About the Author
Stephen Johnston received his Ph.D. from Rutgers University in Plant Pathology in 1977. He is presently Extension Specialist in Plant Pathology at the Rutgers Agricultural and Education Center in Bridgeton, New Jersey. He has responsibility for development and implementation of disease management recommendations and integrated pest management (IPM) tactics for vegetable crops in New Jersey. Johnston was an early participant in IPM program development for vegetables and has chaired the APS Extension and IPM committees. He has served as senior editor of Plant Disease (1995 to present), Associate Editor of Plant Disease from 1989 to 1991, editor of the APS Fungicide and Nematicide Tests from 1988 to 1992, and the Vegetable Crops Section Editor from 1984 to 1987.
Tomatoes and Late Blight
Steve Johnston
Rutgers University
Importance of tomatoes in North America and origin of the crop. Tomatoes are an important crop in North America. Production occurs in Canada, the United States, and Mexico, comprising a total of 775,00 acres. The crop is grown for fresh market and for processing purposes. In the United States, there are 135,000 acres of fresh market tomatoes with an average yield of 26,500 lb and a total value of $1.1 billion. There are 320,000 acres of processing tomatoes with an average yield of 31 Tons/A and a total value of $530 million (2). Total production within North and Central America is 11 million tons, and the per capita consumption in North and Central America is 60 pounds (5).
The center of origin of tomato is in the Andes mountains of Peru, Ecuador, and Chile. Additionally, distinctive relatives of the cultivated tomato are part of the plant life on the Galapagos Islands. Early civilizations of Mexico were the first to cultivate tomatoes outside of the center of origin (5). There is a great diversity of cultivated forms of the tomato in this area. The large amount of diversity within the tomato is significant for future improvements of the cultivated tomato, particularly as it relates to the development of disease resistant cultivars.
Tomato late blight. Late blight, caused by Phytophthora infestans, can be a serious disease of tomato when the weather is consistently cool and rainy. Late blight has resulted in numerous epidemics of tomatoes throughout North America. Tremendous yield reductions result in seasons when late blight epidemics occur. Production areas that have experienced the most severe epidemics in recent years include the Del Fuere Valley in Mexico; the mountain regions of North Carolina, Tennessee, Pennsylvania, New York, New Jersey, Alabama; and in certain coastal regions of California late in the production season. Within the entire United States production area, late blight ranks as the eighth most important tomato disease (2). This ranking is based on a weighted average by acreage. Since the large production states (California and Florida) consider late blight as the 7th to 8th most important disease, the national rankings reflect this factor. However, whenever late blight appears in production areas it has the potential to result in tremendous losses if environmental conditions are favorable for disease spread.
Symptoms. Symptoms of late blight on tomato are first observed as pale green to brown lesions that cover a large portion of the leaves (Fig. 1).
Figure 1. Late blight lesion on a tomato leaf.
Figure 2. Underside of late blight lesion exhibiting sporulation of the pathogen on a tomato leaf.
In moist weather a ring of white, moldy growth is present around the undersides of the lesions (Fig. 2). Infected foliage shortly becomes brown and soon dies resulting in extensive amount of defoliation in the tomato planting (Fig. 3). Fruit lesions appear as dark, olivaceous, greasy spots that eventually turn a chocolate brown color (Fig. 4). Fruit lesions can enlarge to the point of encompassing the entire fruit. A thin weft of white fungal growth can cover fruit lesions during moist weather, and the entire fruit disintegrates. Decaying plants and fruit can be recognized by a foul odor that pervades the air around infested fields.
Figure 3. Defoliation of tomato plants caused by late blight.
Figure 4. Tomato fruit infection exhibiting chocolate-covered lesion.
Pathogen characteristics. Within the past 3 to 4 years new immigrant forms of Phytophthora infestans have appeared on tomatoes within the region, and they have become dominant in the pathogen population. The majority of the new genotypes found in tomatoes recently in North America are US-6, US-7, and US-8. Many of the new genotypes belong to the A2 mating type and have the ability to tolerate applications of the fungicide, metalaxyl (trade name, Ridomil), which is an extremely efficacious fungicide against the late blight pathogen.
In Mexico the predominant genotype reported on tomatoes between 1983 and 1989 is the US-6 genotype that belongs to the A1 mating type (4). In California the US-7 (A2 mating type) began to appear on tomatoes in 1992/1993. US-6 (A1 mating type) was predominant in Monterey County through 1994; and in 1995 a new strain, g11 (A1 mating type and metalaxyl resistant), displaced US-6 in this area (1). In New York US-1 (A1 mating type and metalaxyl sensitive) was predominant on tomatoes in 1992. From 1993-1995 US-7 (A2 mating type and metalaxyl resistant) became the predominant genotype present, and in 1996 US-8 (A2 mating type and metalaxyl resistant) was the predominant genotype on tomatoes. In North Carolina US-7 (A2 mating type) was the predominant genotype present on tomatoes between 1993 and 1995 (3). In New Jersey US-7 was present on tomatoes in 1995 and US-8 was present in 1996.
In North Carolina an extensive study with the new genotypes of Phytophthora infestans indicated that among specific genotypes there was a range of sensitivity to metalaxyl. Therefore, genotype alone cannot be used as a criteria for indication of metalaxyl sensitivity in populations of Phytophthora infestans.
Where both mating types of the pathogen occur in the same tomato field, sexual reproduction is a possibility. Sexual reproduction can lead to the development of additional genotypes of the fungus and the production of a survival spore, the oospore, which will allow the fungus to survive in the soil. Soil survival in tomato fields was not possible when only one mating type of the fungus was present. The subsequent genetic diversity and new survival spore will make disease management efforts more difficult in the future.
Late blight control in tomatoes. Current control measures for late blight in tomatoes involve cultural, chemical, and plant resistance strategies. Cultural controls include the avoidance of introduction of inoculum from nearby potato cull piles by destroying the piles, avoidance of introducing inoculum from transplant production regions via certification programs, and alerting home gardeners to use control measures in order to prevent home gardens from being an inoculum source.
Chemical control measures are an effective means of managing late blight. Protective fungicides, such as chlorothalonil and mancozeb, will provide control if applied with proper coverage; in advance of any symptoms of the disease; and on a strict schedule. Newer fungicides, such as, cymoxanil, dimethomorph and propamicarb, have recently received emergency registrations in some states for use in managing late blight epidemics. These fungicides have advantages over the currently registered fungicides on tomatoes. Cymoxanil/mancozeb (Curzate M-8) has postinfection activity of up to 48 hours. This attribute allows the fungicide to be applied in situations where rainfall prevents application of a protectant fungicide on a specific schedule. Dimethomorph/mancozeb (Acrobat MZ) has antisporulation activity, and can be used in situations where late blight has just appeared in a tomato field. Use of this fungicide will delay the progress of an epidemic. Propamicarb/chlorothalonil (Tattoo C) possesses systemic activity, and can be used during an active growth phase of the tomato crop. In seasons when protectant fungicides cannot keep up with actively growing tomatoes on a 7-day schedule, propamicarb/chlorothalonil can be used to provide the protection of all new growth to prevent late blight infections during this period.
Predictive systems used to time fungicide applications on tomatoes. In the Del Fuere and Guasave Valleys of Mexico (2530 acres of processing tomatoes), Campbell Soup Company has developed a novel approach for the timing of fungicide applications for late blight control on tomatoes. Late blight is a perennial occurrence in this region and has resulted in extensive losses to tomato in the early 1990's. Through the use of eight strategically placed weather stations within the region, starting November 1st each season, disease severity values (DSV) are generated using the WISDOM [utilizes a modification of BLITECAST, (6)] predictive system developed at the University of Wisconsin. Once 18 DSV have accumulated, potato fields in the region are scouted for the presence of late blight. Once late blight is detected in potatoes, the first protectant fungicide spray is applied to tomato fields. Subsequent fungicide applications are made according to the DSV accumulated using the TOMCAST predictive system developed in Ontario, Canada, and used to time fungicide applications on tomatoes for control of early blight (Alternaria solani). An accumulation of a 15 to 20 DSV threshold is used during the active growth phase of tomatoes, and a 25 to 30 DSV threshold is used during slower growth phases (fruit ripening). The use of the combination of two predictive systems to time fungicide applications for late blight control in Mexico has been effective for the past 5 years. Through the use of the system, an average of 5 fungicide applications are made/season compared with 8 applications that would have been applied if a calendar spray schedule were followed resulting in a savings of $122/A.
Plant resistance. Presently there are no commercial tomato cultivars that possess resistance to late blight. However, active breeding programs for late blight resistance in tomatoes are present at North Carolina State University, University of California at Davis and Riverside, and at the Asian Vegetable Research & Development Center in Taiwan. Breeding programs are present in several commercial seed companies also.
Relationship of tomatoes to potatoes concerning late blight. One of the research areas needed to determine the relationship of tomatoes to potatoes concerning late blight is the need to identify the role of late blight infections in tomato transplants, commercial tomato fields, and tomatoes in home gardens in potato late blight epidemics. Additionally, further study is warranted in determining the preference of Phytophthora infestans genotypes attacking tomatoes and potatoes.
Since the late blight pathogen is the same organism on tomatoes as on potatoes, research projects and information generated from research projects should involve tomatoes whenever possible so growers of both commodities can benefit. A prime example where both crops can benefit from research is the area of quick disease identification. Quick identification techniques need to be developed for tomatoes (transplant and field) that can be used by fieldmen, growers, and home gardeners. Another area of coordinated research between tomatoes and potatoes is that research should be conducted to further develop forecasting systems that combine WISDOM and TOMCAST for fungicide scheduling similar to the system in place in Mexico.
Literature Cited
1. Coffey, M. D., Mickler, C. J., Duan, C.-L., Forster, H., and Paulus, A. O. 1996. Emergence of late blight as a major threat to tomato and potato production in California. Phytopathology 86:S5.
2. Davis, R. M., Hamilton, G., Lanini, W. T., and Spreen, T. H. 1996. The importance of pesticides and other pest management practices in U.S. tomato production. USDA, NAPIAP. Document Number 2-CA-96.
3. Fraser, D. E., Shoemaker, P. B., and Ristano, J. B. 1996. Metalaxyl sensitivity, mating type and allozyme genotypes of Phytophthora infestans from tomato and potato in North Carolina. Phytopathology 86:S105.
4. Goodwin, S. B., Spielman, L. J., Matuszak, J. M., Bergeron, S. N., and Fry, W. E. 1992. Clonal diversity and genetic differentiation of Phytophthora infestans populations in northern and central Mexico. Phytopathology 82:955-961.
5. Jones, J. B., Stall, R. E., and Zitter, T. A. 1991. Compendium of Tomato Diseases. APS Press. St. Paul, Minnesota.
6. Krause, R. A., Massie, L. B., and Hyre, R. A. 1975. Blitecast, a computerized forecast of potato late blight. Plant Dis. Rep. 59:95-98.
TOMATE Y TIZON TARDIO
STEVE JOHNSTON
Tomate es un cultivo importante en Norte America. La produccion se encuentra en Canada, EEUU (aproximadamente 400,000 acres) y Mexico con un total de acreajes en produccion en Norte America y Centro America de aproximadamente de 775,000 acres. Se cultiva para consumo fresco y para propositos de procesado. La produccion total de la region es aproximadamente de 11 millones de toneladas con un promedio de rendimiento de 30 Ton/Acre. El consumo per capita en Norte y Centro America es aproximadamente de 60 libras.
El centro de origen del tomate es los Andes del Peru, Ecuador y Chile. Adicionalmente, parientes distintos del cultivo del tomate que son parte de la vida de la planta se encuentra en las Islas de los Galapagos. Civilizaciones tempranas en Mexico fueron los primeros en cultivar tomate fuera del centro de origen. Hay una gran diversidad de formas de cultivo de tomate en esa area. La gran cantidad de diversidad del tomate es significativo para mejorar el cultivo del tomate en un futuro, particularmente referente al desarrollo de variedades resistentes a enfermedades.
Tizon tardio, causado por Phytophthora infestans, puede ser una enfermead muy seria para el tomate cuando las condiciones medio ambientales son consistemente lluviosos y frescos. Tizon tardio ha causado numerosas epidemias a traves de Norte America. La redudcion de de rendimiento es muy grande en ocaciones cuando ocurre epidemias de tizon tardio.
Simtomas de tizon tardio en tomate son primeramente observados como lesiones de color verde palido que tornan cafes y cubren largas porciones de la hoja. En condiciones de clima humedo y lluvioso un crecimiento anular blanco del hongo es presente alrededor y debajo de la superficie de las lesiones. Hojas infectadas en corto tiempo se vuelven cafes y el tejido muere, como resultado existe mucha desfoliacion de las plantas de tomate. Lesiones en el fruto aparecen como manchas gracientas obscuras, olivaceos que eventualmente se tornan de color chocolate cafe. Las lesiones en el fruto pueden alargarse a tal punto que pueden abarcan todo el fruto. Una capa delgada de crecimiento del hongo puede cubrir las lesiones del fruto durante condiciones de clima humedo, y todo el fruto se desintegra. Plantas en decaimiento y el fruto pueden ser reconocidas por un olor fetido que penetra el aire alrededor de los campos infectados.
Dentro de los ultimos 3-4 anos nuevas formas del patogeno de tizon tardio ha estado apareciendo en la region y estas formas se han convertido en un patogeno dominante de la poblacion. La mayoria de los genotipos nuevos encontrados en tomate recientemente en Norte America son US-6, US-7 y US-8. Actualmente estan en camino estudios extensivos en Carolina del Norte para definir en un futuro la dinamica de poblaciones del patogeno de tizon tardio en tomate. Una de las caracteristicas de las nuevas formas del hongo es su habilidad de tolerar aplicaciones de fungicidas, metalaxil (nombre comercial, Ridomil), el cual es un fungicida extremamente eficiente contra el patogeno de tizon tardio. Otra de las caracteristicas de las nuevas formas del hongo es que muchos de ellos son del tipo de apareamiento A2.
Cuando los dos tipos de apareamiento del hongo ocurre en el mismo campo de tomate, la reproduccion sexual es posible. La reproduccion sexual puede permitir el desarrollo de genotipos adicionales del hongo, y de la produccion de oosporas de sobrevivencia, los cuales pueden permitir al hongo sobrevivir en el suelo. La subsequente diversidad genetica y de nuevas esporas de sobrevivencia puede hacer que los esfuersos de manejo de la enfermedad sea mas dificil en el futuro.
Actualmente medidas de control de tizon tardio en tomate envuelve practicas culturales, control quimico, y estrategias de resistencia de la planta. Practicas culturales incluye evitar la introduccion de inoculo destruyendo en los campos vecinos los apilamientos de deshechos de papa, evitar la introduccion de inoculo a traves plantulas de tomate de otras regiones por medio de programas de certificacion, y alertando a los vecinos que tienen jardines en sus casas el de usar medidas de control a fin de prevenir que estos se constituyan en fuentes de inoculo.
Medidas de control quimico son muy efectivos en el manejode tizon tardio. Fungicidas preventivos, tales como, cholorothalonil y mancozeb, pueden controlar la enfermedad si son aplicados apropiadamente y antes de la presencia de los simtomas de la enfermedad, y bajo un extricto programa de aplicaciones. Nuevos metodos han estado siendo desarrollados para modificar los tiempos de aplicacion de los fungicidas de acuerdo al comportamiento del clima. Sistemas de pediccion de tizon tardio, BLITECAST, estan en uso en algunas zonas productoras, como el del estado de Sinaloa de Mexico, para decidir la epoca de la primera aplicacion. Subsecuente aplicacion de fungicidas son realizadas de acuerdo a TOMCAST sistema de prediccion usado para control de tizon temprano (Alternaria solani).
Fungicidas nuevos tales como cymoxail, dimethomorph, y propamicarb, han estado siendo registrados como emergencia en algunos estados para el uso del manejo de tizon tardio.
Actualmente no existe resistencia a tizon tardio en cultivares de tomate. Sin embargo, un programa activo de mejoramiento se esta llevando acabo en la universidad de Carolina del Norte y en muchas companias comerciales de semilla.
