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Disease Cycle and Epidemiology

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The disease cycle of wheat stem rust starts with the exposure of each new wheat crop to spores of Puccinia graminis f. sp. tritici, which are the primary inoculum. The source of the first spores that infect the new wheat crop differs depending on the region in which the wheat is grown. In warm climates, wheat is planted in late fall and harvested in early summer. The first spores to infect the young wheat plants in the fall are urediniospores. They generally come from infected volunteer wheat plants. Seed spilled in the field or on roadsides at harvest time often sprout and produce scattered volunteer plants. These plants can become infected from spores produced on late-maturing wheat plants still in the field. The infected volunteer wheat plants serve as a bridge that carries P. graminis f. sp. tritici through the summer to the next fall-sown crop of wheat.

In regions with temperate climates, wheat may be planted either in the fall (winter wheat) or the spring (spring wheat) depending on the severity of the winters (Figure 12 - Click here to see the map series). For example, few winter wheat varieties can survive well through the severe winters of Minnesota, North Dakota, and Manitoba, so most of the wheat grown there is spring wheat. The first rust spores to infect wheat in the spring in temperate regions may be aeciospores from barberry, the alternate host, or urediniospores from infected wheat in distant regions with milder winters. Therefore, we describe two disease cycles for stem rust - with or without barberry.

Disease cycle, with barberry
Barberry is the most dangerous source of primary inoculum of stem rust in temperate regions. If barberry grows near wheat fields, it will be a consistent source of aeciospores for the earliest infections of wheat in the spring (Figure 13).


Figure 5	Figure 13

Puccinia graminis overwinters as black, thick-walled, diploid teliospores that are produced on wheat or other grass hosts toward the end of the growing season (Figure 5). Karyogamy (fusion of two haploid nuclei to form a diploid nucleus) and meiosis (reduction division to produce four haploid basidiospores) take place in the teliospore. Teliospores are produced in a telium.

In the spring, each teliospore germinates to produce thin-walled, colorless, haploid basidiospores (Figure 14). Basidiospores infect the alternate hosts such as common barberry.


Figure 14	Figure 15

Basidiospores germinate and produce a haploid mycelium which colonizes the leaf tissue. From this mycelium, pycnia are formed inside the leaf but with the tops extending through the surface, usually in the upper surface, of barberry leaves. Pycnia produce receptive hyphae and pycniospores (Figure 15). No further development will occur until the receptive hyphae in the pycnium are fertilized by pycniospores from a pycnium of a different mating type. Pycnia and pycniospores are referred to as spermagonia and spermatia by some authors, but the former are the preferred terms of rust specialists.


Figure 16	Figure 8

Pycniospores (Figure 16) are produced in a sticky honeydew that is attractive to insects and helps ensure that successful cross-fertilization occurs (figure 8). Insects carry pycniospores from one pycnium to another as they forage across the leaves feeding on the honeydew. Splashing raindrops also disperse pycniospores and aid in cross-fertilization. Fertilization of pycnia is critical in the rust fungus life cycle, because it gives rise to the dikaryotic mycelium. After the nucleus of the pycniospore joins that of the receptive hypha, the paired, haploid nuclei divide in tandem in the mycelium throughout the remaining stages of the life cycle. All stem rust infections of wheat or other grasses involve dikaryotic spores and dikaryotic mycelium.

Over a period of days, the dikaryotic mycelium grows through the barberry leaf until a new structure, the aecium, breaks through the lower surface of the leaf to release the dikaryotic aeciospores (Figure 10). Aeciospores, although produced on barberry plants, can infect only wheat or other grass host of P. graminis. Aeciospores (Figure 11) differ from urediniospores, which also infect wheat, in their appearance - slightly warty rather than spiny - and in the way in which they are formed - in chains in an aecium rather than on individual stalks in a uredinium.


Figure 10	Figure 11

On wheat, aeciospores germinate, the germ tubes penetrate into the plants, and the fungus grows as dikaryotic mycelium. Within 1 to 2 weeks, the mycelium in each infection produces a uredinium filled with brick-red, spiny, dikaryotic urediniospores that break through the leaf or stem epidermis (Figure 1).

Figure 1

In heteroecious rusts, this important spore stage is called the "repeating stage," because urediniospores are the only rust spores that can infect the host plant on which they are produced. Under favorable environmental conditions, multiple, repeated infections of the same wheat plant and neighboring wheat plants can result in explosive epidemics.


Figure 5	Figure 6

Toward the end of the growing season, black overwintering teliospores are formed in telia (Figure 5), and the life cycle is completed. Because karyogamy and meiosis take place in the teliospore (Figure 6), this spore stage is an important source of genetic recombination in addition to its role as a survival spore.

Figure 17

Disease cycle, without barberry
In North America, stem rust epidemics can occur in temperate regions even if barberry is not present (Figure 17). In the absence of barberry, the first spores of P. graminis to reach wheat in the spring are windborne urediniospores produced on winter wheat crops to the south (Figure 18). The mild climate along the coast of the Gulf of Mexico allows P. graminis to survive and spread in fields of winter wheat. Prevailing southerly winds in the spring carry the urediniospores north into the central Great Plains where they infect other winter wheat plants. Weather in the central Great Plains is usually too cold to permit stem rust infections during the winter. When spring wheat begins to grow in the northern Great Plains, it may be infected by windborne urediniospores from either the central or southern Great Plains. The stem rust disease cycle in the North ends with the wheat harvest.

Figure 18

In the South, the stem rust disease cycle starts with urediniospores that infect winter wheat seedlings after the fall planting. Most, if not all, of the primary inoculum is local. It comes from volunteer wheat plants that sprouted and became infected in the summer. Spread of urediniospores from north to south is not likely to be important. Spring wheat in the North is harvested in August, long before the new winter wheat crop has emerged in the South, where planting may not start until October or later. Barberry plants do not become infected in the South, so they are not a factor in stem rust epidemics there. This is because P. graminis teliospores will not germinate unless exposed to extended periods of freezing temperatures.

Epidemiology

Stem rust is favored by hot days (25-30^ºC/ 77-86^ºF), mild nights (15-20^ºC/ 59-68^ºF), and wet leaves from rain or dew. Both aeciospores and urediniospores require free water for germination as do the other spore stages. Infections occur through stomata.

Figure 13

The source of inoculum can be predicted from the pattern of the rust disease. If inoculum comes from barberry, a point source, the resulting disease pattern is usually fan-shaped with the alternate host at the apex of the fan (Figure 13). If disease has a more uniform pattern, the inoculum source is usually from a broad area, such as the southern wheat crops (in the northern hemisphere) from which urediniospores are released. Scattered infections mainly on the top leaves in a wheat field indicate that airborne spores were carried into the field from an external source. Rainfall is important for spore deposition during long distance dispersal of the spores.

If disease develops in individual foci within a wheat field, the source of urediniospores is probably overwintering mycelia and/or uredinia. Rusted plants in foci from overwintering sources have heavy infection in lower leaves and less infection in the younger leaves formed higher on the wheat plants.

In the absence of barberry or other alternate hosts, urediniospores are the only functional spores in the disease cycle of P. graminis. In tropical and subtropical climates, mycelium and urediniospores on volunteer wheat and noncrop grass hosts begin epidemics. Urediniospores are generally unable to survive harsh winter conditions. In the Northern Hemisphere, inoculum for spring wheat arrives from southern areas. In the Southern Hemisphere, urediniospores arrive from milder areas in the north. Occasionally, P. graminis can overwinter in wheat volunteers, noncrop grass hosts, and winter wheat, but usually only where snow cover insulates both the wheat leaves and the fungal mycelium. This is most likely to occur where winter wheat is planted directly into wheat stubble from the previous crop.

Urediniospores are produced approximately 7 to 15 days after infection, so there can be multiple generations of inoculum produced during a single growing season. One uredinium can produce at least 100,000 urediniospores. Explosive epidemics can occur during favorable environmental conditions, resulting in losses of 50 to 70% over a region.

Stem rust causes cereal yield losses in several ways. The fungus absorbs nutrients from the plant tissues that would be used for grain development in a healthy plant. As pustules break through the epidermal tissue, it becomes difficult for the plant to control transpiration, so its metabolism becomes less efficient. Desiccation or infection by other fungi and bacteria also can occur. Interference with the vascular tissues results in shriveled grains. Stem rust also can weaken wheat stems, so plants lodge, or fall over, in heavy winds and rain (Figure 19). Where severe lodging occurs, crops cannot be mechanically harvested.

Figure 19

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