Puccinia graminis

Scientific Name Puccinia graminis Pers. f.sp. tritici Erikss. & Henn.
Common Names English: Stem rust, wheat stem rust, black stem rust; Spanish: Roya negra; German: Schwarzrost; French: Rouille noire
Description Stem rust is caused by P. graminis, a basidiomycete belonging to the uredinales. The fungus exists in several races. It is a macrocyclic, heteroecious rust, with five distinct spore stages.

Like most rust fungi it requires two taxonomically diverse hosts to complete its life cycle. The primary host is wheat.

Biology

Damage

First symptoms formed by the uredinia may occur on leaves, stems, leaf sheaths, spikes, glumes, and awns, but stems and leaf sheaths are the main tissues affected. On stems, the uredinia are elongated and reddish-brown; loose epidermal tissue is conspicuous at the margins of the uredinia. The uredinia coalesce to cover large areas of the host tissue in heavy infection. Since the orange-red urediniospores are dehiscent, they are readily released as masses from the uredinia when touched. The teliospores occur in the same tissue, but becomes shiny-black. The pycnial stage appears on the young leaves of the alternate host Berberis vulgaris initially as light, chlorotic areas on the adaxial leaf surface, then become light orange-brown lesions, consisting of individual small cone-shaped eruptions (the pycnia), often occurring in clusters. The aecia develop on the abaxial surfaces of these leaves. When mature, they appear as bright-orange, closely-packed, raised clusters of individual aecia. The aecia are cylindrical in shape and flare out at their apices, appearing as a grouping of rings within the aecial cluster.

Lifecycle

The fungus overwinters as teliospores in colder climates and urediospores on fall planted wheat in warmer climates. The teliospores remain with the straw until spring. The mature teliospore represents the only diploid state of the fungus. The germination of teliospores and subsequent meiosis in the basidium results in the formation of haploid basidiospores. Four basidiospores, two of each opposite mating types, are produced from each basidium. Basidiospores germinate on the surface of the alternate host Berberis vulgaris, penetrate directly through the host epidermis and form a haploid mycelium. The fruiting structure, formed as a result of basidiospore infection, is called a pycnium. The pycnia are normally formed on the adaxial leaf surface, often in clusters. The important features of the pycnia are the formation of flexuous (receptive) hyphae and haploid spermatia, which are exuded in a nectar. The nectar attracts insects, which in addition to splashing rain drops, serve to transport the spermatia to flexuous hyphae of the pycnia of opposite mating types, where fusion occurs. Following union of the opposite mating types, dikaryotization occurs. The spermatial nuclei migrate to the protoaecium, where mitosis occurs, the nuclei reassert into dikaryons and the aecial structure forms. The aecia of P. graminis are elongated, cylindrical structures. The ornamented, dikaryotic aeciospores are produced successively in chains from the aeciosporophores. The aeciospores infect wheat plants, completing the fungal life cycle. The aeciospores are the products of genetic recombination and may differ in their virulence and aggressiveness.

Cause

The uredinial stage is initiated by germination of a urediniospore in the presence of free water, formation of appressoria and penetration through stomata, development of an intracellular mycelium with intracellular haustoria, and subsequent sporulation of uredinia to form new urediniospores. The dikaryotic urediniospores of P. graminis are orange-red, thick-walled and covered with spines. They are elliptical and about 20 x 30 µm. As infected plants mature, teliospore formation commences, either in the same, or in new (telia), fruiting structures. At this stage, the infections become black, hence the name black rust. The teliospores remain attached. The teliospores are dark brown two-celled and somewhat wedge-shaped. They have thick walls, and measure 40 to 60 µm x 18 to 26 µm. The apical cell is rounded or slightly pointed.

Occurrence

Additional Crop Information

About 410 graminaceous species are known hosts for the P. graminis complex. Triticum aestivum L. and T. turgidum L. as cultivated wheats and triticale are the primary hosts of wheat stem rust. The main alternate herbaceous host for all forms of P. graminis is Berberis vulgaris. Allegheny barberries (B. canadensis) and some species of Mahonia also serve as alternate hosts of P. graminis f. sp. tritici.

Agricultural Importance

Stem rust is favored by humid conditions and warmer temperatures of 15° to 35°C. When appropriate conditions such as source and timing of inoculum, susceptible hosts and temperature occur, disease losses may become severe. The distribution of the pathogen is affected by prevailing climatic conditions, the movement of global air masses, geographical features, the availability of alternative grassy hosts or the alternate sexual host, and cropping practices. Urediniospores may be transported by wind over long distances, thus the occurrence of stem rust is only limited by the deposition pattern of the spores, suitable weather conditions and the availability of susceptible hosts.

Control

Integrated Crop Management

Genetic resistance is the most effective, least expensive and most environmentally safe means of control. When adequate genetic resistance to stem rust is achieved, no other control methods are necessary. The use of monitoring and prediction systems for the control of cereal rust diseases has not been very widely practised; the almost universal use of resistant cultivars has reduced the need for such systems. However, under an extreme threat of breakdown of resistance, accurate prediction is useful to devise counter measures. Early maturing cultivars also tend to escape heavy losses as a result of stem rust infection. Planting as early as possible and planting early-maturing cultivars help to reduce the time of exposure of the crop to the pathogen. The success or practicability of this approach depends on a detailed knowledge of the epidemiology of the rust in a particular area. Programmes to eradicate the alternate hosts Berberis vulgaris have had major effects in limiting stem rust epidemics in North America and in Europe. In areas where Berberis is a factor, and in the absence of general eradication programmes, growers should be alert for the presence of Berberis plants on or near their land, and remove them.

Chemical Control

A number of fungicides are highly effective against stem rust and have been used to successfully control the disease. Mostly azole fungicides such as tebuconazole, cyproconazole, fluquinconazole, prothioconazole and triadimenol are used for effective stem rust control. In addition, strobilurins such as trifloxystrobin and fluoxastrobin offer a good potential for the control of cereal rust diseases.
A third fungicide group available for rust control are the amine fungicides such as spiroxamine.

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