Tuesday, September 9, 2008
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.
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.
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).
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.
Detailed outlines for recording stripe rust intensities in cereals are based upon:
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.
tR = Trace severity with a resistant 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
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.
Only the telial and uredinial stages of stripe rust are known.
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.
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.
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
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
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.
INOCULUM SOURCE AND INFECTION:
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.
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.
A. Uredinia of Puccinia graminis f. sp. tritici B. Scanning electron micrograph (SEM) view of a single uredinium
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:
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.
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
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.
Description of infection types and symptoms
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
D.L. Long and J.A. Kolmer..A North American System of Nomenclature for Puccinia triticina. Phytopathology 79:525-529
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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
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.