Sunday, September 7, 2008

Yellow rust or stripe rust

Stripe or yellow rust of wheat caused by P. striiformis f. sp. tritici can be as damaging as stem rust. However, stripe rust has a lower optimum temperature for development that limits it as a major disease in many areas of the world. Stripe rust is principally an important disease of wheat during the winter or early spring or at high elevations.

Symptoms of stripe rust are long stripes of small yellowish orange pustules on the leaves. These pustules consist of masses of rust spores. It can sometimes be confused with leaf rust or stem rust. Note that stem rust can occur on both the stems and the leaves of susceptible varieties. Stripe rust also goes by the name of yellow rust because it is a slightly lighter color than leaf rust or stem rust. Sometimes stripe rust symptoms are confusing on moderately resistant varieties because pustules may be hard to see or absent. In that case, symptoms can resemble bacterial leaf streak (black chaff) or Septoria leaf blotch.



Figure: Moderately Susceptible (typical striped appearance with no browning), Moderately Resistant (long brown lesions but few active pustules), Very Susceptible (stripes coalesced)

Puccinia striiformis has the lowest temperature requirements of the three wheat rust pathogens. Minimum, optimum and maximum temperatures for stripe rust infection are 0°, 11° and 23°C, respectively.





Hosts
Puccinia striiformis is a pathogen of grasses and cereal crops: wheat, barley, triticale and rye.





Alternate hosts
Only the telial and uredinial stages of stripe rust are known.

Life cycle
Puccinia striiformis is most likely a hemiform rust in that the life cycle seems only to consist of the uredinial and telial stages. Uredia develop in narrow, yellow, linear stripes mainly on leaves and spikelets. When the heads are infected, the pustules appear on the inner surfaces of glumes and lemmas.

The urediniospores are yellow to orange in colour, more or less spherical, echinulate and 28 to 34 ┬Ám in diameter. Narrow black stripes are formed on leaves during telial development. Teliospores are dark brown, two-celled and similar in size and shape to those of P. triticina. Stripe rust populations can exist, change in virulence and result in epidemics independent of an alternate host. Urediniospores are the only known source of inoculum for wheat, and they germinate and infect at cooler temperatures.



Disease cycle
Stripe rust over-summers on volunteer wheat. In the fall and winter it may develop in the southern U.S. near the Gulf coast on newly seeded wheat. In the spring, rust spores may blow north to the Central Plains. It is favored by cool, humid weather. Disease development is most rapid between 50 and 60 F. The disease is inhibited when night time temperatures get above 65 F or we have several days in a row in the mid 80's.

In central regions, such as Kansas, seldom have a significant problem with stripe rust for several reasons. First, stripe rust apparently does not over-winter in these areas so it must blow up from the south. In most years, there is not much stripe rust in south areas such as Texas or Oklahoma. Second, most of varieties have good resistance to stripe rust. Third, hot weather in May usually puts a halt to the epidemic before significant economic damage occurs.

Control

Control of stripe rust is through use of resistant varieties. Fortunately, most varieties in the Central Plains have good resistance. Foliar fungicides are essentially never used for stripe rust in most united states. However, Tilt, Quadris, and Stratego fungicides are all labeled for control of rusts, including stripe rust. Stratego has an early cut-off (flag leaf emergence), but Quadris and Tilt can be applied through fully headed.

Disease resistance





Historically Wilhelmina, Capelle-Desprez, Manella, Juliana and Carstens VI genotypes have maintained some resistance for many years. Most cultivars have remained resistant for five years or more, which is about the agronomic lifespan of a cultivar where an active breeding programme exists. However, some cultivars have rusted before they were grown on more than a fraction of the cultivated acreage. In most, if not all the cases, the failures have been due to inadequate knowledge of the virulences present in the pathogen population. In other cases, mutations or perhaps a recombination of existing virulence combinations occurred and rendered the host susceptible. In some instances, the disease screening protocol is inadequate to identify and select the resistant wheat lines. Yellow rust resistant genes are designated as Yr genes.





More about leaf rust


http://www.fao.org/DOCREP/006/Y4011E/y4011e0g.htm

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