Friday, July 6, 2012

Late Blight of Tomato

Written By: Kelly Ivors, Extension Plant Pathologist, NC State University

During this 2012 season, late blight on tomatoes isn’t late. It was already confirmed in commercial potato and tomato fields on the NC coast weeks ago; however, we weren’t sure when we would see it in western North Carolina, where a substantial amount of tomatoes are grown commercially in the mountains. On July 5th, 2012 we confirmed late blight from foliar samples collected on July 3rd in a conventional field of tomatoes in Mills River, NC (Henderson County).

Without proper preventative measures and under the right weather conditions, 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 on tomatoes, 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 loss and/or in the need for intensive use of costly fungicides. The recent spread of aggressive, Ridomil-resistant strains of this pathogen on tomatoes in NC has further aggravated the problem, making the pathogen much harder to control.

The pathogen attacks all above-ground 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.
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.
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. Besides tomatoes, P. infestans can only infect a few other closely related plants including potato, petunia and related solanaceous weeds such as hairy nightshade. 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 80'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.
Figure 5.  Lemon-shaped sporangia of P. infestans attached to sporangiophores.
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, which should be available to growers in the following years.

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; products include 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- usually this means mefenoxam can ONLY be applied for the first application when it is first found in a county. Resistance development to this active ingredient can be very rapid; use of this product after pathogen establishment in the area is not warranted or recommended.

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 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), fluopicolide tank mixed with a protectant (Presidio + chlorothalonil), and mandipropamid (Revus TOP) work well against this pathogen as foliar sprays.

Table 3-55 of the SE Vegetable Crop Handbook provides efficacy ratings against this disease. If you do not have a hard copy of the handbook, it can be found here

In addition, the NCSU tomato spray program website can provide advice on spray schedules and rates. The late blight products come into the spray program at week 9. Click here for more information