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92 Plant Disease / Vol. 95 No. 2. doi: / PDIS The American Phytopathological Society

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During the last two decades, common bunt has re-emerged in low-input and organic wheat, most notably in northern and western Europe. Agriculture in Europe has been moving toward organic and sustainable,
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During the last two decades, common bunt has re-emerged in low-input and organic wheat, most notably in northern and western Europe. Agriculture in Europe has been moving toward organic and sustainable, low-input farming systems with reduced chemical inputs in crop production (74). Fundamental changes in agricultural production systems, such as the lack of chemical seed treatments, have caused the resurgence of many seedborne diseases, including common bunt, that were previously controlled with chemicals. In the United Kingdom, organic seed lots are predominantly contaminated with common bunt spores (94). In the Czech Republic, a 4-year monitoring of bunt incidence from grain samples showed an increase of bunt spores from various locations. This increased bunt incidence was observed in low-input and organic farms, and can also be related to changes that forced farmers to grow winter wheat at a higher percentage in crop rotations (124). Contamination of wheat with common bunt spores has resulted in considerable loss of yield and seed quality. In Romania, if untreated seeds are used, the incidence of common bunt can reach 70 to 80%, with yield losses up to 40% (21). Typically, yield losses almost equal disease incidence because wheat kernels have been replaced with bunt spores. Even cleaning the seed and sowing at higher soil temperatures cannot totally prevent the occurrence of common bunt (126). Given the epidemiology of the disease, it has the potential to cause economic devastation to low-input and organic farmers. The legal requirement for organic seed has compounded the bunt problem in Europe. For many years, it was possible to use conventionally produced seed as long as the cultivars were not of transgenic origin and the seed had not been treated after harvest with synthetic fungicides. All of this changed with Commission Regulation (EC) No. 1452/2003, which stipulated that beginning January 2004, all plant materials used for organic agriculture must be produced under organic farming conditions. With this regulation, a high level of expertise in disease management became requisite for Corresponding author: S. S. Jones, Department of Crop and Soil Sciences, Washington State University Mount Vernon Northwestern Washington Research and Extension Center, Mount Vernon, WA 98273; doi: / PDIS The American Phytopathological Society organic seed production. It is now crucial that seed and planting materials be pathogen-free and of superior quality, since most forms of synthetic chemical protection are not allowed. However, organic seed lots frequently do not make the grade and are often discarded because of contamination with common bunt (76). If this trend continues, there could be shortages of organic and certified seed. The limited supply of organic and certified seed might cause farmers to use seed saved from previous seasons. If farm-saved seed is contaminated with common bunt, the disease will build up further (124), especially if farmers do not monitor each successive crop for this disease (95). In conventional agriculture, common bunt is often exclusively controlled with chemical seed treatments. Given that these seed treatments are prohibited under organic certification standards, alternative treatments are being explored to manage common bunt under organic conditions. Tillecur is one of a few organic seed treatments that are effective, but these seed treatments also vary in efficacy, increase production costs (73), and often cannot be applied on a large scale. Under organic systems, the use of host resistance is a major component for sustainable disease management. However, there are limited numbers of wheat cultivars highly resistant to bunt that are adapted to organic systems. Most of the resistant cultivars have been bred under conventional agricultural systems, and might not be the best cultivars to use in organic farming. Studies have shown that these cultivars could lack important traits required under organic and low-input cropping systems (75,100,129). It is imperative, then, that selection for bunt resistance is conducted under organic farming conditions. Now, more than half a century after common bunt was thought to be vanquished, it has re-emerged in organic wheat. Today s farmers and scientists, like those in the past, are faced with the challenge of managing common bunt, but this time without chemical seed treatments. In this review, we present two main approaches that have been taken in managing the disease under organic systems: host resistance and seed treatments. Much of the research described here was conducted in Europe, Canada, West Asia, and North Africa. Even though common bunt has not yet reemerged in organic wheat in the United States, we believe that it is inevitable if conventionally produced seed will no longer be allowed on organic farms. We conclude this article by making recommendations for the control of common bunt consistent with the principles of organic agriculture. 92 Plant Disease / Vol. 95 No. 2 Fig. 1. Wheat head infected with common bunt, showing kernels replaced by sori or bunt balls. Photo by Margaret Gollnick. The Pathogen and Disease Common bunt is caused by two closely related fungi, Tilletia caries (D.C.) Tul. & C. Tul. (syn. Tilletia tritici (Bjerk.) G. Winter) and T. laevis J.G. Kühn (syn. T. foetida (Wallr.) Liro). The teliospores of T. caries have reticulated walls, whereas those of T. laevis have smooth walls. Although morphologically different, the two species are similar in germination requirements, life cycles, and disease symptoms produced. T. caries and T. laevis, together with T. controversa, the causal agent of dwarf bunt, could be variants of the same species, as proposed by several genetic, biochemical, and molecular studies (18). Common bunt is one of the most destructive diseases of wheat worldwide, causing considerable yield loss and reduction in seed quality. Common bunt is also called stinking smut, due to the production of trimethylamine, which gives the disease a distinct fishy odor even at contamination levels as low as 0.1% by volume (77). There is optimum infection when soil temperatures range from 5 to 10 C, but infection is reduced when soil temperatures are at 22 C (108). Teliospores on the seed or in the soil germinate and produce hyphae that infect the wheat coleoptiles before emergence. The fungus grows systemically in the plant and proliferates in the spikes when ovaries begin to form. The pathogen sporulates in the endosperm tissue until the entire kernel is converted into a bunt ball (sorus) consisting of a dark mass of teliospores. The bunt balls often break during harvest and grain handling, releasing teliospores that contaminate the seed and soil, thus initiating another cycle of infection. History of Bunt Control In 1750, the Royal Academy of Literature, Science and Arts of Bordeaux announced that a prize would be given for the best investigation into the smutting of wheat. Mathieu Tillet, Keeper of the Mint at Troyes, entered the contest. In his seminal experiments, Tillet planted wheat seed that he had dusted with the black spores and other seed that he had not. From the seed coated with black dust, he observed 50% or more smutted heads, while in the rows of clean seed, little or no smut developed (33). Tillet had found the answer. His experiments proved that the smut spores were infective: The outcome of the different experiments I have presented seems sufficient to persuade me that the disease [bunt] was contagious and that the virus was resident in the dust of the bunt balls (122). Tillet not only found the cause of the disease, but also some way to prevent it. He washed the seed grain in water, cattle urine, lye solutions, lime and salt, and finally, copper sulfate. Although none of these eliminated smut entirely, each helped to suppress it. For his groundbreaking experiments, with their remarkable scientific underpinnings, he won the prize. Unknowingly, he also laid the foundation of a new science, plant pathology, and had his name forever linked to smut of wheat: Tilletia. W. J. Farrer is acknowledged to be the first to apply systematic breeding methods to develop wheat cultivars resistant to bunt. He released Florence, which E. F. Gaines crossed with Turkey to produce Ridit, the first bunt-resistant cultivar in the Pacific Northwest (PNW) region of the United States (33). In the early 1900s to the 1960s, common bunt was the most destructive disease of wheat in the PNW, and its management was intensively studied. Pathogen genetics, pathogenic races, survival of spores in soil and spore germination, and the effect of seeding dates, tillage methods, and seed treatments on disease management, were studied (16). In Pullman, WA, work on wheat resistance to bunt began in 1914 (41). There was a concentrated search for resistant cultivars to form the core of a bunt breeding program (123), since the specificity of bunt resistance in wheat was long recognized before the discovery of specialization in the pathogen. In addition to screening wheat germplasm for bunt resistance, Gaines studied the genetics of bunt resistance (39,40). He also established the existence of physiologic races of the pathogen (42,43), as did Flor (34). Their work started decades of effort to gain the upper hand in bunt control. Plant Plant Disease / February breeders and plant pathologists spent years developing bunt-resistant varieties. However, each resistant variety released in the PNW would be subsequently attacked by new, virulent races of bunt (117). In turn, the resistant genes in the wheat cultivars influenced the racial population dynamics of the bunt pathogens (54). From these observations, it became clear that there is a gene-for-gene interaction between wheat and the common bunt pathogens. Dominant bunt resistance genes (Bt) in wheat hosts have corresponding dominant avirulence genes (avr genes) in the fungus. The genetic specificity of the wheat bunt pathogen interaction made it difficult to control the disease by host resistance alone, especially since all of the resistance at that time was race-specific. Since Tillet s time, numerous seed treatment methods have been used to control common bunt, such as salt brine, lime, mixtures of lime, salt, saltpeter, wood ashes, copper sulfate, formaldehyde, copper carbonate, and liquid mercury, all of which were either ineffective or too toxic to the seed, or to humans (91). In addition to these seed dressings, other physical seed treatments were tried, such as hot water treatment, originated by Jensen in 1888, and later, heat treatments in the form of steam (33). As early as 1807, Prevost had demonstrated that bunt could be controlled to some degree by copper sulfate. But it was not until the latter half of the nineteenth century that chemical disease control really started to gain ground (113). The development of the polychlorobenzenes, notable for their high specificity for certain fungi, specifically hexachlorobenzene (HCB), proved to be a powerful weapon in the control of common bunt. HCB was so effective against both seedborne and soilborne spores of bunt (60,107) that efforts toward its integrated control slowed dramatically after the introduction of HCB (16). In a matter of years, the new chemicals were widely adopted. Throughout the PNW and in much of the world, common bunt was finally controlled. The dreaded black harvest was no more. This classic, textbook disease was rarely seen in farmers fields, observed only when untreated or improperly treated seed was used (54,95). Common bunt had become a forgotten disease until its re-emergence in organic wheat. Host Resistance Breeding programs for common bunt resistance no longer exist in most wheat-growing countries. Under the assumption that the disease could be simply controlled by a single chemical seed treatment, breeding for bunt resistance has been given low priority in the United States, Europe, North Africa, and West Asia. Organic and low-input farmers must largely depend on crop cultivars produced for conventional farming (101), for which there is little information on bunt resistance. There is also limited knowledge on pathogenic variability. Therefore, current research on bunt resistance in organic wheat echoes research performed early in the twentieth century: monitoring bunt incidence and pathogen races, screening cultivars for bunt resistance, conducting studies on the mode of inheritance of bunt resistance, and searching for new sources of resistance. Employing molecular techniques, genes involved in resistant host response have been identified (84,85), and resistance genes have been mapped (35,96,127). Quantitative bunt resistance has also been investigated (35). New races and virulence patterns of common bunt isolates. Due to the gene-for-gene interaction that exists between specific bunt avirulence genes and bunt resistance genes in wheat, it is necessary to identify and monitor races of the pathogens. These races can be identified by inoculating them on differential cultivars, monogenic for bunt resistance genes. Their ability to infect specific cultivars within the set of differential cultivars will give a virulence pattern. This virulence pattern is analyzed and compared to the unique virulence patterns of the known races, as reported by Hoffman and Metzger (55). If the virulence patterns are unlike those of the known races, new races could then be postulated. The presence of new races in a certain area or the prevalence of known races in an area would help plant breeders determine what bunt resistance genes to deploy. It would also inform them what resistance genes to use or pyramid when developing new, resistant cultivars. Mamluk (86) reviewed reports on the prevalent common bunt races in Turkey, Egypt, Syria, Tunisia, Lebanon, Iran, and Morocco. In Turkey, 37 races were reported in 1981, and 88 in Five of the prevalent races in Turkey and two in Syria correspond to the North American races. Three isolates from Syria had new combinations of virulence patterns, and were reported to be new races (62). There are more recent reports of new bunt races from Iran, with four new races of T. laevis from the Khorasan Province (2) and nine new races from the Kermanshah Province (22). A systematic survey of common bunt incidence in the different agroecological zones in Iraq was conducted during the season. High disease incidence was observed in the central and southern regions of Iraq for the first time, although common bunt was thought to be restricted to the northern region. Movement of the disease to the central and southern regions could be ascribed to the use of contaminated wheat seed. Results of the survey also showed that T. caries was more widespread in the north, and T. laevis in the south (1). T. laevis is more prevalent in Romania, especially in the south, while T. caries is more common in the northwest, coming with the seed from Europe (104). Due to increased wheat monoculture, inappropriate use of chemical seed treatments, and the continual and rapid evolution in pathogen races, epidemics have become more common in small farms in Romania (104). In Ukraine, the dominant pathogen is T. caries, with the population consisting of 12 races. Seven of these races have virulence patterns similar to those of the known North American races T-1, T-2, T-3, T-7, T-9, T- 17, and T-20. Most of the wheat cultivars grown in the country are susceptible to these races (3). Other European workers have reported the virulence of the local bunt populations to the bunt resistance genes present in their germplasm collection, and against the differential cultivars (Fig. 1, Table 1). Germplasm resistance screenings were conducted for several years, and showed that most of the European bunt populations were virulent against the Bt resistance genes 1, 2, 3, and 7, while these could not attack the Bt genes 5, 8, 9, 10, and 11 (Table 1). In the United States, only five races are virulent on the Bt genes 5, 9, and 10, and none on Bt8, 11, and 12 (48). Screening of wheat germplasm for bunt resistance. Due to limited information on the resistance of registered wheat cultivars to bunt, several resistance screening studies have been performed in the last two decades. For these studies, wheat seed are inoculated by dusting with teliospores before sowing. Inoculated seed Table 1. Virulence of local populations of common bunt against resistance genes (Bt) from differential cultivars and wheat germplasm Source of common bunt population Years of screening Bt genes effective against the local bunt population Reference Hungary Bt5, Bt6, Bt8, Bt9, Bt Europe Bt3, Bt5, Bt6, Bt8, Bt9, Bt11, Bt12, Bt13 5 Austria and Germany Bt4, Bt5, Bt6, Bt8, Bt9, Bt10, Bt11, Bt12, Bt14 61 Poland , Bt4, Bt8, Bt11 72 Romania Bt5, Bt8, Bt9, Bt10, Bt11, Bt12, Bt Latvia Bt4, Bt5, Bt6, Bt8, Bt9, Bt11, Bt Plant Disease / Vol. 95 No. 2 are sown when the soil temperatures are 5 to 10 C, and bunt incidence is observed at plant maturity as the percentage of infected heads. In Canada, Gaudet and Puchalski (46) tested the field reaction of triticale, hard red spring, durum, and soft white spring wheat to common bunt, and found that triticale (wheat rye hybrid) was the most resistant among the cereals tested. Of the classes of wheat, durum (Triticum durum) was the most resistant, followed by hard red, soft white, and Canadian prairie spring wheats. Out of 22 CIMMYT lines and six Canadian wheat cultivars tested for bunt resistance, only four CIMMYT lines and one Canadian cultivar had low infection levels (47). In the PNW of the United States, wheat breeding lines have been screened for bunt resistance for over 25 years (6). However, most of these lines have not been specifically bred for organic farming and had been screened under non-organic conditions. The disease reaction of the cultivars should not change whether the screening was conducted under organic conditions or not. However, in Europe, most of the bunt resistance screening was conducted under organic conditions since the lines were being developed for organic farming. Most of the widely grown, local cultivars in Europe are susceptible to common bunt. In Serbia and Montenegro, only four out of the 12 most widely grown cultivars were resistant to common bunt (109). In the Czech Republic, Dumalasová and Bartoš (25 27) screened winter wheat and spring wheat cultivars for bunt resistance and found that it varied among cultivars, and also across years and locations. Spring wheats generally had less disease, probably due to the warmer soil conditions when the seeds are sown. Recently, they screened 17 newly registered wheat cultivars, and none of these were resistant to common bunt (29). In Lithuania, Liatukas and Ruzgas (78) determined that out of the 26 winter wheat cultivars registered in their country, none was highly resistant, and only two were moderately resistant. In order to initiate a breeding program for organic wheat, they increased the number of cultivars screened for bunt resistance, screening more than 2,000 cultivars over a period of 12 years (1993 to 2004), with some of the cultivars continuously screened for up to 8 years (79). More than 1,000 germplasm lines were screened during 2006 to 2008 against local populations of the pathogen. Their tests showed that only 1% of the genotypes tested were resistant to common bunt (81). In a separate screening of 347 breeding lines, only two lines were resistant (114). Moreover, the resistant lines were agronomically poor and could only serve as donors of resistance genes, indicating the need for a more intensive search for resistant germplasm. Their tests also showed that the local pathogen population in Lithuania possessed virulence to the majority of the genes studied (114). For most of these resistance screenings, workers have observed variation in bunt incidence among replicates and
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