. = 13.0), stylet (22.5 to 24.5, 23.2, 0.6) with anchor-shaped basal knobs, tail (52.5 to 62.5, 57.4, 2.7), and hyaline tail terminal (30 to 38, 33.5, 2.6). The lateral field had four lines of which the inner two were more distinct. Shapes of the tail, tail terminus, and stylet knobs were also consistent with H. filipjevi. Cysts (n = 10) were lemon shaped and light brown. The cyst wall had a zigzag pattern. The vulval cone was bifenestrate with horseshoe-shaped semifenestra. The cysts were characterized by body length including neck (range = 718 to 940 µm, mean = 809.7, st. dev. = 61.8), body width (395 to 619, 504, 71.2), L/W ratio = (1.1 to 2.2, 1.4, 0.3), neck length (75 to 140, 103.2, 22.1) and width (50 to 95, 71.4, 10.9), fenestra length (50 to 65 µm, 56.5, 6.6) and width (27 to 40, 29.0, 3.8), heavy underbridge (60 to 80, 69, 8.5), vulval slit (7.5 to 8.5, 7.8, 0.4), and many bullae. As described for H. filipjevi, cysts hatched much more readily and at lower temperatures than populations of H. avenae. Detection of H. filipjevi in Oregon represents a new record for the occurrence of this species in the United States and for North America. The pathotype and resistance genes for incorporation into wheat, barley, and oat are being identified. References: (1) S. Bekal et al. Genome 40:479, 1997. (2) Z. A. Handoo. J. Nematol. 34:250, 2002. (3) R. Holgado et al. J. Nematol. Morphol. Syst. 7:77, 2004. (4) R. W. Smiley et al. J. Nematol. 37:297, 2005.