I just received a phone call from our plant pathologist, Dr. Kelly Ivors. Late blight has been confirmed in a commercial tomato field in western North Carolina. Now is the time to start protecting your tomatoes. If you are a conventional grower, you can access the tomato spray guides that Kelly has prepared for recommendations.
If you are an organic grower, the product that is most effective is copper. If you are certified organic, please check with your certifier to make sure that you can use copper and how frequently. Kelly recommends that if possible, your first spray should be copper. You can then rotate with other products such as Regalia, Serenade, Sporatec, and Oxidate. eOrganic has an excellent article on Organic Management of Late Blight. There is also a webinar for Late Blight Management on Organic Farms and one for Late Blight Control in Your Organic Garden.
More information from Dr. Kelly Ivors. She is sending this out to growers and it will be posted as a Pest News Alert:
Late blight confirmed on
tomatoes in WNC
Kelly Ivors, Extension Plant Pathologist
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. Today (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 the right weather
conditions, 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 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 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. 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.
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
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- 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, (Jeanine's comment: these product recommendations are for conventional growers) 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.
In addition, the NCSU tomato spray program web site can
provide advice on spray schedules and rates. The late blight products come into
the spray program at week 9.
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.
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