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Oat leaf rust, an emerging threat to Pakistan
A large number of plant pathogens ranging from simple to complex in their mode of action and diversity effects almost all life types on the earth especially the crop plants and their effect ranges from mild symptoms to catastrophes in which large areas planted to food and feed crops destroyed and organisms dependent on these crops for survival suffer severely. These plant pathogens are difficult to control because their populations are variable in time, space and genotype and Pathogen being well aware about its capabilities act as powerful evolutionary force and shape the structure and dynamics of both individual species and of the communities of which they belong. Through shifting in pathogenic race it often overcome the resistance by using its virulence mechanism, changes structural, biochemical and physiological properties of host that may have been the hard-won achievement of the plant breeder. Rusts are the emerging devastating communities of foliar fungal diseases that cause massive crop losses and major famines in the world. Rusts pathogens are difficult to control because they are broadly adopted, rapidly evolve, spread quickly via wind over long distances and are neither seed nor soil born. Among these, leaf rust (puccinia coronata corda var. avenae) is a fungus that attacks to cultivated oat (avenae sativa) also known as crown rust, a most destructive oat disease. The word crown refers to the shape of a type of spore produced by puccinia coronata corda var. avenae and it is not related to disease symptom.
Oat is very important crop world widely but it has special position in Pakistan’s economy by contributing directly or indirectly in its GDP. It is use as cereal or staple food in most parts of the world but in Pakistan it is mostly used as fodder. Oat has many characteristics that made it suitable to local conditions e.g it is high yielding, multicut, short life span for fodder purpose almost 60-90 days of emergence, nutritious, three crops in a year as autumn, winter and spring ,has natural adventage over other crops in waterlogged environment and where tolerance of frost and acidic soils is needed. Above all it cover the lean periods successfully in mid winter(December-January) and in summer(may-June) when all other fodders vanish from field in Pakistan. But know a global threat is found in oat producing areas which are also recently reported in Pakistan i-e crown rust (puccinia coronata corda var. avenae). It uses wild oats and grasses as an alternative host and cause sever infection in cultivated oat (avena sativa). The crown rust fungus life cycle is complex, involving several spore stages i-e urediniospores, basidiospores, aeciospores and teliospores with buckthorn bushes serving as the alternate host.
Crown rust(puccinia coronata corda var. avenae) produces two types of spores during its life cycle i-e appear with two modes of infection (1) urediniospores(orange-yellow bodies) are produced in uredinia on alternative host every 8-10 days and are dissiminated by wind to other oat plants. The urediniospores germinate in the presence of moisture and infect leaf and less frequently leaf sheath, stem and panicle. This cycle will continue indefinitely as long as susceptible oat plants available. (2) a second, but complete, life cycle that utilizes both plant hosts occurs where species of buckthorn( rhamnus carthartica, rhamnus dahurica, and rhamnus lanceolata)are present in climate with low temperature. As oat ripens and during the period of drought, excessive moisture or high temperature telia containing teliospores form in and around uredinia. Teliospores(brown-black bodies) functioning as the mean of survival for fungus through winter, germinate in sprig and form basidiospores that are disseminated by wind to buckthorn and cause infection. Basidiospores produce aeciospores that infect again oat (avena sativa). As crop goes towards maturity telia starts to develop and in some cases leaf epidermis remain intact until after leaves have died and life cycle is successfully completed. Uredinium takes 7-10 days to start reinfection depending upon the host type, its susceptibility and weather conditions. crown rust grows best during mild to warm(20-25 C) sunny days and mild nights(15-20 C) with adequate moisture for due formation winds at mid day is important in moving spores around to infect new oat plants and these conditions are same required to oats for optimum growth. Under ideal conditions, rust spores could travel hundreds of miles having the capability of infection and within one to two weeks, each new infection begins to releasing spores to initiate the next generation of infection but infection is inhibited at temperature above 30C. The peak activity of leaf rust is early to mid autumn and mid to last spring, although it can develop at slower rates in the colder temperatures of winter and hotter temperature of summer providing there are oats growing and there is sufficient dew or rain. Crown rust (puccinia coronata corda var. avenae) is difficult to control because of its short life cycle, rapid changes in its pathogenic race, host resistance breakdown and same requirements of climatic conditions needs for optimum growth of oat and leaf rust and availability of alternative host round the year. oat crops for grain have very clearly defined seasons, limiting the spread of rust, but oats for forage can be grown outside normal or traditional seasonal timeframes, creating what some call the "green bridge" - continuous green forage crops on which the rust inoculums can survive year round.
Pycnia, produced by basidiospores, are the mean by which by which new races of crown rust arises by exchanging genetic material between different fungal strains. Each pycnium produces pycniospores and special mycelia called receptive hyphae. Pycniospores are exuded from pycnia in a thick, sticky, sweet liquid that is attractive to insects. Pycniospores are than splashed by water or carried by insects from one pycnium to another, where they becom attached to receptive hyphae ,the nucleus from the pycniospores enters the receptive hyphae which results in the formation of an aeocium on the lower leaf surface. Aeciospores differ genetically from both pycninspores and receptive hyphae that again active to break host resistance. At least 96 Pc genes have been reported in Avena species. Despite the large number of resistance genes available, the average effective life span of new crown rust resistant oat cultivars is three to five years. The aeciospores disseminated by wind to oat, where infection occurs. Each infection gives rise to a uredinium in which urediniospores are again formed and serve as the repeating infection. The spores produce a germ tube that grows over the leaf surface to a stoma, forms an aprisorium (infection surface) and penetrate through the stoma. the rust grows between the leaf cells, obtaining nutrients by penetrating living host cells with haustoria and damage the leaf by decreasing gaseous exchanges, particularly the flag leaf, reduce photosynthesis. The infected areas, initially discrete, expand with the progress of the infection and symptoms of the disease appear about five days after inoculation, characterized by yellow spots, associated with the presence of mycelium of the fungus within the tissues. At this stage, photosynthesis is slightly reduced in the infected areas, but not in those parts of the leaf without infection, where the process shows levels similar to healthy parts. During sporulation, typically eight days after inoculation, there is a reduction in the rate of photosynthesis throughout the whole leaf, although this reduction is more conspicuous in areas invaded by the fungus. Eleven days after inoculation, "green islands" are formed around those areas of the leaf associated with the mycelium of the fungus and photosynthesis is drastically inhibited in the whole leaf. In areas not infected by the mycelium of the pathogen, photosynthesis is very low, indicating that even in these areas the photosynthetic apparatus is seriously damaged. In the green islands of the leaves, photosynthesis is low, but detectable, indicating that some photosynthetic processes are still occurring but transport of photosynthesized sugars from leaf to developing grain and other parts of body is affected. This limits the plant growth and reduces forage quality and quantity. Badly rusted plants have stunted root system and poor drought tolerance. It also reduces yield and causes thin kernels with low test weights.
In Pakistan more than 35% of the land is under forages. Green yield of oat in rained areas is about 20tonha and 60-80tonha in irrigated areas. Leaf rust is emerging threat in Pakistan that hit some areas in last 4-5 years but no sever epidemic is found but know its inoculums is present in sufficient amount that could cause sever damage to oat fodder any time on the onset of favorable conditions but world widely it gain important position due to its sever damages especially in high oat producing countries e.g., Canada(3.5), USA(1.75), Australia(1.5), china(1.33), Germany(1.20) , Sweden(1)million tons annual yield. Sever cases reported in Spain, new Zeeland, Russia, Poland, Brazil, Argentina and Uruguay but especially for the US oat crop, crown rust is the most damaging of the rust diseases, threatening over 4 million acres and 200 million bushels of production each year. in 2008 record low production with 88.6 million bushels is noticed which accounts for 14% yield loss. Annual world wide oat production ranges between 25-30 million tones which is recently noted to be dropped up to 10-40% in last five years and the main reason is crown rust along with other physiological factors. Initial stage infection cause 8-10 % loss but sever epidemic leads towards 100% clean sweep. Pakistan (45-50 tonha) and India (40-45 tonha) are not big producers of oat area and production wise irrespective of that this crop has major contribution in their national income. And if this high dependency on oat fodder and low actual production potential, is imposed with leaf rust stresses it will left a bad memory in the history of Pakistan.
About the Author
Dr. M. Hammad Nadeem Tahir, associate professor and
Sammina Mahmood, student of MSc(hons)agriculture
Department of plant breeding and genetics
University of Agriculture, Faisalabad
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