Fusarium pseudograminearum is one of the pathogenic fungi that cause Fusarium crown rot (dryland foot rot) of winter and spring wheat. Winter wheat varieties and breeding lines were evaluated for 3 years in inoculated or non-inoculated paired plots at Pendleton and Moro, Oregon. Five pathogen isolates on millet-seed substrate were mixed and dispensed 1in above wheat seed at planting. Wheat seed treated with benomyl to suppress Fusarium damping-off was planted with a John Deere HZ drill equipped with cone-seeder on 14-inch row spacing. Plant stand, disease incidence and severity, and grain yield, test weight, and kernel weights were measured. Inoculation reduced average winter wheat yield at Pendleton by 12, 12, and 61 percent during 1999-2000, 2000-2001, and 2001-2002, respectively. Inoculation reduced winter wheat yield at Moro by 50 percent during 2001-2002. Commercial winter wheat varieties were ranked according to genetic tolerance; e.g., ability to produce grain yield even when infected by the pathogen. No varieties or breeding lines were considered resistant, e.g., ability to prevent infection by the pathogen. Spring wheat varieties and breeding lines were also tested at Moro during 2002. Six Australian varieties representing a range of plant tolerance ratings in that country were tested in four experiments in Oregon during 2000-2002. Performance of these varieties was comparable in Oregon and in Australia. Several Australian and Pacific Northwest wheat lines have levels of tolerance worthy of additional examination in wheat breeding programs.
Fusarium pseudograminearum, genetic resistance, genetic tolerance, spring wheat, winter wheat
Fusarium is a diverse genus of fungi that includes several soilborne plant pathogens. Fungi in several other genera cause similar damage and often are present as a pathogen complex with Fusarium species. These pathogens damage small-grain cereals by rotting seed, seedlings, roots, crowns, basal stems or heads of wheat, barley, oats, corn, grasses, and some broadleaf crops. Damage to spring and winter cereals is often unnoticed until whiteheads appear shortly before the crops mature or until shriveled grain is noted during harvest. Damage can be highly variable within fields. Yield reductions in commercial winter wheat fields have been as high as 35 percent.
The fungi included in this pathogen complex occur in different proportions in each geographic region. The prevalence of each species varies in response to annual climate cycles as well as more static differences among regions. The disease complex is therefore dominated by different pathogens in different areas, and even by different pathogens during successive growing seasons on individual fields. The dominant pathogen region-wide is F. pseudo-graminearum. Other important members of the pathogen complex in the Pacific Northwest include F. culmorum and Bipolaris sorokiniana. These pathogens cause chronic infections. Infected plants are often able to produce normal yields if they are not exposed to stressful environmental conditions during the growing season. Yield reductions become apparent when infected plants are subjected to water stress and/or hot temperature late in the growing season. It is ironic that the most widespread pathogen, F. pseudograminearum, is also well adapted for causing extensive damage in regions of high rainfall and in irrigated fields. These pathogens have therefore evolved to maintain their populations under diverse climatic conditions and management practices.
Diseases caused by these fungi are known by a variety of names, including Fusarium crown rot, dryland foot rot, Fusarium foot rot, dryland root rot, Fusarium root rot, and common root rot. We use the name Fusarium crown rot in this report. In winter wheat/summer fallow rotation, damage by Fusarium crown rot is favored by applying all nitrogen fertilizer into soil before planting, and by planting while the soil is still warm. The disease becomes most damaging in high-residue cropping systems and short rotations, which are conditions particularly applicable to annual-cropped spring cereals.
Management of damage by Fusarium crown rot has been heavily dependent on practices that conflict with preferred agronomic or economic considerations and with uncontrollable events related to weather. Management practices that minimize disease severity for winter wheat include tilling soil to minimize surface residue, planting when the seed-zone soil temperature is below 50oF, and reducing late-season water stress by planting with wide-row spacing and/or low seeding rate, and by splitting fertilizer applications to minimize the amount of nitrogen available to seedlings. Most of these management practices are contrary to practices known to produce highest grain yields when crops are not heavily infected by Fusarium and/or Bipolaris species. It is therefore recognized that control of Fusarium crown rot will only be achieved by crop management systems that include varieties with disease tolerance or resistance.
Genetic tolerance describes the ability of a plant to produce acceptable yield even when that plant is infected by the pathogen. Tolerance does not necessarily limit the ability of the fungus to infect or damage tissue, and it does not necessarily lead to a reduction in numbers of reproductive spores produced by the fungus on infected plants. Tolerance simply allows a plant to withstand the infection. It does not reduce the risk to future crops, and it may fail to adequately protect plants when climatic conditions and crop management are highly favorable for disease development. Tolerance is evaluated by comparing yields from plants grown under field conditions in soils highly or only lightly infested by the pathogen(s). Plants can be tolerant without being resistant to the disease.
Genetic resistance describes the ability of a plant to retard or prevent infection by the pathogen. Resistant plants sustain very little damage to their tissue. Resistant plants also reduce spore production by the pathogen, and therefore reduce the potential risk to future crops. Resistance is likely to be more effective than tolerance when conditions are highly favorable to disease. Resistance can be measured by evaluating the presence or absence of infection, or relative severity of infection, in either seedlings or in plants grown in the field.
The objectives of this study were to (1) determine whether tolerance or resistance was present among winter and spring wheat varieties and breeding lines in the Pacific Northwest, (2) describe the stability of tolerance over time if tolerance was detected, and (3) examine the tolerance in Oregon of wheat varieties used as standards for comparison in Fusarium crown rot breeding programs in Australia.
The relationship of Fusarium crown rot to growth and yield of 18 winter wheat varieties and advanced breeding lines was evaluated for 3 years at the Columbia Basin Agricultural Research Center. Long-term (20-yr) mean annual precipitation is 17.9 and 11.5 inch for stations at Pendleton and Moro, respectively. Soils are Walla Walla silt loams naturally infested with the pathogen. Trials were planted into different fields each year. All fields were maintained as cultivated summer fallow for 14 months following harvests of lentil (1999-2000 trial) or winter wheat (other trials). Seeds were treated with benomyl (Benlate 50W, at 0.75 oz/cwt) and planted at 23-25 seed/ft2 into 5- x 20-ft plots with a John Deere HZ deep-furrow drill equipped with a cone seeder and four openers spaced at 14 inches.
Seed was planted with and without supplemental inoculum consisting of five isolates of F. pseudograminearum from infected wheat crowns. Inoculum was placed in two of the four rows of each drill pass. Inoculum was grown on autoclaved millet seed. Individual isolates were mixed in equal proportions prior to use. Inoculum was dispensed at about 140 millet seed/ft from a Gandy spreader on the seed drill, and was placed 1-inch above the crop seed to force coleoptiles to emerge through the band of inoculum.
Wheat at Pendleton was planted on 24 September 1999, 22 September 2000, and 18 September 2001. Planting depth was 3-inch into marginally moist soil covered with a 1.5-inch dust mulch during 1999 and 2001, and 1.5-inch into moist soil during 2000. Wheat at Moro was planted on 1 October 2001 at 1.5-inch depth into moist soil. Temperature at the depth of seed placement, at the time of planting, was 59oF in 1999, 60oF in 2000, and 73oF in 2001 at Pendleton, and 59oF in 2001 at Moro. The experimental design was a split plot with variety as main plot and inoculum as subplots in blocks replicated four times. Data collected included emergence and stand density (October-December), plant growth and disease incidence and severity (March-May), whiteheads (June) and grain yield, test weight, and kernel weights (July-August). Plants were evaluated for grain yield by two methods. Genetic tolerance was assessed by harvesting plots with a plot combine and comparing yields of inoculated and noninoculated plots. Genetic resistance was assessed by digging plants from 3 ft of one row in each plot prior to machine harvest, and evaluating for each tiller the grain yield and the presence, absence, or degree of basal stem infection.
Effects of Fusarium crown rot were evaluated on early generation lines of winter wheat provided by Drs. Jim Peterson (Oregon State University) and Kim Campbell (USDA-Agricultural Research Service, Pullman, WA). Tests were performed on 75 wheat entries during 1999-2000 and 2000-2001, and 100 entries during 2001-2002. Test protocols and locations were as described above for the commercial varieties. Due to natural cycling of the selection process for early generation germplasm, only 32 of the 75 entries were tested during harvest year 2000 as well as 2001. Only 11 entries were in common for tests during harvest years 2001 and 2002. Only four entries were tested during all three harvest years. Yields were measured only by machine harvest for these early generation entries, e.g., bundle samples were not collected.
Spring wheat varieties or breeding lines (42 entries) were evaluated for reaction to Fusarium crown rot at Moro during 2002. Drs. Kim Kidwell (Washington State University) and Bob Stack (North Dakota State University) provided entries for the screening nursery. The 20-year mean annual precipitation is 11.5 inch at Moro. Seed was planted into cultivated summer fallow. Wheat seed was treated and plots were inoculated and planted as described for winter wheat. Wheat was planted at 1-inch depth into moist soil on 18 March 2002. Seed-zone temperature was 34oF. Data included emergence and stand density (April), plant growth and disease incidence and severity (May), whiteheads (June), and grain yield, test weight, and kernel weights (August).
Six Australian wheat varieties with published characterizations for reaction to Fusarium crown rot were evaluated for this trait in eastern Oregon soils and climates. These varieties are used by the Queensland Department of Primary Industries as “standards” in screening nurseries for crown rot, and have been assigned a numerical “Disease Resistance Index” against which other breeding lines are compared. Drs. Graham Wildermuth (Queensland Department of Agriculture) and Hugh Wallwork (South Australian Research and Development Institute) provided the seed. Reaction to Fusarium crown rot was evaluated during two growing seasons at the Columbia Basin Agricultural Research Center stations near Pendleton and Moro, where 20-year mean annual precipitation is 17.9 and 11.5 inch, respectively. All fields were maintained as cultivated summer fallow for 14 months following winter wheat harvest. Plantings were made as winter wheat during 2000-2001 and 2001-2002 growing seasons, and also as spring wheat during 2002. Wheat seed was treated and plots were inoculated and planted as described earlier. The experimental design was a split plot with variety as main plot and inoculum as subplots in blocks replicated two times during 2000-2001 and four times for trials during 2001-2002. Winter wheat at Pendleton was planted on 22 September 2000 at 1.5-inch depth into moist soil and on 18 September 2001 at 3-inch depth into marginally moist soil covered with 1.5 inch of dust mulch. Winter wheat at Moro was planted on 1 October 2001 at 1.5-inch depth into moist soil. Seed-zone temperature at Pendleton was 60oF in 2000 and 73oF in 2001, and at Moro was 59oF in 2001. Spring wheat was planted at 1-inch depth into moist soil at Pendleton on 14 March 2002 and at Moro on 18 March 2002. Seed-zone temperature was 40oF at Pendleton and 34oF at Moro. Data included emergence and stand density (October-December), plant growth and disease incidence and severity (March-May), whiteheads (June) and grain yield, test weight, and kernel weights (July-August).
Emergence at Pendleton during 1999 was slow and seedlings were stressed by low soil moisture; inoculum was in air-dry soil for 5 weeks until autumn rains began on 28 October. Rain began before planting at Pendleton during 2000 and Moro during 2001, and soils were continuously wet into the winter. Rain (0.36 inch) crusted soil prior to emergence at Pendleton during 2001. Growing-season precipitation (September-August) deviated from the 20-year mean during 1999-2000, 2000-2001, and 2001-2002 by +8, -7, and -27 percent at Pendleton, and -20, -46, and -26 percent at Moro. Spring-season precipitation (March-May) deviated from the 20-year mean during 1999-2000, 2000-2001, and 2001-2002 by +2, -17, and -32 percent at Pendleton, and +9, -22, and -35 percent at Moro.
Inoculum did not affect seedling emergence, plant stand, or plant tillering during the first 2 years at Pendleton or during 2001-2002 at Moro (p < 0.05; Table 1). Plant stands were strongly affected by an inoculum x soil crusting interaction at Pendleton during 2001; stand counts were 52 percent lower in inoculated than noninoculated plots (6.5 vs 13.4 plants/ft row, respectively; p < 0.001). Compared to plants affected by native levels of Fusarium, additional inoculum of the pathogen in all tests increased the incidence of plants with rotted crowns, the incidence and severity of lesions on subcrown internodes, and the percentages of whiteheads (Table 1). Inoculum reduced grain yield and usually also reduced test weight (Table 1).
Inoculation reduced yields of individual entries (Table 2) at Pendleton by 2-28 percent (11 percent mean) in 1999-2000, 0-22 percent (12 percent mean) in 2000-2001, and 32-85 percent (58 percent mean) in 2001-2002, and at Moro by 35-70 percent (55 percent mean). The greatest damage from Fusarium crown rot occurred on ‘Coda’, ‘Connie’, and ‘Lewjain’ (Table 3). Wheat varieties with overall best performance were ‘Brundage’, ‘Gene’, and ‘Weatherford’. However, even the best varieties were damaged by extraordinary drought that occurred during 2001-2002, and especially when fungus spores were present near the cotyledon at a time when emergence was retarded by soil crusting before seedling emergence.
All yield observations noted above represented genetic tolerance or intolerance to infection, based on machine harvest. None of the entries retarded the ability of the pathogen to infect crowns and stem bases, as assessed by evaluating disease and yield parameters on individual tillers collected from 3-ft-row sections in each plot. Therefore, genetic resistance to F. pseudograminearum was not detected among the 18 winter wheat entries in these tests.
Rainfall and soil crusting were as described for the commercial varieties. Inoculum did not significantly (p < 0.05) affect seedling emergence, plant stand, or tillering during 1999-2000 and was not evaluated during 2000-2001. Inoculation increased the incidence of plants with rotted crowns, lesions on subcrown internodes, and whiteheads, and also increased the severity ratings for lesions on subcrown internodes. Inoculation reduced grain yield (110 vs 101 bu/acre in 1999-2000 and 86 vs 78 bu/acre in 2000-2001), and grain test weight during 1999-2000 (58.9 vs 58.2 lb/bu) but not 2000-2001 (mean of 58.6 lb/bu). Inoculation reduced grain yields 9 percent (range of 2-29 percent) in the year with a dry autumn but overall wet season (1999-2000) and 11 percent (range of 0-28 percent) in the year with a wet autumn but overall dry season (2000-2001). Most of the 32 entries tested during both seasons exhibited variable yield responses to F. pseudo-graminearum each year. However, several entries had minimal yield depression from inoculation both years (Table 4), including ‘Altar 84’ (5 and 0 percent), ‘KS93U104’ (2 and 1 percent), ‘KS93U134’ (5 and 0 percent), ‘KS93U161’ (2 and 0 percent), ‘OR942494’ (7 and 6 percent), and ‘OR942504’ (0 and 0 percent). Some entries also had highly negative yield responses to F. pseudograminearum during both years, including ‘N96L1226’ (10 and 21 percent), ‘OR941345’ (19 and 10 percent), ‘OR9800919’ (18 and 21 percent), and ‘Stephens’ (23 and 13 percent).
Four of the 32 entries described above were tested at Pendleton and Moro during 2001-2002. None of those entries represented the highly tolerant entries tested during the two previous years. Likewise, 11 entries were examined during both 2000-2001 and 2001-2002. There was no commonality in response during the years of moderate and high disease pressure.
Fusarium crown rot reduced the grain yield for all market classes of wheat (data not shown). Average yield reductions were almost identical for market classes soft white common, soft white club, hard red, hard white, and soft red; reductions of 7-11, 10-12, and 51-61 percent occurred in 1999-2000, 2000-2001, and 2001-2002, respectively. Durum wheat had higher levels of grain yield reduction during 2 years (18 percent in 1999-2000 and 69 percent in 2001-2002) and a lower level of reduction (8 percent) during 2000-2001.
Growing-season precipitation (September-August) at Moro deviated from the 20-year mean during 2000-2001 and 2001-2002 by -46 and -26 percent, respectively. Spring-season precipitation (March-May) deviated from the 20-year mean during 2002 by -35 percent. Under these highly stressful conditions, inoculum had a particularly strong influence in elevating the disease severity and reducing grain yield. Entries had highly variable responses to Fusarium crown rot (Table 5). Some entries were heavily damaged (e.g., ‘WA7902’, ‘WA7901’, ‘Butte 86’) and others had little measurable impact (e.g., ‘Parshall’, ‘WA7925’, ‘ND744’). Since this report includes only one site and year for the spring wheat trial, it remains unknown whether these responses will vary over years and locations.
Rainfall conditions were as described for the winter and spring wheat tests. Inoculum did not affect seedling emergence, plant stand, or plant tillering (p < 0.05). Inoculum increased the incidence of rotted crowns, lesions on subcrown internodes, and whiteheads, and reduced grain yield. The varieties in Australia are classed as very susceptible (‘Puseas’100), susceptible (‘Vasco’91 and ‘Hartog’71), moderately resistant (‘Sunco’48 and ‘2-49’45), or resistant (‘Gala’49). Numbers in superscripts are numeric indices assigned to describe the relative level of resistance in Australia. All comparisons are made with respect to the response by ‘Puseas’, the “standard” for anticipated highest level of susceptibility for entries planted into each test. ‘Puseas’ was also the most susceptible variety in these Oregon-based tests, and ‘2-49’ and ‘Gala’ were most tolerant (Table 6). During four site-years, including inoculated and noninoculated stands grown as either winter or spring wheat, the Australian indices for six “standards” were correlated with percentage yield reduction caused by inoculation with F. pseudograminearum in Oregon (Fig. 1). Varietal responses to Fusarium crown rot in Australia and in Oregon were therefore similar; those varieties with highest resistance indices in Australia also had the lowest levels of differential yield reduction when additional pathogen was added to the soil in Oregon.
Genetic tolerance to Fusarium crown rot is important during years when the disease pressure is moderate. Tolerance is not needed when disease pressure is low, and is ineffective when disease pressure is high. Tolerance is, however, a very effective means for reducing economic damage during many years in areas where this disease causes chronic damage. Table 3 presents relative disease tolerance rankings for commercial varieties grown during years of diverse rainfall patterns. The best varieties were ‘Brundage’, ‘Gene’, and ‘Weatherford’. Others, including ‘Madsen’, ‘Temple’, and ‘Tubbs’ are also reasonable choices if they have more desirable performance criteria in a particular production area, compared to the most disease tolerant varieties.
Preliminary evidence suggests that germplasm exists that is capable of yielding very well in the presence of Fusarium crown rot. This was especially true for the early generation lines ‘OR942504’, ‘KS93U104’, and ‘KS93U161’. However, these lines were not tested during 2001-2002 when crown rot was most severe. Nevertheless, these lines should be examined further for their potential to serve as parents in breeding programs.
Durum wheat is noted internationally for being exceptionally susceptible to damage by Fusarium crown rot. Although few durum varieties have been tested in Oregon, this market class was represented by the most susceptible of winter wheat varieties tested (e.g., ‘Connie’), and by highly susceptible early generation breeding lines. However, great variability occurred among entries in every market class. It appears from these results that the heritage of individual lines may be more important than market class per se, with respect to damage that can be anticipated by Fusarium crown rot.
Spring wheat lines and varieties also differed greatly in response to Fusarium crown rot. These lines have not been tested for enough years to determine if the performance at Moro during 2002 is a stabile feature for these entries.
Australian wheat varieties were imported to determine if Pacific Northwest varieties were near the top, middle, or bottom of the range in crown rot susceptibility described overseas. Our tests show that local varieties span the full range of susceptibilities described in Australia. Also, these tests show that our tests can be compared to results of future research in Australia if the Australian varieties continue to be included in tests conducted in the Pacific Northwest. ‘Gala’ and ‘2-49’ are particularly interesting as potential parents for genetic tolerance in breeding programs. ‘Sunco’ performed quite well and is also a potential parent for breeding because it is used as a “standard of excellence” in end-use quality testing programs overseas. Each of these Australian varieties may prove useful in Pacific Northwest wheat improvement programs. Each of these varieties has produced much higher yields when grown as winter than spring wheat in the Pacific Northwest, and each has survived all winters tested thus far (from 1999-2000 to 2002-2003) at Pendleton, Moro, and Lind (WA).
As described earlier, genetic tolerance is useful during many years. But tolerance is likely to fail during years when protection is most urgently required. Also, it is not feasible to screen all or even high numbers of entries at the early-generation level. A program of that type would be much too costly and breeding programs cannot retain germplasm long enough for thorough testing for this disease alone. Breeding programs make judgments for a multitude of selection criteria. It became clear that entries with apparent tolerance to Fusarium crown rot were usually dropped from breeding programs based on other selection criteria. It was also clear that the number of entries tested in the field each year (<100) was much too small to have a reasonable chance to detect genetic resistance, which may occur in only a fraction of one percent of the germplasm. This screening program for tolerance to Fusarium crown rot therefore needed to be redirected.
Current efforts with Fusarium crown rot are being shifted to a search for genetic resistance. Field tests are not well suited to this pursuit. We are therefore adopting a system currently being used in Australia and Syria. That system involves testing large numbers of individual plants in small pots. Seedlings are started in the greenhouse and then are moved outdoors onto a sandbed for further growth and maturation. But in fact, almost all plants will die before maturing when seedlings are challenged by one or more of these pathogens. Thus, the numbers are reduced rapidly. Seed is collected from the few surviving plants and the test is repeated to determine if the seedling simply escaped being infected, as opposed to demonstrating true resistance. If the promising lines survive a second generation of screening, they are advanced to head row tests in the field, in a manner that we currently use to evaluate genetic tolerance. Lines that perform well in head row tests are transferred back to breeding programs for use in crossing blocks. Progeny of the crosses are then re-examined in the greenhouse screening system, or are examined using DNA marker-assisted breeding techniques to detect the gene(s) that confer resistance. This search for genetic resistance is being accelerated by developing collaborative testing protocols with international wheat research testing programs that are already using tests of this type to screen for resistance to Fusarium crown rot in winter and spring wheat.
This research was funded by the Oregon Agricultural Experiment Station and a grant from the USDA-Agricultural Research Service (CSA 58-5348-9-100 “Control of Root Diseases of Wheat and Barley”).
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