CASE STUDY REPORT

Late Blight in Potato and Its Sustainable Management
Kiran Parajuli
Principles of Sustainability (FS 536)
Professor Dr. Gregor Moller
Washington State University
Pullman, WA

ABSTRACT
Phytophora infestans causes the devastating diseases in potato crop called late blight. It has two mating type population ie; A1 and A2. While A1 type is reproduced asexually, A2 type is reproduced sexually. The main habitat of these pathogens is moist and damp places where they complete their cycles and completely destroy the potato crop. The potato growers are seeking the sustainable late blight management techniques to increase the yield of potato crop. The integrated disease management process would be the cornerstone of sustainable late blight management. Cultural practices, chemical controls and late blight resistant cultivars would be the integrated late blight management techniques to increase the yield of potato to fulfill the current demand of potato as dietary supplements.

INTRODUCTION
At first potato was introduced in South America through Andes and was grown in Europe at the initial stage (Haverkort et al. 2008). Potato became one of the best crops worldwide behind the rice, wheat and maize since its worldwide distribution from the Europe in the sixteen century (Spooner et al., 2010). This crop is cultivated in more than 100 countries of the world and more than a billion people from these countries are benefitted (Varshney & Tuberosa, 2013). Potato is a significant dietary food and it diversifies based on location and time (Haverkort et al., 2008). It is an important crop in Central, Eastern and North Western Europe. These parts of Europe have the highest per capita production (Hijmans & Spooner, 2001). This crop indicated its accelerated expansion in Eastern and Southern Asia over the past few decades. The highest potato production occurred in China in comparison to the other countries of the world (Hijmans & Spooner, 2001). This is the fastest growing vital crop in developing countries, but its growth is limited by the economic sources of poor families (Forbes et al. n.d.). Farmers also need to spend millions of dollars per year for fungicide use, which also limits the productivity of potato crop due to the absence of enough economic resources (Forbes et al., n.d.). The yield of potato production is higher in developed countries in comparison with developing countries. The average production of potato in Western Europe and North America is 40 T/ha, Ecuador is 12 T/ha and in Africa is lower than the Ecuador (Forbes et al., n.d.). During its cultivation from place to place, the fungus like disease hit the potato crop in Ireland of Europe in 1845 (Haverkort et al., 2008). The pathogen P. infestans causes the fungus like disease called late blight (Oosumi et al., 2009). The tubers and foliages of potato were attacked by this disease. In Ireland, the total potato crops were destroyed by devastating late blight disease in 1845 and 1846, which is called Irish Potato Famine. Initially one million people died and other million people migrated from Ireland due to the Irish Potato Famine (Varshney & Tuberosa, 2013).
Late blight causes not only total estimated cost $1.3 million for the management of this disease in Columbian Basin in 1995 but also resulted in 4-6% cutback in harvested yields (Johnson et al., 1997). The total annual loss of $3 billion was estimated in worldwide potato production in 1999 by Duncan (Duncan, 1999). After ten years of this study, another study was performed regarding the cost due to late blight in European Union countries by Haverkort and colleagues in 2008 (Duncan, 1991). It was estimated the cost €1 billion for crop damage and chemical control and €6 billion for the value of potato production (Duncan, 1991).
Late blight has been creating vital problem in potato crop since 1845 in global market. This crop has more chances to attack by diseases due to its vegetative propagated reproductive character (Varshney & Tuberosa, 2013). Vegetative propagated crop is attacked either by the causal organism of infected seed tuber from the transmission of one generation to another generation or through direct attacks of causal organism during vegetation season (Varshney & Tuberosa, 2013). Late blight could occur at any time during the cultivation period but more favorable season is late summer and early autumn (Bevacqua, n.d.). This disease spreads more rapidly during cool and moist conditions than in dry conditions (Duval, n.d.).The ideal conditions for the contagion and grow of the disease are: relative humidity near 100%, daytime temperature between 60 -70o F, and night time temperature between 50-60oF (Bevacqua, n.d.). Potato is affected by late blight in different aspects from cultivation to storage. Integrated management system must be applied by all kinds of producers, pesticides and equipment manufacturers and suppliers, crop consultants, extension specialists and government agencies for sustainable management of late blight disease (Integrated, 1996). This paper summarized the different control methods of late blight in potato which can be used side by side to control this disease. Two or more than two tactics can be used as integrated pest management program for the effective result of controlling this disease. This kind of integrated pest management program could be the cornerstone of sustainable management of late blight in potato in global agricultural field. OBJECTIVE
The objective of this paper is to deliver concise information regarding late blight pathogen, i.e., P. infestans and its sustainable management tactics in potato crop. OVERVIEW OF LATE BLIGHT
Social context plays a vital role for the effective implementation of integrated late blight management system (Forbes et al. n.d.). Most of the growers of developing countries do not have knowledge about the biology and ecology of disease cycle and pathogen in comparison to developed countries due to the absence of formal training program (Forbes et al., n.d.). If the growers have good knowledge of pathogen’s biology and ecology, growers will be able to make the right decision during the disease management cycle (Ministry, n.d.). The symptoms, disease cycle and mating type of P. infestans are discussed as below: (a) Late blight symptoms on potato:
The initial symptoms of late blight on potato leaf appear as small, dark, circular and irregularly shaped necrotic lesions within 3-5 days of infection. Symptoms usually begin from the lower mature leaves where the humidity is favorable for late blight pathogen (USA blight, 51 n.d.). These pathogens may appear on upper leaves if favorable weather condition occurs and transfers into the other different parts and field through air currents. These lesions occur on the compound leaves near by the petiole, at the edge of leaf where dew is stored longest (Ministry, n.d.). At favorable condition, the lesions become larger, black, or dark brown spots on the tips, veins and edges of leaf. As a result, these lesions can damage the whole leaf due to its expansion through out the leaf surface (Kirk et al., n.d.). The late blight pathogen colonies develop within the internal tissue of the leaves and surface of stems with the expansion of lesions and produces the sporangia. As the expansion of diseases, the entire infected parts of plant will decay. The white and velvety growth develops on contaminated stems and leaves during cool and damp weather in early morning (Ministry, n.d.).

Figure1: Symptoms of late blight disease (USA blight, 51, n.d.).

This growth helps the growers to distinguish the late blight disease with other foliage disease. The plant produces the odor like as chemical vine-kill when it is affected by severe late blight disease (Ministry, n.d.; Kirk et al., n.d.). The purplish-brown lesions appear on the potato tuber due to the infection of late blight pathogen. The reddish brown, granular, dry rot can be observed into the tuber. The <1 inch lesions can be observed by cutting the tuber. Sometimes secondary invaders may get in these rotted tubers, which make the study of symptoms more difficult (USA blight, 51 n.d.; Ministry, n.d.; Kirk et al., n.d.).

(b) Life Cycle of P. Infestans:
P. infestans is not called real fungus, but it is a kind of water mold. This belongs to the Oomycetes under classification (Kirk et al., n.d.). Late blight infected potato tubers have mycelium, which infects seedling (Duval, n.d.). Sporangiophores are produced on infected seedlings, which have clear, large, and lemon-shaped spores known as sporangia. These can be observed with the help of 100 times magnifying power of microscope although they are comparatively larger in size than real fungi (Kirk et al., n.d., Bush et al., 2012). Sporangiophores have periodic distinct swellings during the formation of sporangia. Each sporangium produces 8-12 motile zoospores at the favorable temperature range 44-55oC, where water films are available on leaves (Kirk et al.,n.d., Duval, n.d.). Sporangia also form single germ tube at the temperature between 55-70 o F with high relative humidity (Kirk et al., n.d., Duval, n.d.). These zoospores float independently in water layer and encysted on the surface of leaf. The germ tubes of zoospores penetrate the surface of leaf through the stomata or epidermal cell wall (Duval, n.d.). The zoospores and sporangia move down from the foliage to the soil due to irrigation or rainwater. Tubers become infected when these are in contact with washed off irrigation or rainwater (Duval, n.d.; Kirk et al., n.d., Bush et al., 2012).
Figure 2: Life cycle of P. infestans (Duval, n.d. )
(c) Mating type of P. infestans: P. infestans consists of two mating type, i.e, A1 and A2 populations (Ministry, n.d.). The P. infestans population is generally distributed geographically in different places of the world. Latin America has both mating type A1 and A2 but sexual reintegration is mostly occurs in Mexico (Forbes et al., n.d.). Both mating types are present in US and Canada but A1 is mostly found in western regions and A2 is mostly found in Eastern regions (Ministry, n.d.). Sexual reintegration of P. infestans do not occur in Asian countries even they have both mating type (Forbes et al., n.d.). The mating types of P. infestans population are distributed in our present world as below.
Table 1: Distribution of P. infestans population (Forbes et al., n.d.; USA blight, 52, n.d.) Name of Country | P. infestans Population | United States, Canada | A1 and A2 | Mexico | A1+ A2 | Colombia, Honduras, Venezuela, Costa Rica, Panama | A1 | Chile, Ecuador, Peru | A1 | Bolivia | A2 | Argentina, Brazil, Paraguay, Uruguay | A1 and A2 | Bangladesh, Sri Lanka, Philippines and Taiwan | A1 | China, Nepal, Thailand, India, Japan, Pakistan, Indonesia, Korea. | A1 and A2 | South Africa | A1 | Morroco | A1 and A2 | Ethiopia, Kenya, Uganda | A1 |

INTEGRATED MANAGEMENT SYSTEM
The integrated management practices of late blight can be discussed as below: Cultural practices, late blight resistant cultivars and chemical controls. (a) Cultural Practices:
Cultural practices are the front line of defense to control the spread of disease and infection in potato during cultivation, harvesting and storage (Integrated, 1996). These practices can be discussed as below: (i) Elimination of primary source of inoculum: Mostly, the late blight infected volunteer potatoes, seed tubers and potato in cull piles are the primary source of inoculum (Cultural, n.d.). Thus it is significant practices to eliminate the late blight infected sources from the cultivated field. Cull piles should be made small pieces as much as possible to freeze them earliest (Kirk et al., n.d.). Waste potatoes can be used as a supplemental fertilizer in the field at a rate of about 400cwt/A during fall season. These waste potatoes should be left on the cultivated field to freezing after pulverized (Kirk et al., n.d.). The 39 % of the survey conducted in Netherlands from 2003 to 2005 showed that the widespread of diseases driven by contaminated seed (Lehtinen & Hannakkala, 2004). Oospores of late blight pathogen is also the one source of primary inoculum. The 18 % of the survey conducted in Netherlands from 2003 to 2005 showed that the widespread of diseases driven by these oospores (Lehtinen & Hannakkala, 2004). Suspicious oospore derived foci were found in a field of previous potato crop affected by late blight in Sweden (Andersson et al., 1998). The best practices to decrease the effect of oospores are to reduce the growing of pathogen in the early crop and to prevent the volunteers (Lehtinen & Hannakkala, 2004). (ii) Avoiding conditions that favor late blight: The development of late blight disease mainly depends on the weather condition (Kirk et al., n.d.). The favorable conditions for the late blight can be expressed as: temperature range from 55oF to 80oF, rainy weather, heavy dew formation and high relative humidity (~100 %) (Cultural, n.d.). These favorable conditions enhance the infection, disease development and spore production. We cannot control the weather conditions manually due to its natural phenomena (Kirk et al., n.d.). The appropriate field and water irrigation could help to minimize the development of diseases even though the weather conditions are behind the manual key (Cultural, n.d.). Irrigation can be used between 8 a.m. to midnight to reduce the existence time length of water on leaf. Because leaves are covered by dew during these periods (Kirk et al., n.d.). Alternatively, the favorable time for irrigation to potato crop is daylight two hours before the beginning of dark. Thus leaves could have enough time to dry after irrigation (Kirk et al., n.d.). The fields having good drainage features and water infiltration can be selected for planting potato crop to avoid the late blight (Integrated, n.d.). (iii) Choosing the right site and environment: The field having sandy acid soils is better soil to yield the growth of potato with minimal effect of late blight than the heavy soils (Duval, n.d.). It is not recommended that the growing of potatoes in other soil than the sandy acid soils due to their humidity retain property. Sandy acid soils do not have humidity retain property like other soils, which do not favor for late blight (Duval, n.d.). The excessive use of nitrogen fertilizer is not the best practice for potato cultivation because it develops large foliar canopies that support leaf wetness, develops succulent tissue which is more likely to infection (Bush et al., 2012). It is recommended any practices that diminish leaf wetness of potato plant (e.g., planting in area with good air circulation, irrigating in every morning to enhance foliar drying, staking plants, plant canopy size should be limited and strictly prohibiting the overhead irrigation) (Bush et al., 2012, Bevacqua, n.d.). (iv) Selection of disease free transplants and potato seed tubers: It is advised to carefully inspect any potato transplants and seed tubers for symptoms of disease before buying and planting. It is recommended to buy certified seed tubers and transplants from approved growers and suppliers and examine them cautiously upon receiving and before planting (Bush et al., 2012; Integrated, 1996). (v) Forecasting techniques and scouting systems: It can be used forecasting with respect to the local weather circumstances to identify the favorable conditions for disease development. This method can be helpful to schedule the appropriate fungicide application and reduce the application of unwanted fungicide (Integrated, 1996; Duval, n.d.) (vi) Sanitation and cull clean up: Good sanitation is one of the best control methods that can be applied against late blight. Growers must be attentive in the field for effective sanitation. Sanitation program can be applied for potato storage facilities and the equipment used during planting, harvesting and storage to remove the sources of late blight disease (Cultural, n.d.; Bevacqua, n.d.). Leaving tubers, leaves, debris, any part of plant and seed left over after harvesting must be avoided (Cultural, n.d.). Potato culls are the vital source of late blight. These potato culls need to be disposed per local authorities regulation. These can be fed to animals, buried, composted, burned before planting another season potato plant [Cultural, n.d., Integrated, 1996). (vii) Crop rotation: Volunteer potatoes generate the late blight development in the grower field. Late blight problem due to volunteer potatoes could eliminate by crop rotation (Bush et al. 2012). Crop rotation should be restricted among the tomato, eggplant and pepper because these are the best hosts for late blight pathogen. Late blight pathogen is highly mobile organism. This character of pathogen limits the efficacy of crop rotation (Kuepper & Sullivan, 2004; Bush et al., 2012). (b) Late blight resistant cultivars:
Many potato cultivars are used as host resistance for decades and some of them are down by newly developed pathogen. The primary late blight disease management strategy is host resistance potato cultivars (Forbes, n.d.). Late blight resistant cultivars are not believable in all cases (Bush et al., 2012) so appropriate fungicide must be used in order to keep away from the late blight infection (Duval, n.d.). Early maturing potato cultivars are preferred for use to late maturing cultivars because early one needs the application of less fungicide spraying than the late one (Bevacqua, n.d.). Some of the late blight resistant cultivars are discussed as below on the basis of previous work by researchers. Hundreds of potatoes were in cultivation among the several potato cultivars selected from S. tuberosum in the nineteenth century; they also had very good local importance (Toxopeus, 1964). It was reported in 1840 at the time of potato famine that only few cultivars cultivated in Europe showed some resistance while another cultivars were completely damaged (Glendinning, 1983). The two species Solanum demissum and Solanum stoloniferum were brought as Mexican resistant cultivars in the beginning of 20th century (Salaman, 1937). The hypersensitive reaction of Phytophthora infestans to Solanum demissum was reported around 1920. This reaction made the interspecific crosses and successive backcrosses between the Solanum demissum and Solanum tuerosum and coetaneous choice as resistant cultivars (Haverkort et al., 2008). The four R-genes used among the major resistance R-genes in different varieties either as a single or as in combination (Haverkort et al., 2008). The different combination of more than 4- genes was reported in Solanum demissum (Black & Gallegly, 1957). The wild potato Solanum bulbocastanum used as a resistance to Phytophthora infestans and these were tested on a large scale (Oosumi et al., 2009). Two different loci RB/Rpi-b1b1 and Rpi-b1b2 had multiple RGAs. These loci produced the another functional genes. These genes functioned as a resistant factor to other disease and late blight pathogen. The genomic cDNA produced the transgene Rpi-bt1. These transgenes also functioned as a resistant factor for late blight pathogen in transgenic potatoes (Oosumi et al., 2009).
The P. infestans pathogens were asexually reproduced and also had A1 mating type. The population of these pathogens was dominated till the mid of 1970s in the United States and Europe (Varshney & Tuberosa, 2013). These old populations of pathogens were replaced by more aggressive and virulent new A1 and A2 mating type pathogens. These new pathogens were capable for both sexual and asexual reproduction (Spielman et al., 1991). Because of the virulent activity of new mating type pathogens, the R-gene containing S. demissum species did not work as durable resistant potato cultivars. Thus the breeding centers and breeders from different places changed the single R-gene to more developed and advanced broad genetic base quantitative traits, which have effective resistance against broad spectrum of the races of pathogens (Thurston, 1971). The various solanum species such as: S. demissum, S. stoloniferum, S. verrucosum, S. phureja etc were found as resistant cultivars to late blight pathogen (Thurston, 1971). No one study has completely resistant potato varieties to Phytophthora infestans; however, few study had demonstrated the resistant cultivars. These are still under the research in breeding program to identify more sustainable resistant cultivars. Thus we can use the resistant cultivars as one part of integrated disease control program for effective disease management.
(c ) Chemical Controls:
Fungicide is one important and widely used element of integrated disease management program because of the limited use and discovery of host resistant potato cultivars in present time (Forbes et al., n.d.). Fungicide mostly used to control the late blight pathogen from potato cultivation- harvesting- storage. We can consider the following factors during selection and application of fungicides. i. Label must be checked for the fungicides application restrictions and procedure, harvest limitation time, and worker exposure safety because this can be used by aircraft, fumigation or ground equipment (Chemical, n.d.; Ministry, n.d.). ii. The appropriate fungicide and spray program must be selected depending on the size and shape of field, tillage practices, irrigation method, site of obstacles, which help to enhance the yield of potato crop [Ministry, n.d.; Chemical, n.d.]. iii. The similar fungicides or fungicides derived from the same chemical must not be used repeatedly which helps to raise the resistance power of pathogen to fungicides. Fungicides must be selected based on their performance, chemical group, mode of reaction and resistance management criteria (Ministry, n.d.; Integrated, 1996). iv. At first, fungicides spray must be applied before any effect appears on crop and timely manner depending on the residual effect of every fungicide (Ministry, n.d.).
The successful application of different type of fungicides was proved from previous studies, which are discussed in the following paragraphs. The biocontrol fungicides Bacillus pumilus and Bacillus subtilis could be used with other products to improve the management of storage pathogen (Gachango et al., 2012). Phosphoric acid was applied to control the development of late blight on potato tubers after harvested if the late blight pathogen attacks to potato tubers (Miller et al., 2006). It has been reported that the Bacillus subtilis and Trichoderma harzianum used to control the decay seed piece caused by late blight pathogen under optimal re-storage condition (Relative humidity at 95% and 12-180C with ventilated force air) (Wharton et al., 2012). The single foliar application of folpet, captafol, acetate, chlorothalonil, mancozeb, fosetyl Al, metalaxyl, fentin is less effective than the mixture of these contact and systemic fungicide (Smoucha and Cohen, 1986). The Curzate M-8 (cymoxanil + mancozeb) was applied as most effective fungicide from the group of Curzate M-8 (cymoxanil + mancoze), Acrobat MZ (dimethomorph + mancozeb) and Tatto C (chlorothalonil + propamocarb hydrochloride) fungicides to control the tuber borne inoculum of late blight pathogen in field and greenhouse studies (Powelson & Inglis, 1999). Fungicides usually control the transmission of this pathogen from tuber to tuber during harvesting and storage but do not control this disease in the field. The mixture of Dimethomorph (9%) and mancozeb (60 %) used to control the transmission of this pathogen from tuber to tuber and foliage to foliage during handling through the harvesting and storage and in the field (Caldiz et al., 2007). The fungicide selection and timing are most important for effective control of Phytophthora infestans in potato crop. Some of the fungicides provide effective action against late blight pathogen when applied in combination with another fungicides (Chemical, n.d.) CONCLUSION
Potato is the fourth important crop worldwide and billions of people from the world are benefitted day by day to fulfill their dietary supplements. The yield of this crop dominated by devastating late blight pathogen ie; Phythophthora infestans. The sustainable management of late blight is the current demand of real world to increase the yield of potato. The sustainable management of late blight would be promoted by educating the potato growers about the biology and ecology of P. infestans and their host responses. The decreases of potato yield by these pathogens are associated with the development of new mating type pathogen to overcome the different host responses, increasing fungicide resistance and unavailability of effective cultural practices during cultivation, harvesting and storage. Development of highly effective fungicides, use of the mixture of more than one fungicides instead of single fungicide, development of highly resistant potato cultivars, continuous skill and knowledge development training program regarding cultural practices, habitat and disease cycle of P. infestans, and implementation of integrated disease management program will be the sustainable management of late blight in potato. ACKNOWLEDGEMENT
I am grateful to my respected Professor Dr. Gregor Moller for his valuable instructions, sincere comments and encouragements throughout this work. It was almost impossible to bring this case study in the present form without his kind perusal and thorough observation. I am heartily indebted to his nice cooperation throughout the case study development.
I am thankful to Washington State University online library and interlibrary loan system for continued support to provide reference materials throughout the development of this paper. I am also thankful to WSU library staff for providing reference materials via mail. REFERENCES
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