|Scientific Name||Puccinia triticina Erikss. & Henn., Puccinia tritici-duri V. Bourgin|
|Synonyms||Puccinia recondita f.sp. tritici Dietel & Holw. [obs.]|
|Common Names||English: Brown rust, leaf rust; Spanish: Roya anaranjada del trigo; German: Braunrost; French: rouille brune du blé|
|Description||Leaf rust is caused by P. triticina, a basidiomycete belonging to the uredinales. It is a macrocyclic, heteroecious fungus, with five distinct spore stages. Like most rust fungi it requires two taxonomically diverse hosts to complete its life cycle.|
The disease is recognized by yellowish-brown to cinnamon-brown pustules scattered on the upper leaf surface. Small secondary pustules may develop in circles around older pustules on susceptible host cultivars. A halo of pale green or yellow appears around the pustules when host resistance is incomplete. It occurs mainly on the leaf blades, although leaf sheaths can also be infected under favorable conditions, high inoculum densities, and extremely susceptible cultivars. When the temperature increases, some pustules turn black due to the production of teliospores. Telia remain covered by the host epidermis and are blackish-brown in color. On the alternate host plant plants e.g. meadow rue (Thalictrum spp.speciosissimum) the pycnia are clustered in small groups on slightly swollen yellowish to reddish-brown areas on the upper leaf surface. Aecia are usually in clusters on gall-like areas on the abaxial surface of the leaf.
Urediniospores are orange to light brown, echinulate and globoid to broadly ellipsoid, 13-25 x 16-34 µm, with 6-10 scattered germ pores. They initiate germination 30 minutes after contact with free water at temperatures of 15° to 25°C. The germ tube grows along the leaf surface until it reaches a stoma and an appressorium is formed, followed immediately by the development of a penetration peg and a sub-stomatal vesicle from which primary hyphae develop. A haustorial mother cell develops and the first haustorium is formed inside the living host cell. Secondary hyphae develop resulting in additional haustorial mother cells and haustoria. The fungus may survive as mycelia when temperatures are near or below freezing. The teliospores of P. triticina are formed under the epidermis with unfavorable conditions or senescence and remain with the leaves. They are dark brown, two-celled with thick walls and rounded or flattened at the apex, 32-60 x 11-24 µm, mostly oblong-clavate, usually slightly constricted. Epidemics in areas where alternate hosts are functional occur when the overwintered telia from wheat produce basidiospores and infect the young leaves of Thalictrum spp. The mature teliospore represents the only diploid state of the fungus.
Germination of teliospores and subsequent meiosis in the basidium results in the formation of haploid basidiospores. Basidiospores are formed and released under humid conditions, which limit their spread. Basidiospores are also hyaline and sensitive to light, further limiting travel to probably tens of metres. Pycnia bearing the pycniospores and receptive hyphae are produced on the upper surface of Thalictrum leaves in 7-10 days. These gametes are heterothallic and are carried between pycnia by insects. Fertilization takes place when compatible spermatia come in contact with receptive hyphae; aecia are formed on the lower side of the infected leaves. The binucleate aeciospores are capable of infecting nearby wheat plants, completing the fungal life cycle.
Aerial dispersal occurs as urediniospores are passively released from uredinia on wheat leaves. The urediniospores become airborne and may be transported up to several hundred kilometres in an air mass before being deposited by gravity or washed out of the atmosphere by rain. Infections may also occur from urediniospores that survive between wheat crops or during the dormancy stage of winter wheat on volunteer wheat or native grasses. Urediniospores are spread from these areas by wind to cause annual recurrence of the disease in areas a few hundred kilometres away.
A second leaf rust caused by P. tritici-duri V. Bourgin occurs around the Mediterranean Sea and infects both durum and bread wheat. The pathogen is a member of the P. recondita complex and occurs in traditional agriculture systems where the alternate host plant Anchusa italica L. serves as the primary source of inoculum. This pathogen produces few urediniospores, which generally appear on the lower leaf surface. Epidemics are rather local, due to the lack of urediniospores. Abundant telia are produced in a ring around the initial uredinium.
Additional Crop Information
The primary host of P. triticina is T.riticum aestivum. (common bread wheat). It has generally been of lesser importance on T. turgidum L., (durum wheat), except in the Mediterranean, the Middle East, Ethiopia, Chile and India, where durum wheats are more extensively cultivated. Wheat leaf rust would also appear to be a major threat to triticale (X Triticosecale Wittmack).
Alternate hosts for P. triticina include Thalictrum speciosissimum, and species of Isopyrum, and Clematis, which are all in the Ranunculaceae family. Puccinia tritici-duri is a pathogen of T. turgidum and some T. aestivum cultivars. This leaf rust is limited to regions around the Mediterranean Sea where A. italica is common and where traditional methods of cultivation allow the perennial Anchusa to survive in wheat fields.
Under favorable weather conditions, one spore-uredinia-spore cycle may be completed in 7-8 days on a susceptible host. The disease develops rapidly at temperatures between 10° and 30°C. Leaf rust occurs to some extent wherever wheat is grown. Losses in grain yield are primarily attributed to reduced floret set and grain shrivelling. In highly susceptible genotypes, florets, tillers and plants can be killed by early (pre-heading) epidemics. Losses due to leaf rust are usually less than 10 %, but can be severe (30 % or more).
Integrated Crop Management
Strategies being used to manage rust epidemics include escape mechanisms, elimination of volunteer wheat, gene deployment, use of adult plant resistance, stacking of genes, or combinations of these strategies. Leaf rust control has been achieved mainly through the development of disease-resistant varieties. The recurrent varietal breakdown, known as the 'boom and bust' cycle, has led to the identification of several physiological races or virulences within P. triticina.
There are several fungicide classes available which are well suited to control brown rust infections.
Most commonly triazoles are used which have preventive but mostly also good curative activity.
Standard triazole fungicides for rust control are tebuconazole, cyproconazole, fluquinconazole, prothioconazole and triadimenol.
Alternatively, QoI fungicides such as trifloxystrobin and fluoxastrobin offer a good efficacy against brown rust. Further, amine fungicides such as spiroxamine show a considerable efficacy against rust diseases.
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