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Authors
Introduction
Symptoms/Signs
Causal Organism
Disease Cycle
Diagnostic Methods
Disease Management

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Figure 1. The first symptoms of late blight on tomato leaves are irregularly shaped, water-soaked lesions.

Figure 2. During humid conditions, white cottony growth of P. infestans may be visible on the underside of affected leaves.

Figure 3. P. infestans can cause leaves to turn brown, shrivel and die.

Figure 4. Infected fruit are typically firm with spots that eventually become leathery and chocolate brown in color.

Figure 5.  Lemon-shaped sporangia of P. infestans attached to sporangiophores.

Author

K.L. Ivors, Extension Plant Pathology

Department of Plant Pathology
North Carolina State University

Introduction

Diseases can be important limiting factors during fresh market tomato production. Without proper preventative measures, some diseases like late blight can completely defoliate and destroy a crop within two to three weeks. Due to moderate temperatures, frequent rainfall, and heavy morning dew during the growing season, late blight, caused by Phytophthora infestans, can be severe in the mountains of North Carolina, as well as in late plantings in the piedmont. Despite intensive efforts for over 150 years to control P. infestans, it remains one of the world's most costly plant pathogens, concerning either direct losses and/or in the need for intensive use of costly fungicides. The recent spread of aggressive, fungicide -resistant strains of this pathogen on tomatoes in NC has further aggravated the problem, making the pathogen much harder to control. 

Symptoms/Signs

The pathogen attacks all aboveground parts of the tomato plant. The first symptoms of late blight on tomato leaves are irregularly shaped, water-soaked lesions (Figure 1); these lesions are typically found on the younger, more succulent leaves in the top portion of the plant canopy. During humid conditions, white cottony growth may be visible on the underside of affected leaves (Figure 2). As the disease progresses, lesions enlarge causing leaves to brown, shrivel and die (Figure 3). Late blight can also attack tomato fruit in all stages of development. Rotted fruit are typically firm with greasy spots that eventually become leathery and chocolate brown in color (Figure 4); these spots can enlarge to the point of encompassing the entire fruit.

Causal Organism

Late blight of tomato is caused by the fungus-like organism Phytophthora infestans. The pathogen is best known for causing the devastating Irish potato famine of the 1840's, which killed over a million people, and caused another million to leave the country. P. infestans is an oomycete, which is a group of organisms sometimes referred to as "water molds" that are more closely related to brown algae than to fungi. P. infestans produces microscopic, asexual spores called sporangia that are found growing on stalks called sporangiophores (Figure 5). These stalk-like structures aid in air dispersal of the sporangia. Sporangia are clear, lemon-shaped and usually 20-40 um long. Under conditions of high relative humidity or in free water, these sporangia produce numerous motile zoospores that can swim towards host tissues through chemotaxis.

Disease Cycle

Besides tomatoes, P. infestans can only infect a few other closely related plants including potato and related solanaceous weeds such as hairy nightshade. Unlike some plant pathogens, P. infestans does not survive in soil or dead plant debris. For an epidemic to begin, the pathogen must survive the winter in potato tubers (culls, volunteers), be reintroduced on seed potatoes or tomato transplants, or viable spores must blow in from nearby infested fields. The pathogen is favored by cool, wet weather; clouds protect the spores from exposure to UV radiation by the sun, and wet conditions allow the spores to infect when they land on leaves. Nights in the 50's and days in the 70's accompanied by rain, fog or heavy dew are ideal for late blight infection. Under these conditions, lesions may appear on leaves within 3-5 days of infection, followed by white cottony growth soon thereafter (Figure 2). This white cottony growth is a sign of rampant spore (sporangia) production. Although spores may also be produced on tomato fruit, they are more commonly produced on leaves. Sporangia can be spread readily by irrigation, equipment, wind and rain and can be blown into neighboring fields within 5-10 miles or more, thus beginning another cycle of disease.

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Diagnostic Methods

Late blight detection is typically based on symptomology and presence of sporangia on the underside of infected leaves. The branched sporangiophore, with swellings at the points where sporangia are attached, is distinctive and useful for identification of this pathogen. In addition, the mycelium is hyaline with relatively few septa.

Disease Management

Host resistance
Plant resistance is not currently an integral component in late blight management for commercial production of fresh market tomatoes. However, new breeding lines resistant to some strains of P. infestans have recently been developed at the Mountain Horticultural Research and Extension Center in Fletcher, NC by tomato breeder Dr. Randy Gardner. A new campari-type (small fruited) variety called 'Mountain Magic' that has resistance to some strains of P. infestans, in addition to early blight, should be available to growers in the future.

Chemical
There are several diseases that attack tomato leaves and fruit in this region. Therefore it is necessary to use a combination of different products in a spray program to optimize management of these diverse pathogens, including strobilurins, mancozeb, and chlorothalonil. One consideration is that different products have different preharvest intervals (PHI). A product with a PHI greater than 1 day such as mancozeb (PHI = 5 days) cannot be used when harvests are done 2 or more times per week. Another important consideration is fungicide resistance management. For example, pathogens may develop insensitivity (resistance) to the strobilurins (i.e. Amistar, Cabrio, Quadris or Tanos) if these products are used too frequently.

The application of fungicides plays a significant role in the control of late blight of fresh market tomatoes; however mefenoxam resistant strains of the pathogen have been identified throughout the southeast. Fungicides containing mefenoxam are recommended only when weather favors disease development and resistant populations have not been identified in the area that season. Commercial growers in western NC should apply protectant products since controlling late blight preventatively is better than after infection. Before late blight infection occurs, mancozeb products such as Maneb, Dithane, Manzate, and Penncozeb work well early in the season before harvest (5-day PHI); chlorothalonil products (Bravo, Equus) work best during fruit growth (0-day PHI). In addition, several other chemistries such as cyazofamid (Ranman) and mandipropamid (Revus) work well against this pathogen. Check the NCSU tomato spray program web site for help on spray schedules and rates:
http://www.ces.ncsu.edu/fletcher/programs/plantpath/

The information given herein is supplied with the understanding that no discrimination is intended and no endorsement by North Carolina Cooperative  Extension Service is implied.

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March 2008