Ruth Dill-Macky

The Effect of Previous Crop Residues and Tillage on Fusarium Head Blight of Wheat

Effects of previous crop residues and tillage practices on Fusarium head blight (FHB) of wheat were examined. Fusarium head blight was monitored in plots of the FHB-susceptible spring wheat cultivar Norm following crops of corn, wheat, and soybeans in 1995, 1996, and 1997. Moldboard plow, chisel plow, and no-till treatments were imposed perpendicular to crop strips to establish a range of residue levels in each of the previous crop residues. Fusarium head blight incidence and severity were greatest when wheat followed corn and least when wheat followed soybeans. Incidence and severity were lower in moldboard plowed plots than in either chisel plowed or no-till plots, although differences among chisel plow and no-till treatments were not apparent. Yields of wheat were approximately 15% lower in plots where wheat followed corn or wheat than in wheat following soybeans and were 10% greater in moldboard plowed plots than in either chisel plowed or no-till treatments. The deoxynivalenol (DON) content of harvested grain was significantly correlated with FHB incidence and severity. The DON level in wheat following soybeans, averaged across tillage treatments, was 25% lower than in wheat following wheat and 50% of the level in wheat following corn. These findings suggest that changes in regional tillage practices, principally the move toward conservation tillage and reduced-till systems, contributed to the recent FHB epidemics in the Upper Midwest. Because differences in the type and quantity of crop residues in small plots affected disease development, it is likely that local sources of inoculum, such as those within a growers field, contribute directly to the inoculum load and disease potential. The implication of these findings is that selection of cultural practices aimed to reduce inoculum-borne residues will assist in the control of FHB.

Quantitative trait loci associated with resistance to Fusarium head blight and kernel discoloration in barley.

Abstract Resistance to Fusarium head blight (FHB), deoxynivalenol (DON) accumulation, and kernel discoloration (KD) in barley are difficult traits to introgress into elite varieties because current screening methods are laborious and disease levels are strongly influenced by environment. To improve breeding strategies directed toward enhancing these traits, we identified genomic regions containing quantitative trait loci (QTLs) associated with resistance to FHB, DON accumulation, and KD in a breeding population of F4:7 lines using restriction fragment length polymorphic (RFLP) markers. We evaluated 101 F4:7 lines, derived from a cross between the cultivar Chevron and an elite breeding line, M69, for each of the traits in three or four environments. We used 94 previously mapped RFLP markers to create a linkage map. Using composite interval mapping, we identified 10, 11, and 4 QTLs associated with resistance to FHB, DON accumulation, and KD, respectively. Markers flanking these QTLs should be useful for introgressing resistance to FHB, DON accumulation, and KD into elite barley cultivars.

Engineering deoxynivalenol metabolism in wheat through the expression of a fungal trichothecene acetyltransferase gene

Abstract.Fusarium head blight occurs in cereals throughout the world and is especially important in humid growing regions. Fusarium head blight (FHB) has re-emerged as a major disease of wheat and barley in the U.S. and Canada since 1993. The primary causal agents of FHB, Fusarium graminearum and Fusarium culmorum, can produce deoxynivalenol (DON), a trichothecene mycotoxin that enhances disease severity and poses a health hazard to humans and monogastric animals. To reduce the effects of DON on wheat, we have introduced FsTRI101, a Fusarium sporotrichioides gene formerly known as TriR, into the regenerable cultivar Bobwhite. TRI101 encodes an enzyme that transfers an acetyl moiety to the C3 hydroxyl group of trichothecenes. Four different transgenic plants carrying the FsTRI101 gene were identified. Although expression levels varied among the four lines, all of them accumulated FsTRI101 transcripts in endosperm and glume. TRI101-encoded acetyltransferase activity was detected in endosperm extracts of a single plant that accumulated FsTRI101 mRNA. Greenhouse resistance tests indicated that the accumulation of FsTRI101-encoded acetyltransferase in this plant confers partial protection against the spread of F. graminearum in inoculated wheat heads (spikes).

Overexpression of defense response genes in transgenic wheat enhances resistance to Fusarium head blight

Fusarium head blight (FHB) of wheat, caused by Fusarium graminearum and other Fusarium species, is a major disease problem for wheat production worldwide. To combat this problem, large-scale breeding efforts have been established. Although progress has been made through standard breeding approaches, the level of resistance attained is insufficient to withstand epidemic conditions. Genetic engineering provides an alternative approach to enhance the level of resistance. Many defense response genes are induced in wheat during F. graminearum infection and may play a role in reducing FHB. The objectives of this study were (1) to develop transgenic wheat overexpressing the defense response genes α-1-purothionin, thaumatin-like protein 1 (tlp-1), and β-1,3-glucanase; and (2) to test the resultant transgenic wheat lines against F. graminearum infection under greenhouse and field conditions. Using the wheat cultivar Bobwhite, we developed one, two, and four lines carrying the α-1-purothionin, tlp-1, and β-1,3-glucanase transgenes, respectively, that had statistically significant reductions in FHB severity in greenhouse evaluations. We tested these seven transgenic lines under field conditions for percent FHB disease severity, deoxynivalenol (DON) mycotoxin accumulation, and percent visually scabby kernels (VSK). Six of the seven lines differed from the nontransgenic parental Bobwhite line for at least one of the disease traits. A β-1,3-glucanase transgenic line had enhanced resistance, showing lower FHB severity, DON concentration, and percent VSK compared to Bobwhite. Taken together, the results showed that overexpression of defense response genes in wheat could enhance the FHB resistance in both greenhouse and field conditions.

Survival and Inoculum Production of Gibberella zeae in Wheat Residue

Survival and inoculum production of Gibberella zeae (Schwein.) Petch (anamorph Fusarium graminearum (Schwabe)), the causal agent of Fusarium head blight of wheat and barley, was related to the rate of wheat (Triticum aestivum L.) residue decomposition. Infested wheat residue, comprising intact nodes, internodes, and leaf sheaths, was placed in fiberglass mesh bags on the soil surface and at 7.5- to 10-cm and 15- to 20-cm depths in chisel-plowed plots and 15 to 20 cm deep in moldboard-plowed plots in October 1997. Residue was sampled monthly from April through November during 1998 and every 2 months through April to October 1999. Buried residue decomposed faster than residue placed on the soil surface. Less than 2% of the dry-matter residue remained in buried treatments after 24 months in the field, while 25% of the residue remained in the soil-surface treatment. Survival of G. zeae on node tissues was inversely related to the residue decomposition rate. Surface residue provided a substrate for G. zeae for a longer period of time than buried residue. Twenty-four months after the initiation of the trial, the level of colonization of nodes in buried residue was half the level of colonization of residue on the soil surface. Colonization of node tissues by G. zeae decreased over time, but increased for other Fusarium spp. Ascospores of G. zeae were still produced on residue pieces after 23 months, and these spores were capable of inducing disease. Data from this research may assist in developing effective management strategies for residues infested with G. zeae.

Transgenic wheat expressing a barley class II chitinase gene has enhanced resistance against Fusarium graminearum

Fusarium head blight (FHB; scab), primarily caused by Fusarium graminearum, is a devastating disease of wheat worldwide. FHB causes yield reductions and contamination of grains with trichothecene mycotoxins such as deoxynivalenol (DON). The genetic variation in existing wheat germplasm pools for FHB resistance is low and may not provide sufficient resistance to develop cultivars through traditional breeding approaches. Thus, genetic engineering provides an additional approach to enhance FHB resistance. The objectives of this study were to develop transgenic wheat expressing a barley class II chitinase and to test the transgenic lines against F. graminearum infection under greenhouse and field conditions. A barley class II chitinase gene was introduced into the spring wheat cultivar, Bobwhite, by biolistic bombardment. Seven transgenic lines were identified that expressed the chitinase transgene and exhibited enhanced Type II resistance in the greenhouse evaluations. These seven transgenic lines were tested under field conditions for percentage FHB severity, percentage visually scabby kernels (VSK), and DON accumulation. Two lines (C8 and C17) that exhibited high chitinase protein levels also showed reduced FHB severity and VSK compared to Bobwhite. One of the lines (C8) also exhibited reduced DON concentration compared with Bobwhite. These results showed that transgenic wheat expressing a barley class II chitinase exhibited enhanced resistance against F. graminearum in greenhouse and field conditions.

Daily inoculum levels of Gibberella zeae on wheat spikes

The inoculum level of Gibberella zeae on wheat spikes was measured during 1995 and 1996 in nine locations of Canada and the United States prone to Fusarium head blight of wheat. Spikes were exposed after exsertion and until kernel milk or soft dough stage in fields with wheat or corn residue as a source of inoculum; other spikes were exposed in a location remote from any obvious inoculum source; and in 1995 only, control plants remained in a greenhouse. After 24 h, spikes were excised and vigorously shaken in water to remove inoculum. Propagules were enumerated on selective medium and identified as G. zeae from subcultures. Significantly more inoculum was detected from fields in epidemic areas than from remote sites in an epidemic and from fields in nonepidemic areas. The median inoculum level was 20 CFU of G. zeae per spike per day in fields experiencing an epidemic, 4 CFU in locations remote from epidemic fields, 2 CFU in nonepidemic fields, and 1 CFU in locations remote from a source of inoculum in non-epidemic areas. In an epidemic region, inoculum levels near corn stubble reached up to 587 CFU of G. zeae per spike per day, and the median inoculum level of 126 CFU was significantly higher than the median of 13 CFU found near wheat residue. Inoculum was not detected or occurred sporadically during extended dry periods. While inoculum increased during rainy periods, timing of increased levels was variable. Fusarium head blight epidemics were associated with multiple inoculation episodes and coincident wet periods.

Colonization of the Residues of Diverse Plant Species by Gibberella zeae and Their Contribution to Fusarium Head Blight Inoculum

The presence of Fusarium spp. was examined in the residues of wheat, barley, corn, sunflower, pasture, and gramineous weed species common in wheat and barley cropping systems collected from no-tillage and reduced-tillage plots from February 2001 to March 2003 in Uruguay. Gibberella zeae was recovered from residues of wheat, barley, corn, sunflower, fescue, and the gramineous weeds Digitaria sanguinalis, Setaria spp., Lolium multiflorum, and Cynodon dactylon, except from birdsfoot trefoil or white clover. Of the Fusarium spp. obtained, G. zeae was the most frequently recovered from wheat and barley residues, while other species were more common in other crops. G. zeae declined over time in all residues examined. Wheat and barley residues produced more ascospores of G. zeae than corn or other gramineous residues. Sunflower residue did not support ascospore production, indicating that it probably did not contribute to primary inoculum. Wheat and barley residues supported G. zeae colonization longer in no-till than in reduced-tillage production systems and, thus, may represent major contributors to Fusarium head blight (FHB) inoculum in Uruguay. The presence of G. zeae in the gramineous components of pastures, weed species, and sunflower should be considered when implementing control strategies for FHB.

Pathogen population structure and epidemiology are keys to wheat crown rot and Fusarium head blight management

This paper summarises the key findings from recent research on the population genetics and epidemiology of Fusarium pathogens causing head blight and crown rot of wheat in Australia and how this information has enabled the screening and selection of wheat germplasm with improved resistance to Fusarium. By relating new findings to the current state of knowledge, the paper serves as a timely and critical review of the international literature. In Australia, both Fusarium pseudograminearum and F. graminearum can cause both crown rot and Fusarium head blight under artificial inoculation. However, the former species is more widespread and is predominantly associated with crown rot whereas F. graminearum is mainly associated with Fusarium head blight, with limited geographical distribution in and around the Liverpool Plains in northern New South Wales. Studies of population structure and genetics have revealed that both species are genotypically diverse with similar levels of genetic recombination despite Gibberella zeae, the teleomorph of F. graminearum, being homothallic and G. coronicola, the teleomorph of F. pseudograminearum, being heterothallic. A high-throughput and reliable crown rot bioassay has been developed and used to screen over 1500 wheat germplasms to select 17 lines with putative crown rot resistance. Key differences in pathogen biology and epidemiology between Australia and the USA have emerged from other recent collaborative studies, which show that macroconidia constitute the bulk of aerial Fusarium head blight inoculum in Australia, whereas ascospores are the dominant primary inoculum for Fusarium head blight worldwide. The limited spread of splash-dispersed macroconidia of F. graminearum probably explains the restricted geographical distribution of this species in Australia. Other research collaboration has compared the aggressiveness, mycotoxin production and genotypic polymorphisms of the pathogen population from Australia and the USA. These and other differences in pathogen adaptation emphasise that research outcomes from elsewhere must be tested for relevance before applying them to Australian farming systems.

Efficacy and Stability of Integrating Fungicide and Cultivar Resistance to Manage Fusarium Head Blight and Deoxynivalenol in Wheat

Integration of host resistance and prothioconazole + tebuconazole fungicide application at anthesis to manage Fusarium head blight (FHB) and deoxynivalenol (DON) in wheat was evaluated using data from over 40 trials in 12 U.S. states. Means of FHB index (index) and DON from up to six resistance class-fungicide management combinations per trial (susceptible treated [S_TR] and untreated [S_UT]; moderately susceptible treated [MS_TR] and untreated [MS_UT]; moderately resistant treated [MR_TR] and untreated [MR_UT]) were used in multivariate meta-analyses, and mean log response ratios across trials were estimated and transformed to estimate mean percent control ( ) due to the management combinations relative to S_UT. All combinations led to a significant reduction in index and DON (P < 0.001). MR_TR was the most effective combination, with a of 76% for index and 71% for DON, followed by MS_TR (71 and 58%, respectively), MR_UT (54 and 51%, respectively), S_TR (53 and 39%, respectively), and MS_UT (43 and 30%, respectively). Calculations based on the principle of treatment independence showed that the combination of fungicide application and resistance was additive in terms of percent control for index and DON. Management combinations were ranked based on percent control relative to S_UT within each trial, and nonparametric analyses were performed to determine management combination stability across environments (trials) using the Kendall coefficient of concordance (W). There was a significant concordance of management combinations for both index and DON (P < 0.001), indicating a nonrandom ranking across environments and relatively low variability in the within-environment ranking of management combinations. MR_TR had the highest mean rank (best control relative to S_UT) and was one of the most stable management combinations across environments, with low rank stability variance (0.99 for index and 0.67 for DON). MS_UT had the lowest mean rank (poorest control) but was also one of the most stable management combinations. Based on Piephos nonparametric rank-based variance homogeneity U test, there was an interaction of management combination and environment for index (P = 0.011) but not for DON (P = 0.147), indicating that the rank ordering for index depended somewhat on environment. In conclusion, although the magnitude of percent control will likely vary among environments, integrating a single tebuconazole + prothioconazole application at anthesis with cultivar resistance will be a more effective and stable management practice for both index and DON than either approach used alone.