Field Resistance to the Ug99 Race Group of the Stem Rust Pathogen in Spring Wheat Landraces

Wheat landraces provide a source of genetic variability for breeding. The emergence and spread of highly virulent races of the stem rust pathogen (Ug99 race group of Puccinia graminis f. sp. tritici) threaten wheat production globally. Spring wheat landraces were screened for resistance in eight field seasons at the Kenya Agricultural Research Institute, Njoro, where the Ug99 race group has become endemic. Accessions showing resistance in one season were retested and screened with molecular markers associated with resistance genes Sr2, Sr24, Sr36, and Lr34/Yr18; two height-reducing genes; and a photoperiod insensitivity allele. Of 2,509 accessions tested, 278 were categorized as resistant based on results from at least two seasons. Of these resistant accessions, 32 were positive for one or more markers for Sr2, Sr36, Rht-B1b, or Rht-D1b, indicating that they do not fit the definition of "landrace" because these genes were likely introduced via modern breeding practices. Thus, 246 resistant "landrace" accessions were identified. Of countries with more than five tested accessions, Afghanistan, Iran, Portugal, Ethiopia, Uzbekistan, Greece, Tajikistan, Bosnia and Herzegovina, and Serbia had at least 10% of tested accessions that were resistant to the Ug99 race group. Future research will characterize the resistance to determine its novelty and incorporate novel genes into improved lines.

Identification of New Pathogenic Races of Common Bunt and Dwarf Bunt Fungi, and Evaluation of Known Races Using an Expanded Set of Differential Wheat Lines

Pathogenic races of Tilletia caries and T. foetida, which cause common bunt of wheat (Triticum aestivum), and Tilletia contraversa, which causes dwarf bunt of wheat, have been identified previously by their reaction to 10 differential wheat lines, each containing single bunt resistance genes Bt1 through Bt10. The reactions of races to the differential wheat lines follow the classic gene-for gene system for host-pathogen interactions. The pathogens are closely related and resistance to both diseases in wheat is controlled by the same genes. To better define pathogenic races, six additional wheat lines containing the genes Bt11 through Bt15 and a wheat line with a resistance factor designated as Btp were added to the set of 10 differentials and tested with all named U.S. races of common bunt and dwarf bunt. In addition, new isolates of dwarf bunt, and common bunt from hybrids and field collections, were tested with all 16 differentials for race identification. Six new races of T. caries, five new races of T. foetida, and two new races of T. contraversa were identified. Races of common bunt virulent to Bt8 or Bt12, and dwarf bunt races virulent to the combinations of Bt11 and Bt12, and Bt8, Bt9, Bt10, Bt11, and Bt12, were identified for the first time. Comparison of the reactions of the common bunt races with the Bt14 and Bt15 differentials grown in different environments after initial infection showed that these genes are temperature sensitive, indicating they should be excluded from the set of differential lines to avoid ambiguity in determining virulent or avirulent reactions. In the previous list of bunt races, there were races that had the same reaction to the set of 10 differentials but were designated as different races. These races were not differentiated further with the six additional differentials, indicating that the duplicate races should be dropped from the list of pathogenic races. The new races of common bunt and dwarf bunt identified have unique patterns of virulence that allow specific targeting and elucidation of bunt resistance genes in wheat and will aid the development of bunt-resistant wheat cultivars.

Analysis of Induction and Establishment of Dwarf Bunt of Wheat Under Marginal Climatic Conditions

Dwarf bunt caused by Tilletia contraversa is a disease of winter wheat that has a limited geographic distribution due to specific winter climate requirements. The pathogen is listed as a quarantine organism by several countries that may have wheat production areas with inadequate or marginal climate for the disease-in particular the People's Republic of China. Field experiments were conducted in the United States in an area of Kansas that is a climatic analog to the northern winter wheat areas of China to evaluate the risk of disease introduction into such areas. The soil surface of four replicate 2.8 × 9.75 m plots, planted with a highly susceptible cultivar, was inoculated with six teliospore concentrations ranging from 0.88 to 88,400 teliospores/cm². A single initial inoculation was done in each of three nurseries planted during separate seasons followed by examination for disease for 4 to 6 years afterward. Any diseased spikes produced were crushed and returned to the plots where they were produced. One nursery had no disease during all six seasons. In two nurseries, the disease was induced at trace levels at the three highest inoculation rates. Disease carryover to the second year occurred during one year in one nursery in plots at the highest inoculation rate, but no disease occurred the following three seasons. A duplicate nursery planted in a disease conducive area in Utah demonstrated that the highest rate of inoculum used in the experiments was sufficient to cause almost 100% infection. This study demonstrated that in an area with marginal climatic conditions it was possible to induce transient trace levels of dwarf bunt, but the disease was not established even with a highly susceptible cultivar and high levels of inoculum. Our results support the conclusions of the 1999 Agreement on U.S.-China Agricultural Cooperation which set a tolerance for teliospores in grain, and supports the Risk Assessment Model for Importation of United States Milling Wheat Containing T. contraversa.

Survival of secondary sporidia of floret-infecting Tilletia species: implications for epidemiology

Secondary sporidia of Tilletia horrida, T. indica, and T. walkeri initiate local infection of rice, wheat, and ryegrass florets, respectively, leading to disease in seed. Secondary sporidia are considered to be fragile and short lived. To examine this, secondary sporidia from agar cultures of these species were naturally discharged onto petri dish lids and were air-dried and maintained in the laboratory at 10 to 20% relative humidity (RH) at 20 to 22 degrees C, and at 40 to 50% RH at 18 degrees C. Lids were periodically inverted over fresh agar to determine viability of dried sporidia. Sporidia held 31 to 49 days at 10 to 20% RH and 56 to 88 days at 40 to 50% RH regenerated rapidly. Commonly, 18 h after lids with dried sporidia were inverted over agar, newly produced secondary sporidia had discharged onto the agar and produced extensive hyphal growth. There was no difference in the viability of sporidia that were initially dried rapidly or dried slowly over 10 h. Sporidia of T. horrida or T. indica dried on petri dish lids placed in the lower canopy of barley or wheat fields in Idaho and Arizona during early flag leaf to soft dough stages and held until crops were near or beyond maturity regenerated rapidly despite temperatures up to 46 degrees C and several days of RH < 20%. These results suggest that sporidia produced well prior to susceptible growth stages of the host can lay dormant in very dry field environments and then rapidly regenerate under humid rainy conditions associated with the diseases.

Effect of biofumigation with volatiles from Muscodor albus on the viability of Tilletia spp. teliospores

Volatile organic compounds produced by the fungus Muscodor albus inhibit or kill numerous fungi. The effect of these volatiles was tested on dormant and physiologically active teliospores of the smut fungi Tilletia horrida, Tilletia indica, and Tilletia tritici, which cause kernel smut of rice, Karnal bunt of wheat, and common bunt of wheat, respectively. Reactivated rye grain culture of M. albus was used to fumigate dormant teliospores in dry Petri dishes and physiologically active teliospores on water agar for up to 5 days at 22 degrees C. Teliospores of all 3 species were incapable of germination when fumigated on agar for 5 days. When T. tritici on agar was fumigated only during the initial 48 h of incubation, viability was reduced by 73%-99%. Fumigation of dry loose teliospores of T. tritici caused a 69%-97% loss in viability, whereas teliospores within intact sori were not affected. Dormant teliospores of T. horrida and T. indica were not affected by M. albus volatiles. It appears that M. albus has potential as a seed or soil treatment for controlling seedling-infecting smuts where infection is initiated by germinating teliospores prior to seedling emergence. The volatiles were not effective for postharvest control of teliospores under conditions used in these experiments.

Susceptibility of Wheat to Tilletia indica During Stages of Spike Development

ABSTRACT Karnal bunt of wheat is caused by the fungus Tilletia indica, which partially converts kernels into sori filled with teliospores. Despite minor overall yield and quality losses, the disease is of considerable international quarantine concern. Plant development stages reported susceptible to infection vary considerably. A study was designed to better define the susceptibility period by inoculating wheat spikes at different growth stages with naturally liberated secondary sporidia under optimal conditions for disease development. Spikes of a resistant and susceptible cultivar were inoculated at eight growth stages from awns emerging to soft dough. Spikes became susceptible only after emerging from the boot and continued to be susceptible up to soft dough stage at which low levels of disease occurred. Disease severity in both cultivars peaked when spikes were inoculated after complete emergence, but before the onset of anthesis. Disease levels tapered off gradually in spikes inoculated after anthesis. The results broaden the known susceptibility period of wheat to T. indica to include stages long after anthesis, and indicate that infection from airborne inoculum is not possible during boot or awns emerging stages, which are commonly referred to as the most susceptible stages.

Nonsystemic bunt fungi--Tilletia indica and T. horrida: a review of history, systematics, and biology

The genus Tilletia is a group of smut fungi that infects grasses either systemically or locally. Basic differences exist between the systemically infecting species, such as the common and dwarf bunt fungi, and locally infecting species. Tilletia indica, which causes Karnal bunt of wheat, and Tilletia horrida, which causes rice kernel smut, are two examples of locally infecting species on economically important crops. However, even species on noncultivated hosts can become important when occurring as contaminants in export grain and seed shipments. In this review, we focus on T. indica and the morphologically similar but distantly related T. horrida, considering history, systematics, and biology. In addition, the controversial generic placement and evolutionary relationships of these two species are discussed in light of recent molecular studies.

Relationship Between Soilborne and Seedborne Inoculum Density and the Incidence of Dwarf Bunt of Wheat

The incidence of dwarf bunt of wheat as a function of inoculum density was studied in a susceptible and a partially resistant cultivar at three disease-conducive locations for three seasons. Prior to seeding, plots were fumigated with methyl bromide to eliminate residual inoculum. Each cultivar was seeded into two 1.2-m rows in four replicates. The soil surface was inoculated with 0, 16 × 10², 16 × 10³, 16 × 10⁴, 16 × 10⁵, and 16 × 10⁶ teliospores of Tilletia controversa per row, or seed was inoculated with 0, 2 × 10², 2 × 10³, 2 × 10⁴, 2 × 10⁵, and 2 × 10⁶ teliospores per gram. To determine maximum possible infection, two 3.1-m rows of each cultivar were soil-surface inoculated at 10× the highest treatment rate. In the soil-inoculated plots, a minimum of 16 × 10³ teliospores/row was needed to cause trace amounts of disease (0.6% maximum), even when the positive indicator treatment had up to 88% incidence. Only trace amounts or no disease occurred below the 16 × 10⁵ rate. In the seed-inoculated plots, infection was rare and occurred only at inoculation rates of 2 × 10⁵ teliospores/g or higher; the highest incidence was 0.4%.

Nuclear behavior during teliospore germination and sporidial development in Tilletia caries, T. foetida, and T. controversa

Germinating teliopspores of Tilletia caries, T. foetida, and T. controversa were stained with acetic orcein and examined with light microscopy. The general nuclear events were the same in the three species. Nuclei in teliospores stained only after a period of incubation. Teliospores contained a single interphase nucleus, which underwent meiosis and then zero to two synchronous mitotic divisions. At metaphase I – anaphase I, approximately four chromosomes were visible in each species. The nuclei migrated from the teliospore into the promycelium and then each migrated into a primary sporidium. At this stage there were usually fewer nuclei than primary sporidia. The nuclei divided almost synchronously in the primary sporidia, after which one of the daughter nuclei in each primary sporidium migrated to the tip of the promycelium and then into any remaining anucleate primary sporidia. Supernumerary nuclei remained in the promycelium or in the primary sporidia and eventually lysed. Mature primary sporidia usually contained one interphase nucleus. When T. caries and T. foetida were germinated at 5 as compared with 15 °C, the nuclei in most teliospores underwent one less synchronous division and the number of primary sporidia was reduced.

Spindle pole body fusion in the smut fungus Tilletia foetida

Fusion of double-structured, interphase spindle pole bodies (SPBs) occurred before nuclear fusion in heterokaryotic secondary sporidia. The SPBs of two separate nuclei were juxtaposed with their long axes perpendicular to each other. Also, SPBs were observed oriented with their long axes parallel and fused to each other at both ends. Fusion apparently continued toward the midportion of the SPBs. Nuclei were observed joined together in a narrow region. These nuclei appeared to share a single SPB that was located opposite to a protuberance on both nuclei. Following fusion, the SPB apparently returned to an interphase structure.

Ultrastructure of mitosis and the spindle pole body cycle in the smut fungus, Tilletia foetida

The interphase nucleus in secondary sporidia of Tilletia foetida consists of mostly diffuse chromatin, one or two nucleoli, and an area of heterochromatin located opposite an electron-dense, extranuclear spindle pole body (SPB). The interphase SPB is an oval- to bar-shaped, double-structured disc that has a crystallinelike substructure. During nuclear migration into nascent sporidia, SPBs and nucleoli are randomly oriented. At the onset of division, chromatin begins to condense and the SPB becomes located on a nuclear protuberance. Cytoplasmic microtubules terminate at the SPBs and multivesicular bodies surround the SPBs from the early stages of SPB division to early postdivision. SPB discs become spheroid and each develops a medial, dense layer. Then, a basal, dense layer develops and elongates as the SPBs separate and become positioned on opposite sides of the nuclear protuberance. The nuclear membrane opens opposite the SPB during SPB division. The nucleolus is extruded into a nuclear bleb and degenerates. SPBs migrate to opposing sides of the nucleus and become diffuse as a microtubular spindle develops between them. Some spindle microtubules terminate at dense chromatin patches that are contiguous with the major mass of chromatin surrounding the spindle. During late division stages, spindle microtubules often appear to be closely juxtaposed. Except for polar openings adjacent to the SPBs, the nuclear membrane is entire until late division when it degenerates in the midregion of the nucleus. During early postdivision, the SPB condenses into a small, dense sphere as the chromatin and heterochromatin opposite the SPB become diffuse. The SPB then elongates into a dense bar and SPB material increases, except at the midportion, reforming the double structure of interphase.