Tuesday, September 9, 2008

soybean-rust important disease in USA

tan spot - symptoms and control

Tan Spot
Causal organism - Pyrenophora tritici-repentis

Wheat but can also attack barley, rye and some grasses.
Tan spot can be seed borne and infect seedlings, resulting in small tan to light brown flecks on young leaves. Symptoms are normally seen later in the season in the middle and upper canopy. First symptoms of infection are small tan to light brown flecks, with a chlorotic halo, often with a dark spot at the centre. Later these develop into light brown oval lesions with slightly darker margins with a light coloured spot at the centre. Under wet conditions the lesions produce spores which can make lesions darker in colour. Under ideal conditions these lesions coalesce to produce large areas of dead tissue.

Cultural control
The disease is greatly favoured my minimum tillage systems as the disease survives mainly on crop debris left on the soil surface. Disposal of crop debris by ploughing can help prevent early infection. Many varieties are susceptible.

Chemical control
Seed infection is controlled by most seed treatments. It is essential to protect the upper leaves from disease by appropriate fungicide sprays when wet weather occurs between GS32 and GS39. Sprays applied against Septoria will normally control tan spot although timings are more critical as tan spot has a very short latent period (5-7 days).

Disease resistance

Life cycle

Fig: Pseudothecia developed in wheat stem

Tan Spot survives mainly as dormant mycelium on stubble and crop debris. This produces pseudothecia on stubble which produce ascospores for long distance spread. In the absence of crop debris, initial infections in the autumn or spring may result from seed borne infection but this is not thought to be a major source of inoculum. Under warm, wet conditions, leaf spots produce dark conidia which are spread up the plant under wet conditions. The disease can infect the ear and cause discoloration of the glumes and the grain. Symptoms on the head are indistinct but can cause brownish glumes. Infected grains can have a reddish appearance, similar to fusarium infection. The disease develops over a wide range of temperatures but has quite a high optimum (20-28oC) and is favoured by long periods (18 hours or more) of dew or rain.

scoring stripe rust

Scoring stripe rust

Detailed outlines for recording stripe rust intensities in cereals are based upon:
Severity (percentage of rust infection on the plant) and

Field response (type of disease reaction).
Severity is recorded as a percentage, according to the modifed Cobb scale. This recording process relies upon visual observations, and it is common to use the following intervals: Trace / 5 / 10 / 20 / 40 / 60 / 100 percent infection.

Field response is recorded using the following letters:
O - No visible infection on plant.
R- Resistant: visible chlorosis or necrosis, no uredia are present.
MR- Moderately Resistant: small uredia are present and surrounded by either chlorotic or necrotic areas.
M- Intermediate: variable sized uredia are present; some with chlorosis, necrosis, or both.
MS- Moderately Susceptible: medium sized uredia are present and possible surrounded by chlorotic areas.
S- Susceptible: Large uredia are present, generally with little or no chlorosis and no necrosis.
Severity and field response readings are usually combined.
For example:
tR = Trace severity with a resistant field response.
5MR = 5% severity with a moderately resistant field response.
60S = 60% severity with a susceptible field response.

Peterson R. F., Campbell A. B. and Hannah A. E. 1948. A diagrammatic scale for rust intensity on leaves and stems of cereals. Can. J. Res. 26, 496–500.

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.

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 µm 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 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


Cereal Rusts and their hosts

Bread Wheat(Triticum aestivum) / Durum Wheat(Triticum turgidum)

stem rust - Puccinia graminis f.sp. tritici - Barberry (Berberis vulgaris, B. canadensis)

leaf rust - P. triticina (P. recondita f. sp. tritici) - Meadow Rue (Thalictrum speciosissimum = T. flavum ssp. glaucum)

leaf rust - P. tritici-duri - Anchusa italica . Occurs only in the Mediterranean Area

stripe rust - P. striiformis f. sp. tritici - unknown

stripe rust - P. striiformis f. sp. hordei (rare)

Rust resistant genes

Wheat rusts

Leaf rust - Stem rust-yellow rust

Updated nomenclature of rust resistant genes (Lr, Sr, Yr)


Sequenced / mapped rust resistant genes - grain gene database

Nomen clature systems of rusts -
Stem rust- Phytopathology 78:526-533

Leaf rust - Phytopathology 79:525-529

Race surveys in united states

stem rust

Stem rust

Stem rust was once the most feared disease of cereal crops. It is not as damaging now due to the development of resistant cultivars, but outbreaks may occur when new pathogen races arise against which the existing kinds of resistance are ineffective. Stem rust remains an important threat to wheat and barley and, thus, to the world food supply. Anton deBary first demonstrated the heteroecious life cycle of a rust fungus with Puccinia graminis, the causal agent of stem rust.

COMMON NAME:Stem rust, black rust

SCIENTIFIC NAME: Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. E. Henn.

SYMPTOMS: Uredinia generally appear as oval lesions on leaf sheaths, true stem, and spike. Uredinia can appear on the leaves if other diseases have not killed them. Uredinia are brick red in color and can be seen to rupture the host epidermis, on the leaves uredinia generally penetrate to sporulate on both surfaces. Infected areas are rough to the touch.

ENVIRONMENTAL CONDITIONS: Stem rust is favored by hot days 25-30 C, mild nights 15-20 C with adequate moisture for night time dews. Wind can effectively disperse urediniospores over great distances. Rain is necessary for effective deposition of uredinospore involved in regional spore transport.


Aeciospores from Berberis vulgaris are currently rare, but historically it was an important source of inoculum in northern North America and Europe.

Mycelium or uredinia on volunteer wheat, are the most important source of inoculum in tropical and subtropical climates. Windblown urediniospores are usually from earlier maturing wheat from the south in the northern hemisphere, or from the north in the southern hemisphere.

Urediniospores and aeciospore germinate when in contact with free water. Infection by penetration through the stoma. Penetration requires at least a low light intensity. Germination optimum is 18 C, latent period varies from 10 to 15 days in the field with temperatures of 15-30 C.

HOSTS: wheat and barley, common barberry (and some additional Berberis, Mahoberberis, and Mahonia spp.)

SURVIVAL: Stem rust can survive as teliospores during winter when aeciospores are a major source of inoculum. It generally survives as mycelium or uredinia on volunteer wheat during the non-wheat growing season. Uredinospore can be spread by wind into disease-free areas. Sporulating uredinia are active in tropical and some subtropical areas throughout the winter. Occasional dormant mycelium may survive beneath the snow pack in more northern temperate regions.

METHOD OF DISSEMINATION:Urediniospores and aeciospores are wind borne. Teliospores remain with the straw.

HOST RANGE:Stem rust is generally confined to Triticum species, although naturally infected plants of Secale cereale, Hordeum vulgaris, H. jubatum, H. pusillum, Elymus junceus occur. Many genera of the tribe Hordeae are infected when artificially inoculated. Other formae specialis of P. graminis attack many cereals and related grasses, and many species are susceptible to more than one formae specialis.

Life cycle

On wheat and other grass hosts:

Plants do not usually show obvious disease symptoms until 7 to 15 days after infection when the oval pustules (uredinia) of powdery, brick-red urediniospores break through the epidermis (Figures). Microscopically, these red spores are covered with fine spines (Figures). The pustules may be abundant and produced on both leaf surfaces and stems of grass hosts. Later in the season, pustules (telia) of black teliospores begin to appear in infected grass species. Microscopically, teliospores are two celled and thick walled (Figure).

A. Uredinia of Puccinia graminis f. sp. tritici B. Scanning electron micrograph (SEM) view of a single uredinium

Urediniospores of Puccinia graminis A. normal B. SEM figure

Telia on wheat plants and Teliospores of Puccinia graminis f. sp. tritici. Note dark color and thick cell walls.

On barberry and other alternate hosts:

Pycnia appear on barberry plants (Figure) in the spring, usually in the upper leaf surfaces. They are often in small clusters and exude pycniospores in a sticky honeydew (Figure). Five to 10 days later, cup-shaped structures filled with orange-yellow, powdery aeciospores break through the lower leaf surface (Figure). The aecial cups are yellow and sometimes elongate to extend up to 5 mm from the leaf surface (Figure). Microscopically, aeciospores have a slightly warty surface (Figure).

Pycnia with honeydew on barberry leaf

Aecial cups on barberry leaf.

Aecial cups and aeciospores on barberry leaf

Aeciospores are produced in chains in the aecium

Disease Management
Barberry eradication:

Once the life cycle of P. graminis was determined, the potential effects of the removal of the barberry alternate host became clear. An expensive and extensive barberry survey and eradication program was initiated in 1918 in the U.S. and continues to a limited extent today.

It was originally hoped that the program would eliminate stem rust as a significant disease in North America, because the basidiospores would have no barberry hosts to infect, and urediniospores could not usually survive harsh winter conditions. The importance of continental spread of stem rust epidemics was not understood until later. Urediniospores overwinter in wheat fields in the southern U.S. and northern Mexico and are then airborne northward via what is now called the "Puccinia Pathway" (Figure). If the weather is favorable for stem rust development in the South, urediniospores will arrive in time and in sufficient numbers to cause epidemics in northern wheat-growing areas.

Each year urediniospores move northward via the "Puccinia Pathway."

Despite this problem, barberry eradication has had significant positive effects on the control of stem rust epidemics. First, it removed a significant, early source of inoculum. A single barberry plant can produce as many as 64 billion aeciospores. Second, it reduced the genetic variation in the fungal population by eliminating the sexual cycle, leaving only asexual urediniospores to maintain the fungus. Mutation is now the primary source of genetic variation. Consequently, there are no longer so many different races of wheat stem rust against which wheat breeders must seek resistance. Finally, epidemics are delayed by several weeks in many of the major wheat producing areas of the U.S. and Canada because aeciospores were released before the first arrival of urediniospores from the south.

Cultural practices- It has long been known that moisture on leaves and excessive foliar nitrogen favor infections by rust fungi. Farmers consider these factors in spacing, row orientation, and fertilizer schedules. Recent changes in production practices may have effects on stem rust. In some areas, summer wheat crops are irrigated, which may increase the survival of infected volunteer plants. In addition, many farmers are practicing no-till or minimum tillage. This increases the probability that rust fungi may successfully overwinter in the protective layer of stubble from the previous crop.

Use of earlier-maturing wheat varieties in the central Great Plains of the U.S. has helped reduce the threat of stem rust epidemics. Modern wheat varieties in that region mature about 2 weeks earlier than older varieties. This limits the length of time for stem rust epidemics to develop in the central Great Plains as well as the numbers of urediniospores that can contribute to epidemics farther north.

Genetic resistance- Genetic resistance is the most commonly used and the most effective means to control stem rust. Its success is directly linked to the reduced number of races present in the fungal population following the barberry eradication program. Because funding for the program has been reduced in recent years, scientists fear that the remaining barberry bushes will continue to spread into wheat-growing areas to serve both as a source of inoculum and as a means by which the fungus can complete its sexual cycle. The currently used resistance genes should not be expected to remain effective as new races of the fungus begin to appear.
Even without the presence of alternate hosts, the fungus is capable of overcoming resistance genes, primarily through mutation. For this reason, plant pathologists monitor the race populations each year and advise wheat breeders about which resistance genes will best protect the wheat crop in various areas. Wheat breeders use a combination of vertical resistance genes against specific races of P. graminis and horizontal resistance genes that slow the development of the epidemic by offering some resistance to all pathogen races.

Chemical control- In some areas where disease pressure is high, fungicides are applied to wheat to control rust diseases. Fungicides that inhibit the synthesis of sterols [i.e., sterol biosynthesis inhibitors (SBIs) or demethylation inhibitors (DMIs)] are particularly effective, but the cost of application is generally prohibitive for routine use in most wheat-growing areas in the U.S.

Potential approaches to management- Urediniospores infect wheat only through stomata. Scientists have studied how germinating urediniospores locate stomata on leaf surfaces. Although several factors are involved, the germ tube is able to detect the guard cells by their physical dimensions relative to the epidermal cells. Once a stoma is found, an appressorium is produced and infection begins. In the future, it may be possible to breed wheat resistant that is resistant to urediniospore infection because it has epidermal patterns that are not recognized by the fungus.

rust symptoms - comparision

Symptoms - close up

Leaf rust (brown rust)

Yellow rust / stripe rust
CO: Puccinia striiformis Westendorp f. sp. tritici

Leaf / brown rust Stem rust

Yellow or strip rust

Figure . Relative resistances of wheat to stripe (left) and leaf rust (right): R = resistant, MR = moderately resistant, MS = moderately susceptible, and S = susceptible.

Saturday, September 6, 2008

leaf rust problem

Leaf rust -

Leaf rust is one of the most common and most important wheat diseases in wheat growing areas of the world. Leaf rust is caused by a parasitic fungus called Puccinia recondita f. sp. tritici.

Leaf rust causes very small (about 1/32 inch long by 1/64 inch wide), orange pustules that erupt through the leaf surface. In some cases, pustules are surrounded by a narrow yellow or white halo. The pustules contain masses of powdery orange spores of the rust fungus. Spores may spill out of pustules and form a grainy orange dust on the leaf surface around the pustule.

As leaves age, pustules begin to produce dark black spores instead of orange spores. These black pustules look like tar spots and are most easily seen on the lower leaf surface and leaf sheaths. Although leaf rust may initiate tiny orange spots on culms and heads, it does not form large, open pustules on these organs. This helps distinguish leaf rust from stem rust. Stem rust is uncommon and usually only occurs late in the season because it requires warm temperatures. Leaf rust pustules occur randomly across the leaf; this distinguishes leaf rust from stripe rust, which has narrow yellow stripes of pustules. Stripe rust is rare in warm areas because it requires very cool weather.


Alternative hosts are not important in the leaf rust life cycle. Although some strains of the leaf rust fungus can survive on jointed goatgrass or wheatgrass, they appear to be different from the strains that attack cultivated wheat. Many rusts have special alternate hosts for completion of the sexual cycle. Meadow rue (Thalictrum spp.) is the alternate host for wheat leaf rust in Eurasia. However, the sexual cycle on meadow rue apparently does not occur in North America. Therefore, the leaf rust population in the U.S. is composed of distinct races that do not cross with each other. This slows the development of new races because mutation is the only means of genetic change.

The leaf rust fungus can only survive in living leaf tissue. It is not soilborne or borne in crop residue. In the summer, it survives on volunteer wheat. In the fall, spores blow to newly planted wheat. Early planted wheat sometimes sustains heavy rust infection and may turn yellow in the fall. This does not seem to cause winterkill of the wheat. Leaf rust can survive the winter as latent infections if green leaves survive the winter. In the early spring, pustules erupt and fresh spores blow to new leaves. If rust does not survive through the winter in Kansas, spores eventually blow up from Oklahoma or Texas. However, the delay often reduces the final severity of the disease. The rust fungus moves back to volunteer wheat around harvest time.

Leaf rust epidemic severity increases exponentially over time. That's why rust epidemics appear to suddenly "explode" during favorable weather. Rust development in the spring is favored by daytime temperatures between 60 and 75F. The infection process requires moisture, which can be provided by rain or dew. Heavy rain is unfavorable for rust because it tends to wash the spores off the leaves. Infection can occur in as little as four hours during favorable weather. Dispersal of spores to upper leaves and between fields is favored by dry, windy conditions.

Description of infection types and symptoms

0 Low No uredinia or other macroscopic sign of infectiton
0; Low Few faint flecks
; Low No uredinia, but hypersensitive necrotic or chlorotic flecks present
1 Low Small uredinia often surrounded by a necrosis
2 Low Small to medium uredinia often surrounded by chlorosis
Y Low Ordered distibution of variable-sized uredinia with largest at leaf tip
X Low Random distibution of variable-sized uredinia
3 High Medium-sized uredinia without chlorosis or necrosis
4 High Large uredinia without chlorosis or necrosis

For nomenclature of genes refer to -

D.L. Long and J.A. Kolmer..A North American System of Nomenclature for Puccinia triticina. Phytopathology 79:525-529


Resistant varieties possess one or more special leaf rust resistance genes called Lr genes. Currently there are more than thirty different Lr genes available, but most varieties have only a few Lr genes. In order to be virulent on a given variety, the leaf rust fungus must be able to defeat all the Lr genes in the variety. Different races of leaf rust can defeat different combinations of Lr genes in the wheat. The prevalence of different rust races is always changing in response to the popularity of different wheat varieties with different Lr genes.

abiotic stress tolerance - interconnected?

Abiotic stresses such as extreme temperatures, low water availability, high salt levels and mineral deficiency and toxicity are frequently encountered by plants in both natural and agricultural systems. In many cases, several classes of abiotic stress challenge plants in combination. For example, high temperatures and scarcity of water are commonly encountered in periods of drought, and can be exacerbated by mineral toxicities that constrain root growth.

Higher plants have evolved multiple, interconnected strategies that enable them to survive abiotic stress. However, these strategies are not well developed in most agricultural crops. Across a range of cropping systems around the world, abiotic stresses are estimated to reduce yields to less than half of that possible under ideal growing conditions.

Upland rice : useful genetic resource for drought tolerance

Upland rice, known as Ghaiya Dhan in Nepali, is mainly grown on Tars, and also in, marginal hillside terraces or hillsides newly cleared of forest cover in Nepal. Tars are actually ancient alluvial fans now formed into flat basins with aAt least 9% (constituting a total of about 126,000 ha) of the total rice area in Nepal is upland rice gricultural importance.

There were many landraces of upland rice in Nepal, has been disappeared now. Some of genotypes grown in Lamjung area of Nepal include - Sunakhari, Seto Tauli, Gegari Seto, Korsali, Kalnathre, Choto, Chobo, Pakhesali, Thantar, Charinagre (Khoplange), Karlunge, Mangaltar, Parampyuri, Dalle Sete, Kanaki, Phalame, Korsali, Banspate, Gegari rato, Dalle Sete, Karlunge, Thantar, Panthe, Subi, Tauli, and Pakhejhuwa.

Drought stress screening at rain-shelter culture
"To conserve and promote utility of these valuable sources, we have collected more than 30 genotypes from different part of Lamjung, Gorkha, Tanhu and Kaski" - says Rosyara. U.R. Rosyara and B.B. Adhikari in IRRI collaboration with IRRI and NAST has been working to promote diversity and productivity of rice in upland rice. Rice assessions are evaluated in farmers field and under greenhouse conditions for drought tolerance and other properties.

University-Farmer collboration
International Rice Research institute and IAAS are collaborating together to increase food security of uplands of Nepal. Participatory varietal selection, drought stress screening, crop management counselling has been continously done from 2003 to date. Once the rice was almost disappeared from farmers' field has been revitalized.

IRRI-IFAD project newsletter

varieties for new environments

The earth’s climate system is experiencing a warmer phase. Increase in temperature and atmospheric CO2 concentration are two major effects of climate change, besides increase or decrease in the local rainfall. Higher temperatures are expected to improve or retard seed germination, plant growth and/or plant development, depending on the relative sensitivity or tolerance of crop genotypes. The increased CO2 concentration will have a positive effect on productivity, albeit in a crop genotype-dependent manner.

The new crop varieties will have to be tolerant to high temperature throughout their life cycle. To take advantage of faster growth under higher temperatures, the new varieties, especially of the rabi cropping season should have characteristics of early flowering (photo- and temperature-insensitivity, but development-related onset of flowering) and early maturity and high produce. Wheat, mustard, chickpea, lentil, pigeonpea and potato varieties should have alternate genetic make-ups to fit into area- and need-specific cropping patterns and schedules.

There will be requirement for the so-called upland rice varieties that can be cultivated aerobically with irrigation, not requiring standing water conditions like those for conventional rice varieties, without major compromise in yield. In the changed climate scenario, at places where assured irigation facilities exist despite rain-water deficit, with the availability of suitable varieties, it may be possible to take up to four crops in a year, instead of three, two or one in the past.

C4 plants may be favored over C3, using raised level of CO2 levels.

Crop breeding programmes to develop temperature- and drought-tolerant highyielding cultivars of the identified crops should be initiated urgently, so that the desired kinds of varieties are available when climate change effects are experienced consistently. The genetic resources, especially land races from areas where past climates mimicked the projected future climates for agriculturally prime areas of the world.

climate change and crop productivity

Climate change threatens to increase crop losses, increase the number of people facing malnutrition, or worse, and may change the development patterns of animal diseases and plant pests, the United Nations agricultural agency says in a new report.

The UN Food and Agriculture Organization (FAO), in collaboration with the International Institute of Applied Systems Analysis (IIASA), has developed the Agro-Ecological Zones (AEZ) methodology, a worldwide spatial soil and climate suitability database for use in quantifying regional impacts and geographical shifts in agricultural land and productivity potentials.
Using this data, FAO says in the report, presented during the 31st session of the Committee on World Food Security, that the northern industrialized countries could increase their crop production potential as a result of climate change.

On the other hand, "in some 40 poor, developing countries, with a combined population of 2 billion, including 450 million undernourished people, production losses due to climate change may drastically increase the number of undernourished people, severely hindering progress in combating poverty and food insecurity," the report says.

Sixty-five developing countries, home to more than half the developing world's total population in 1995, risk losing about 280 million tons of potential cereal production, valued at $56 billion, as a result of climate change. This loss would be equivalent to 16 per cent of the agricultural gross domestic product (GDP) of these countries in 1995 dollars, it adds.

Among these countries, India could lose 125 million tons, or 18 per cent, of its rainfed cereal production, while China’s rainfed cereal production of 350 million tons is expected to rise by 15 per cent, it says.

In Africa, 1.1 billion hectares of land have a growing period of less than 120 days, it says. By 2080 climate change could result in an expansion of this area by 5 to 8 per cent, or by about 50 to 90 million hectares, FAO says.

Wheat scab or fusarium head blight

Wheat scab or Fusarium head blight (FHB) is a devastating disease for wheat production worldwide, the causal organism is Fusarium graminearum Schevabe [(telomorph: Giberella zeae Schw. (Petch)].

Arthur, in 1891, reported that a wheat field which was expected to yield 35–40 bushels/acre yielded only 8 bushels/ acre in 1890, a season in which there was a severe epidemic of Fusarium head blight (FHB). The damage attributed to FHB has been well documented periodically throughout the past 120yr. The disease has frequently caused low to severe wheat crop losses in the United States, and with increased frequency and severity coinciding with the recent widespread adoption of reduced soil tillage for purposes of soil conservation and reduced input costs of crop production.

The widespread FHB (scab) epidemic causing extensive damage in the wheat and barley production areas of the Northern Great Plains of the U.S. is well documented. In 1993, scab in spring wheat caused losses of approximately $80 million in South Dakota alone. It is a floral-infecting disease caused by the fungus Fusarium species, with Fusarium graminearum Schwabe, telomorph Gibberella zeae (Schw.) Petch, as the predominant causal organism in the U.S. Infected wheat florets and spikelets are often destroyed. The fungus readily colonizes florets, spreading through the rachis to adjacent spikelets. The fungus produces mycotoxins, including deoxynivalenol (DON), causing Fusarium infected grains to be toxic to animals and humans. Deoxynivalenol (DON) has been linked to livestock feed refusal and depression of the immune system, nausea, and vomiting in humans. Concerns over food safety have led the FDA to impose a strict 1 µg g-1 (1 ppm) standard for finished wheat food products.


The earliest and most conspicuous symptom of scab occurs soon after flowering. Diseased spikelets turn light-straw colored and have a bleached appearance due to premature death of tissues. Healthy spikelets on the same head retain their normal green color. One or more spikelets may be infected, or the entire head may be diseased. When the fungus infects the stem immediately below the head the entire head may die. Infected spikelets of oats are ash-grey and those of barley are light brown.

Several days after infection masses of pink to salmon-colored spores and mycelium may form on the margin of the glumes of individual spikelets, especially near the base of the kernel. The pink spore masses are easiest to see early in the morning before the dew dries. Infected kernels are generally shrunken, wrinkled, and light in weight, with a rough, scabby appearance. These kernels range in color from light-brown to pink to grayish white. The extent of shriveling and discoloration of the kernels depends on the time of infection and the weather conditions following infection.

If the fungus invades and kills the rachis or main axis of the spike, the spikelets above that point die. The result is no grain at all or small, shriveled kernels that are lost during the threshing process. Heads with diseased spikelets may become speckled with dark purplish-black fruiting bodies (perithecia) of the fungus if the weather remains cool and moist until harvest. These perithecia are a sign of the sexual stage, the Gibberella stage of the fungus.


1. There are few varieties of wheat, oats or barley highly resistant to scab, but in greenhouse tests some varieties restrict the development of the disease to one, or only a few, florets per head. In the field, some varieties appear more resistant than others because they flower earlier or later than other varieties, or because they shed their anthers more quickly than other varieties. These varieties look resistant because they have escaped infection by avoiding rains that supply free water on the surface of the heads for germination of the spores. Differences in susceptibility may also be due to physical barriers to infection of spikelets.

2. Plant cereals as far away as possible from old corn fields if stalk residues are left on the soil surface. No-till wheat seeded in old corn residues greatly increases the chance of scab. If conventional tillage is used, clean, deep plowing of all infested stubble and straw of cereals and weed grasses, corn stalks and rotted ears is recommended. Complete coverage of crop residues reduces head blight infection by reducing inoculum levels. Manure containing infested straw or corn stalks may harbor the fungus and should not be put on fields planted to small grains. When possible, plant wheat following a legume crop (soybean) and maintain a rotation with 2 to 3 years between wheat crops.

Friday, September 5, 2008

Drought tolerance in wheat

Drought tolerance refers to the degree to which a plant is adapted to arid or drought conditions. Desiccation tolerance is an extreme degree of drought tolerance. Drought is one of the most important environmental challenges growers have to face around the world. Droughts are the cause for large grain losses every year, especially in developing countries, and the current trend in global climate change will likely lead to further losses.

At least 60 million ha of wheat is grown in marginal rainfed environments in developing countries. National average yields range from 0.8 to 1.5 t/ha, approximately 10 to 50% of their
theoretical irrigated potential. Rainfall distribution patterns vary considerably among locations and years, and additional stresses may include heat and cold stress, soil micro-element deficiency or toxicity, and a range of biotic stresses. Physiological assessment of drought tolerance characteristics in the field is therefore a complex task.

Traits that improve drought tolerance

1) Large seed size. Helps emergence, early ground cover, and initial biomass.
2) Long coleoptiles. For emergence from deep sowing.This is practiced to help seedlings reach the receding moisture profile, and to avoid high soil surface temperatures which inhibit germination.
3) Early ground cover. Thinner, wider leaves (i.e., with a relatively low specific leaf weight) and a more prostrate growth habit help to increase ground cover, thus conserving soil moisture and
potentially increasing radiation use efficiency.

4) High preanthesis biomass. Potential for vigorous growth prior to heading provides the
opportunity to take advantage of relatively good growing temperatures and moisture availability earlier in the cycle.
5) Good capacity for stem reserves and remobilization. Stored fructans can contribute substantially to grain filling, especially when canopy photosynthesis is inhibited by drought. Traits that may contribute include long and thick stem internodes, with extra storage tissue perhaps in the form of solid stems. In studies where crosses where made between lines contrasting in the solid stem trait, the solid-stem progeny contained more soluble carbohydrate per unit of stem length, though total stem carbohydrate was unaffected due to narrower and shorter stems.
6) High spike photosynthetic capacity. Spikes have higher WUE than leaves and have been shown to contribute up to 40% of total carbon fixation under moisture stress. Awns contribute substantially to spike photosynthesis and longer awns are a possible selection criterion.

7) High RLWC/Gs/CTD during grain filling to indicate ability to extract water. A root system that can extract whatever water is available in the soil profile is clearly drought adaptive, but difficult to measure. Traits affected by the water relations of the plant, such as relative leaf water content (RLWC) measured pre-dawn, stomatal conductance (Gs), or canopy temperature depression (CTD), during the day, and C13 discrimination or ash content of grain or other tissues, can give indications of water extraction patterns.

8) Osmotic adjustment. Adjustment will help maintain leaf metabolism and root growth at relatively low leaf water potentials by maintaining turgor pressure in the cells. Some research suggests that the trait can be assayed relatively easily by measuring coleoptile growth rate of seedlings in polyethylene glycol (PEG) solution.

9) Accumulation of ABA. The benefit of ABA accumulation under drought has been demonstrated. It appears to pre-adapt plants to stress by reducing stomatal conductance, rates of cell division, organ size, and increasing development rate. However, high ABA can also result in sterility problems since high ABA levels may abort developing florets.
10) Heat Tolerance. The contribution of heat tolerance to performance under moisture stress needs to be quantified, but it is relatively easy to screen for.

11) Leaf anatomy: waxiness, pubescence, rolling, thickness, posture. These traits decrease radiation load to the leaf surface. Benefits include a lower evapotranspiration rate and reduced risk of irreversible photo-inhibition. However, they may also be associated with reduce radiation use efficiency, which would reduce yield under more favorable conditions.
12) High tiller survival. Comparison of old and new varieties have shown that under drought older varieties over-produce tillers many of which fail to set grain while modern drought tolerant
lines produce fewer tillers most of which survive

13) Stay-green. The trait may indicate the presence of drought avoidance mechanisms, but probably does not contribute to yield per se if there is no water left in the soil profile by the end of the cycle to support leaf gas exchange. It may be detrimental if it indicates lack of ability to
remobilize stem reserves. However, research in sorghum has indicated that staygreen is associated with higher leaf chlorophyll content at all stages of development and both were associated with improved yield and transpiration efficiency under drought.