The biology of Phytophthora colocasiae and implications for its management and control.

Transcription

1 The biology of Phytophthora colocasiae and implications for its management and control. R.A. Fullerton 1 J.L. Tyson 1 The Horticulture and Food Research Institute of New Zealand Mt Albert Research Station Private Bag Auckland, New Zealand rfullerton@hortresearch.co.nz Abstract Defining features of the life cycle of Phytophthora colocasiae include: capacity for endemic survival during extended dry periods, the ability to sporulate and infect within the same night, sporangia germinating to produce multiple zoospores under cool conditions, a very short life cycle (less than 3 days) and, the ability to progress from endemicity to a destructive epidemic within days of onset of wet weather. These characteristics determine, to a large degree, the efficacy of various strategies available for its control. Field sanitation (through the removal of spotted leaves) is effective in small plots at the endemic phase but is less effective, and largely impractical, in large plots under epidemic conditions. Fungicidal control will be most effective during the endemic phase when inoculum levels are relatively low, and between-plant spread is limited, but less effective in the epidemic phase. The use of tolerant cultivars with horizontal resistance is the only viable long- term solution. Where climatic conditions favour frequent disease epidemics, visual assessments in the field are relatively effective in detecting resistant genotypes in populations of progeny. Under conditions of high temperatures and dry weather, field assessments can be supplemented by laboratory assays using inoculated leaf discs and defined incubation conditions. Field evaluations of elite selections in different climatic zones is essential to ensure the reliablity of horizontal resistance in epidemic prone localities. 1 The Horticulture and Food Research Institute of New Zealand Limited. 120 Mt Albert Rd, Private Bag 92169, Auckland New Zealand. Tel , Fax jtyson@hortresearch.co.nz rfullerton@hortresearch.co.nz 1

2 INTRODUCTION Taro Leaf Blight (TLB), caused by Phytophthora colocasiae Raciborski, is the most destructive fungal disease of taro (Colocasia esculenta L (Schott)). It is considered to have originated in South East Asia (Trujillo, 1967; Zhang et al., 1994) and is widely distributed throughout the tropical regions of the world (CMI, 1997). Typical symptoms are large, necrotic, zonate spots on the leaves, often coalescing to destroy large areas of leaf. The margin of the lesion is marked by a white powdery band of sporangia and numerous droplets of orange or reddish exudate. The disease can cause rapid and complete defoliation of susceptible varieties. Under some circumstances the disease can invade harvested corms and cause heavy losses during storage (Jackson & Gollifer, 1975). The organism has a very limited host range; most commonly affecting species of Colocasia, Xanthosoma (Xanthosoma saggitifolia is immune) and Alocasia macrorrhiza. It has also been recorded as the cause of foot rot on Piper betle (McRae, 1934). In this paper, key features of the biology of the organism are outlined and their implications for various control options are discussed. LIFE CYCLE AND EPIDEMIOLOGY Rainfall, humidity and temperature are the key factors controlling the disease cycle and epidemiology of P. colocasiae. The primary reproductive unit of P. colocasiae is the sporangium. It is convenient to take this as the starting point for the life cycle. Germination Sporangia require free water in which to germinate and can germinate in either of two ways, depending on the temperature. 1. Indirect. Under cool conditions (20-22 C) cytoplasm within each sporangium differentiates into 15 to 20 zoospores. The terminal pore of the sporangium dissolves and the zoospores ooze out and swim off into the water film. This is a very rapid process. Zoospore formation usually commences within minutes of being cooled at 20 C. From the first signs of movement within the cytoplasm to zoospore release takes less than a minute. Within about 10 minutes the zoospores settle onto the leaf surface, lose their flagella and form a rounded cyst. Cysts germinate to form a fine germ tube within 5-10 minutes. This mode of germination provides a strong ecological advantage to the fungus. It provides for up to a 15-fold increase in inoculum, allows dispersal in dew or rainwater and, because new infections can be initiated within an hour of a sporangium being formed, the fungus can continue sporulate and infect during short periods of leaf wetness. 2. Direct. Under warmer conditions (28-30 C), sporangia germinate directly by germ tubes that can infect the leaf. This is a relatively slower process than zoospore production as it can take 5-6 hours for a sporangium to germinate. The proportion of sporangia germinating directly is is generally much lower than for those forming zoospores (Putter, 1976; Trujillo, 1965). 2

3 Infection Infection can occur on both surfaces of the leaf. Germ tubes can either penetrate the epidermis directly or enter via stomata and spread inter- and intra-cellularly through the leaf tissue. Most infections occur between midnight and dawn with the majority over the period hr (Putter, 1976). The conditions of cooler temperatures and free moisture from rain or dew over that period are also those that promote zoospore production by sporangia. Daytime infections only occur during continuously wet conditions. Symptom development Initial symptoms appear within 36 hours of infection as small water-soaked flecks on the leaf surface. The fungus is normally most active during the night and each morning lesions have a distinctive water-soaked margin of newly invaded tissue bearing a white mass of sporangia, and orange liquid droplets. Under dry conditions the water-soaked margin dries out during the day and the process is repeated the next night. During cooler, rainy days the lesions can also continue to expand during the day. The growth of the fungus within the tissues of the plant is strongly affected by temperature. The optimum temperature for growth of the fungus in vitro is approximately 25 C. In detached leaf tissue, the rate of symptom development is greatest at temperatures in the range 25 C-30 C; at 35 C symptom development is halted (Fullerton & Tyson, 2001a). During hot, dry weather it is common for lesions in the field to stop expanding, and for the necrotic centres to drop out. Many of these shot holes expand no further; others will resume development (often from one point at the margin) under conditions of heavy rain. The most rapid expansion of lesions occurs when cool, showery weather allows fungal growth in tissues both night and day. Sporulation Sporulation normally occurs only in the zone of active fungal growth at the margin of the lesion. Sporangiophores emerge either through the stomata or directly through the epidermis on both surfaces of the leaf. Under optimum conditions (relative humidity approaching 100%, temperature C) sporulation can take place at the margin of a lesion in less than 3 hours (Trujillo, 1965). Sporulation normally peaks between midnight and dawn with no sporangia produced during the day (Putter, 1976). Dispersal and spread Sporangia and zoospores in rain splash and wind blown spray are the principal sources of spread within and between plants. Rivulets of dew and exudate droplets carrying sporangia and zoospores are important mechanisms of spread on and between leaves of the same plant (Putter, 1976). Sporangia are not released into the air on drying. Long-distance dispersal of the organism occurs only by movement of infected plant material (leaves, petioles, infected corms). 3

4 Survival The primary mode of survival is the continuous recycling of the pathogen, often on single plants within the crop. This is accomplished by the ability of the fungus to sporulate and reinfect within the same night utilising dew as the moisture source. It is not well adapted to long-term survival in the absence of living host material. The fungus does not survive as hyphae in soil (Sitansu, Ghosh & Pan, 1994). Sporangia on leaves dehydrate rapidly during the day. Sporangia in vegetative material (e.g. tops used for planting) seldom survive more than a few days, though some have been shown to survive for up to two weeks (Gollifer, Jackson & Newhook, 1980). Under normal circumstances large numbers of sporangia are washed to the soil. Most of these discharge zoospores or lyse within the first five days. However, a small proportion develops thick walls, forming chlamydospores that are able to survive in soil for up to three months (Quitugua & Trujillo, 1998). The importance of soilborne chlamydospores in the epidemiology of the disease has not been established but they could allow survival of the pathogen between crops. In situations where vegetative material dies off due to drought or cold conditions, the fungus most likely survives between seasons as vegetative mycelium in infected corms (Butler & Kulkarni, 1913). Endemicity and Epidemic Development The first lesions to occur in a new crop will be the result of inoculum carried from lesions in adjacent plots or wild plants. In the absence of regular rainfall, conditions favourable to reinfection occur on most nights ensuring regular cycling and survival on infected plants (endemicity). Those plants represent foci of infection scattered throughout the crop. When those conditions are supplemented by daytime rainfall, the disease can rapidly increase to epidemic proportions, spreading both on and between all plants throughout the crop. Trujillo (1965) determined that epidemics will occur when night temperatures and relative humidity are optimal for 6-8 hours for 3-4 consecutive days, and light rains or dews prevail in the morning. Under conditions of endemic survival, the distribution of infected plants in an area, and the severity of symptoms on those plants, is often irregular. With continuous night time sporulation and infection during the endemic phase it is possible for some plants to become severely diseased while there may be little or no disease on plants immediately adjacent. On infected plants it is common for the older (lower) leaves to be more severely affected than younger leaves. This is due to a constant supply of inoculum deposited by runoff water or dew from above, a more favourable microclimate for the fungus lower in the canopy, and also because the less waxy cuticles of older leaves allows better adhesion of spore-bearing water drops. 4

5 Heterothallism and Genetic Variability P. colocasiae is a heterothallic fungus, requiring the presence of opposite mating types (A 1 and A 2 ) for the formation of oospores. In other Phytophthora species, sexual reproduction is associated with increased genetic variation, including increased variability in virulence and aggressiveness. The oospores may also provide a source of long-lived inoculum. Strains of both mating types have been found in Hainan (Zhang et al., 1994). In a recent study of strains from the Pacific region (Tyson & Fullerton, 2003) only one mating type (A 2 ) was found throughout the region, including Guam, Hawaii, Indonesia, Philippines, Papua New Guinea and Samoa. Strains of neuter (A 0 ) mating type (no oospores formed with either tester) were found also from Indonesia, Thailand and Papua New Guinea - the majority of isolates from PNG were A 0. While oospore formation can be readily induced between opposite mating types in culture there is no evidence that this occurs regularly in nature. Despite the apparent lack of a sexual cycle, P. colocasiae has a high degree of genetic variability. Isoenzyme and RAPD analyses of strains from Thailand, Vietnam, Philippines, Indonesia and Papua New Guinea (TANSAO, 2001) have revealed extensive genetic variation amongst strains both within and between countries. Furthermore, the different genotypes (classified by zymotype) were unique to each country suggesting that the fungus has the capability for significant genetic change in the absence of sexual recombination. Although pathogenic variability may be inferred from a high degree of variability determined by enzyme or molecular analysis, this has not yet been demonstrated. IMPLICATIONS FOR CONTROL STRATEGIES The epidemiological characteristics of P. colocasiae, viz. capacity for endemic survival under dry conditions and rapid transition to epidemic development under wet conditions have an impact on the likely success of control measures. Exclusion. The organism is unlikely to be dispersed over long distances by fungal propagules. Outbreaks of the disease in new areas distant from known centres of infection are most likely due to the introduction of infected planting material. In countries that do not have the disease constant vigilance is needed to ensure that it is not imported. Sanitation. Putter (1976) showed that the removal of infected leaves was highly effective in controlling the disease in subsistence taro gardens, particularly when plots were relatively well separated from one another. This strategy would be most effective when the disease is in an endemic phase with a relatively low and restricted disease incidence. When the disease is in an epidemic phase, the removal of all leaves with lesions would quickly lead to almost complete defoliation of the crop with consequent effects on yield. This was in fact the experience of growers in Samoa (Adams, 1999), and sanitation was largely abandoned as a disease management strategy. 5

6 Fungicides. Successful control of taro leaf blight is technically possible with fungicides. While a range of fungicides have been evaluated, mancozeb (e.g. Dithane M45), copper (e.g. copper oxychloride), metalaxyl (e.g. Ridomil MZ containing mancozeb) and phosphorus acid (e.g. Phoschek) are amongst those most commonly recommended. Mancozeb and copper have protectant activity only. Metalaxyl and phosphorus acid are specific for Phytophthora (and Pythium) diseases; metalaxyl is prone to the development of resistance. However, results with chemical control can be variable. Jackson et al. (1980) found that mancozeb did not control the disease in Solomon Islands. Trujillo (1996) reported that copper gave little control in Hawaii. The efficacy of fungicidal control of any foliar disease is strongly governed by the severity of the disease at the time, and the prevailing weather conditions. In principle, fungicides are most effective when the target disease is present at low incidence, thereby limiting inoculum levels in the crop. When diseases enter an exponential phase, efficacy of control is reduced. On that basis, fungicides might be expected to be most effective against taro leaf blight when applied regularly during its endemic phase. Because of the rate at which taro leaf blight can progress from an endemic to an epidemic state, and the frequency of epidemic promoting conditions in many localities, fungicidal control can be both difficult and expensive. For example, in Samoa fungicides were frequently applied at higher rates and with greater frequency than label recommendations in an attempt to maintain control (Adams, 1999; Semisi, Mauga & Chan, 1998). In cashless, subsistence agriculture, fungicidal control is not a viable option. Resistant cultivars. Resistant cultivars represent the only sustainable solution to taro leaf blight in most production systems. To ensure maximum durability of resistance, the TaroGen project (AusAID/SPC Taro Genetic Resources: Conservation and Utilisation, ) adopted a strategy of breeding for horizontal resistance utilising recurrent mass selection techniques (Anon., 1998). The merit of that course is supported by the recent demonstration of a high degree of genetic variability of the organism within and between countries (TANSAO, 2001). Robinson (1998) proposed three rules for breeding for horizontal resistance: 1. Screen for yield, as this is correlated with freedom from parasites. 2. Inoculate to ensure that the high yield is due to resistance and not due to chance escapes. 3. Use the one pathotype technique to ensure resistance is horizontal. To date there is no evidence of strain-specific resistance in taro, or of matching pathotypes in P. colocasiae. Because horizontal resistance is not pathotype specific, failure to identify different pathotypes is not a limiting factor to the strategy. A major challenge however, is the reliable identification of the the least susceptible individuals in the population for use in the next cycle of intercrossing. In areas where weather conditions favour frequent epidemics, field evaluations by eye remain the most convenient method of selection. This technique has proved to be very successful in the breeding programme in Samoa. While there is the chance of selecting escapes in single genotype populations, these can normally be eliminated in the second round of evaluations when small clonal plots can be assessed. 6

7 In areas where there are often extended periods of endemicity, the irregular distribution of the disease in the field can cause difficulties in discriminating between susceptible and resistant genotypes, and escapes. Under those conditions more objective screening methods may need to be employed. In a comparison of different methods (inoculations in screenhouse, nursery, field and water-bed ), Ivancic et al. (1996) found that only the water bed-method allowed detection of differences in resistance and susceptibility in breeding progeny. The method involved floating leaves of test plants on the surface of a water bath in which leaves with sporulating lesions had been previously been washed. This method has disadvantages in that the concentration of inoculum is unknown and would have a natural bias towards selection of plants with immunity (or vertical resistance). Wall et al. (1998) used spray inoculation and high humidity incubation in a greenhouse and rated plants on percentage of leaf area damaged after 6-8 days. This method proved to be able to distinguish the most resistant clones within the test population (29 cultivars). This method is less practicable for screening large populations as all plants would have to be propagated in pots for testing. Fullerton et al. (1999; 2000) developed a field inoculation method using tabs of blotting paper infiltrated with sporangia and fixed to the leaf overnight with wide adhesive tape. The tape and tabs were removed the following morning and numbers of lesions and lesion size measured after three days. The method was effective for identifying hypersensitive reactions (HR)(indicative of vertical resistance). However, field reactions of non-hr plants varied between repeat inoculations. Laboratory studies on leaf discs showed that the rate of lesion development was strongly affected by temperature and that lesion development slowed above 30 C and ceased by 35 C. Thus, assessments by this method are strongly influenced by daily weather and therefore unreliable. A laboratory test using leaf discs held on water agar augmented with wetting agent and benzimidazole (to prevent senescence), inoculated with agar plugs of P. colocasiae and incubated at 25 C in the dark gave consistent and reliable results (Fullerton & Tyson, 2001b). The method was able to differentiate between the most susceptible and least susceptible genotypes and was also highly effective in identifying hypersensitive reactions. The technique could also be used to screen genotypes generated in a country that does not have leaf blight provided that fresh leaves can be rapidly delivered to a laboratory where testing can be carried out. With horizontal resistance breeding strategies, it is normal to generate many progeny of good agronomic quality but which differ widely in their degree of disease resistance. Such a range of material provides the opportunity to match the degree of resistance to the potential risk of disease. For example, selections of moderate resistance but of superior agronomic quality could be utilised in climatic zones less prone to epidemics. For this reason genotype x environment evaluations are critical to ensure maximum efficiency and output of the breeding programmes. 7

8 References Adams, E. (1999). Farmers use both chemical and cultural methods to control TLB. IRETA's South Pacific Agricultural News 16, 1,4,7. Anon. (1998). In AusAID/SPC Taro Genetic Resources: Conservation and Utilisation. Taro Breeding Workshop, pp. 21. Secretariat of the Pacific Community, Noumea, New Caledonia, Suva, Fiji, August Butler, E. J. & Kulkarni, G. S. (1913). Colocasia blight caused by Phytophthora colocasiae Rac. Memoirs of the Department of Agriculture in India, Botanic Series 5, CMI. (1997). Commonwealth Mycological Institute, Distribution Maps of Plant Diseases, Map No. 466, Edition 3. Phytophthora colocasiae. Commonwealth Agricultural Bureau, Wallingford, Oxfordshire, UK. Fullerton, R. A. & Tyson, J. L. (2001a). Plant pathology progress report: TaroGen Review. March 2001, pp. 7. HortResearch, Auckland. Fullerton, R. A. & Tyson, J. L. (2001b). Plant pathology progress report: TaroGen Review. November 2001., pp. 8. HortResearch, Auckland. Fullerton, R. A., Tyson, J. L. & Gunua, T. G. (1999). Plant pathology progress report: TaroGen Annual Review. 22 October 1999, pp. 13. HortResearch, Auckland. Fullerton, R. A., Tyson, J. L. & Iramu, E. (2000). Plant pathology progress report: TaroGen Review November 2000, pp. 13. HortResearch, Auckland. Gollifer, D. E., Jackson, G. V. H. & Newhook, F. J. (1980). Survival of inoculum of the leaf blight fungus Phytophthora colocasiae infecting taro, Colocasia esculenta in the Solomon Islands. Annals of Applied Biology 94, Ivancic, A., Kokoa, P., Gunua, T. & Darie, A. (1996). Breeding approach on testing for resistance to taro leaf blight. In The Second Taro Symposium. Proceedings of an International Meeting, pp , Faculty of Agriculture, Cenderawasih University, Manokwari, Indonesia, November Jackson, G. V. H. & Gollifer, D. E. (1975). Storage rots of taro (Colocasia esculenta) in the British Solomon Islands. Annals of Applied Biology 80, Jackson, G. V. H., Gollifer, D. E. & Newhook, F. J. (1980). Studies on the taro leaf blight fungus Phytophthora colocasiae in Solomon Islands: control by fungicides and spacing. Annals of Applied Biology 96, McRae, W. (1934). Foot-rot disease of Piper betle L. in Bengal. Indian Journal of Agricultural Sciences 4, Putter, C. A. J. (1976). The phenology and epidemiology of Phytophthora colocasiae Racib. on taro in the East New Britian province of Papua New Guinea. Thesis submitted for the degree of Master of Science thesis, University of Papua New Guinea. Quitugua, R. J. & Trujillo, E. E. (1998). Survival of Phytophthora colocasiae in field soil at various temperatures and water matric potentials. Plant Disease 82, Robinson, R. (1998). Horizontal resistance and its relationship to plant pathosystems. In AusAID/SPC Taro Genetic Resources: Conservation and Utilisation. Taro Breeding Workshop. Secretariat of the Pacific Community, Noumea, New Caledonia, Suva, Fiji, August Semisi, S. T., Mauga, T. & Chan, E. (1998). Control of the leaf blight disease, Phytophthora colocasiae Racib in taro, Colocasiae esculenta (L.) Schott with phosphorous acid. Journal of South Pacific Agriculture 5,

9 Sitansu, P., Ghosh, S. K. & Pan, S. (1994). Effect of temperature, moisture and soil amendment on the survival ability of hyphae of Phytophthora colocasiae in soil. Journal of Mycopathological Research 32, TANSAO. (2001). Taro: Evaluation and breeding for rainfed cropping systems in South East Asia and Oceania, pp INCO-DC : International Cooperation with Developing Countries. Trujillo, E. (1996). Taro leaf blight in Micronesia and Hawaii. In Taro Leaf Blight seminar. Proceedings., Alafua, Western Samoa, November, (pp ). Unpublished. Trujillo, E. E. (1965). The effects of humidity and temperature on Phytophthora blight of taro. Phytopathology 55, Trujillo, E. E. (1967). Diseases of the genus Colocasia in the Pacific area and their control. In Proceedings of the International Symposium ontropical Root Crops. Volume 2. St. Augustine, Trinidad. University of the West Indies., University of the West Indies, St. Augustine, Trinidad, 2-8 April (IV 13-IV 19). Tyson, J. L. & Fullerton, R. A. (2003). Mating types of Phytophthora colocasiae strains from the Pacific region, India and South -East Asia. In 8th International Congress of Plant Pathology., vol. Abstracts of offered papers. Abst , 359, Christchurch, New Zealand, 2-7 February Wall, G. C., Wiecko, A. T. & Trujillo, E. E. (1998). Evaluation of resistance to taro leaf blight in 29 Colocasia esculenta cultivars. Phytopathology 88, S123. Zhang, K. M., Zheng, F. C., Li, Y. D., Ann, P. J. & Ko, W. H. (1994). Isolates of Phytophthora colocasiae from Hainan Island in China: evidence suggesting an Asian origin of this species. Mycologia 86,