Puccinia triticina (wheat brown rust)

On wheat, the disease is recognized by small to relatively large, yellowish-brown to cinnamon-brown pustules scattered on both leaf sides, and rarely on the stem, leaf sheath or ears. Small secondary pustules may develop in a circle around older pustules on susceptible host cultivars. On resistant cultivars, pustules may be small or appear only as necrotic spots which do not develop spores. A halo of pale green or yellow appears around the uredinium when host resistance is incomplete (McIntosh et al., 1995). After several cycles of uredinial stages, some blackish-brown, oblong telia appear, usually on tissues that are dying, yellowed or brownish, and shrivelled. Telia remain covered by the host epidermis.

On Thalictrum, pycnia are clustered in small groups on slightly swollen, yellowish to reddish-brown areas on the upper leaf surface. Aecia are usually cupulate in clusters on gall-like areas on the undersurface of the leaf.

Host-Plant Resistance

Genetic resistance is the preferred method to reduce losses from leaf rust, and more than 70 leaf rust resistance (Lr) genes have been designated in wheat (McIntosh et al., 2008). Most Lr genes confer race-specific resistance in a gene-for-gene manner. However, wheat varieties relying on race-specific resistance often lose effectiveness within a few years by imposing selection for virulent leaf rust races (Bolton et al., 2008). Virulence surveys conducted worldwide bring information on the effectiveness of Lr gene combinations. Non-specific resistance, based on Quantitative Trait Loci, has largely been investigated over the past 30 years as a more durable and efficient source of resistance to be used by breeders.

Introduction

Leaf rust seldom kills wheat, but it is capable of causing 35-50% yield loss in endemic areas on susceptible cultivars, where severity levels of 25-40% are reached at the tillering stage and 100% at the flowering stage. The disease causes more damage worldwide than other wheat rusts. Quarantine is of no relevance as leaf rust is of worldwide occurrence and virulences spread freely between nations and zones. Crop losses are dependent on the genetic resistance of each cultivar, pathogen virulence and environmental conditions. Losses caused by leaf rust particularly originate from reductions of the wheat photosynthetic area. Infected plants normally produce a lower number of tillers, lower amounts of grains per head and smaller grains. The earlier the epidemic in the cropping season, the higher the yield losses. Mathematical models for estimating disease severity and crop losses have been developed based on multiple-point disease recording at different physiological stages of the plant (Burleigh et al., 1972; Eversmeyer and Kramer, 1998, 2000).

North America

When Huerta-Espino et al. (2011) claim that leaf rust causes serious production losses in almost all wheat production areas of the USA nearly every year, Oerke et al. (1994) mentions maximum losses of 3% during 1984-1988. For the same area, loss percentages mentioned in the literature can vary greatly according to the year. For example, in Kansas, they were estimated at 11-29% under favourable conditions (Oerke et al., 1994), at 5, 9 and 4%, in 1985, 1986 and 1987, respectively (Roelfs et al., 1992) and at 13.9, 4.7 and 1.37% in 2007, 2008 and 2009, respectively (Appel et al., 2009).

In Canada, estimated losses in 1970 were 5.1% in Manitoba and 3.6% in Saskatchewan (McDonald et al., 1971). Between 2000 and 2009, leaf rust occurred every year but severity varied from trace amounts to 22% flag leaf infection over this period. In unprotected fields, yield losses varied from trace to 10%. However, yield losses were minimized because foliar fungicides were commonly applied, primarily to control leaf rust and Fusarium head blight (Huerta-Espino et al., 2011).

In the Sonora and Toluca Valley of north-west Mexico, yield losses of >40% were recorded on the bread wheat cultivar Jupateco 73 following a strong epidemic in 1975-1976 (Dubin and Torres, 1981). Later on, the deployment of bread wheat cultivars possessing slow rusting resistance prevented leaf rust epidemics (Huerta-Espino et al., 2011). On durum wheat, epidemics during the 2000-2003 crop seasons caused losses estimated at $32 million (Singh et al., 2004a). Yield losses in the southern part of the state of Sonora in north-western Mexico during the 2008-2009 growing cycle were estimated at $40 million, including the cost of two fungicide applications.

South America

In South America, changes in the leaf rust pathogen population during 1996-2003 affected 10 cultivars and caused an estimated $172 million in yield losses in the region (German et al., 2004). Potential yield losses in areas with favourable weather conditions for the development of leaf rust in the Southern Cone of South America can exceed 50% if fungicides are not applied (Huerta-Espino et al., 2011). In Argentina, throughout the period 1982 to 1994, the annual leaf rust severity ranged from 0.4 to 43.1%, with a maximum at 80% (Galich and Galich, 1996a). In Chile, recorded losses were 5% in 1972-1973, and 36% in 1982 (Oerke et al., 1994). In southern Brazil, Barcellos (1982) determined wheat losses of 50%, comparing yields of 600 kg/ha in plots without fungicides, with 1200 kg/ha in treated plots. Yield losses up to 50% were reported by Reis (1994) in the state of Rio Grande do Sul.

Asia

The rust diseases of wheat pose a constant threat to sustainable wheat production in Asia. If susceptible wheat cultivars are grown, approximately 60 million hectares could experience periodic epidemics of leaf rust (Singh et al., 2004).

In China, wheat leaf rust occurs annually on about 15 m ha. Disease severity in commercial wheat fields is commonly 10-30% at the ripening stage, but can surpass 60% in some locations. Annual yield losses due to leaf rust are estimated to be 3 million tons. Fungicide application has become the principal control measure (Huerta-Espino et al., 2011).

Most areas of wheat cropping in India, Pakistan, Bangladesh and Nepal are at risk of leaf rust losses. Losses were negligible over a 5-year period due to widespread use of resistant varieties, especially in the more rust-prone areas (Huerta-Espino et al., 2011). In 1973, leaf rust intensities in Pakistan ranged from 40 to 50%, with 100% infection occurring on susceptible wheat varieties (Hassan et al., 1973). In 1978, a severe leaf rust epidemic in Pakistan caused a 10% production loss estimated at $86 million (Hussain et al., 1980). During 1971-1972 and 1972-1973 there were severe outbreaks of leaf rust in north-western India. Losses of 5.9, 24.1 and 2.0%, respectively, were estimated for varieties Kalyansona, K68 and Sonalika (Joshi et al., 1975).

In Russia, leaf rust causes yield losses especially in the Volga Basin, and the North-Caucasian and Central Chernozem regions, where it occurs annually and quite often reaches epidemic levels (Huerta-Espino et al., 2011). Losses up to 60% were recorded in Ukraine (Lesovoi et al., 1981) and 50% in Kazakhstan (Polyakov and Tanskii, 1980).

Europe

Damaging leaf rust levels have been noted regularly in western Europe (except the UK), Italy, and eastern and south-eastern Europe (namely former Yugoslavia and Romania) (Oerke et al., 1994). During the past 30 years, particularly in western European countries, fungicides have been used to control diseases including leaf rust in most years. Insignificant crop losses <0.1% due to leaf rust were recorded in the UK during 1970-1975 (Oerke et al., 1994) and during 1981-1988 (James et al., 1991). In Poland and Czech Republic, losses of 6.7% were recorded during 1983-1985, and in former Yugoslavia losses of 11-15% were recorded in 1985 (Oerke et al., 1994). In France, the latest strong epidemic in 2007 caused 30-40% yield losses (Maufras et al., 2014).

Africa

Leaf rust is an important disease in South Africa. However, low infection levels have been observed recently in farmers’ fields due to lower inoculum levels resulting from fungicide applications to control stripe rust, host resistance, and a non-conducive environment (Terefe et al., 2009). In Zimbabwe, losses of 6 to 16% were recorded (Pretorius and Kemp, 1988).

Leaf rust is also important in northern Africa, particularly in Morocco, Egypt and Tunisia (Huerta-Espino et al., 2011). In Egypt, yield losses of 11 to 23% (Nazim et al., 1984) or up to 50% (Abdel-Hak et al., 1980) were estimated. In Tunisia, potential losses were estimated at 30% (Deghais et al., 1999).

Oceania

Leaf rust of wheat occurs in all wheat-growing regions of Australia. Watson and Luig (1961) estimated losses in highly susceptible wheat cultivars at about 10%, whereas other studies indicated that the disease had the potential to cause much higher losses (Keed and White, 1971; Rees and Platz, 1975). Although historically more frequent in northern New South Wales and Queensland, the disease was controlled in this region for many years by resistant varieties. Foliar applications of fungicides have occasionally been used to control wheat leaf rust in Australia (Huerta-Espino et al., 2011).