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Late Blight – A serious disease killing tomatoes and potatoes this year

Late blight lesions on tomato stems

Update: 4/13/2010: See new post for 2010 season, Avoid the late blight blues.

Update: 1/7/2010: Late Blight Q&A from the Northeast IPM Center.

Update: 8/11/2009: New factsheet from NYSIPM program: Late Blight: A Serious Disease of Potatoes and Tomatoes.

Update: 8/10/2009: Fungicide info for home gardeners added to Meg McGrath’s late blight photo gallery.

Update: 7/29/2009: Cornell Plant Disease Diagnostic Clinic late blight factsheet has been updated to list chlorothalonil products registered for home garden use in New York State.

Update: 7/20/2009: View new Late Blight FAQ and Late Blight: Corrections of Misinformation by Meg McGrath.

Update: 7/10/2009: Photos of late blight on potatoes added to Meg McGrath’s plant disease photo gallery.

Update: 7/10/2009: View late blight webinar for home gardeners hosted on July 2 by Rutgers and Cornell Cooperative Extension. (Webinar uses Elluminate. Allow program to download to your computer and a PowerPoint presentation will appear on your screen. Make sure your volume is on. While the timer shows that the recording is already 38 minutes into the session, it is actually very early in the presentation when the recording begins. You haven’t missed much.)

Update: 7/2/2009: See also: Cornell Chronicle article.

Update: 6/30/2009: See also: Late blight factsheet and webinar announcement [.pdf]

Update: 6/29/2009: Late blight webinar, Thursday July 2, 7:30 to 9:30 p.m. See details at end of post.

Via Meg McGrath, Department of Plant Pathology and Plant-Microbe Biology, Cornell University.

Right: Late blight lesions on tomato stems

More late blight images:

Home gardeners need to be on the lookout for Late Blight – a very destructive and very infectious disease that’s killing tomato and potato plants in gardens and on commercial farms in the eastern U.S.

Late blight is the same disease that caused the Irish Potato Famine in the 1840s. It has never occurred this early and this widespread in the U.S. One of the most visible early symptoms of the disease is brown spots (lesions) on stems. They begin small and firm, then quickly enlarge, with white fungal growth developing under moist conditions that leads to a soft rot collapsing the stem. (See image above. More images.)

Classic symptoms are large (at least nickel-sized) olive-green to brown spots on leaves with slightly fuzzy white fungal growth on the underside when conditions have been humid (early morning or after rain). Sometimes the border of the spot is yellow or has a water-soaked appearance. Spots begin tiny, irregularly shaped and brown. Firm, brown spots develop on tomato fruit.

You need to act quickly to protect your garden-grown tomato and potato plants and to make sure that your plants don’t become a source of spores that could infect commercial farms, as late blight spores are easily dispersed by wind.

Here are the steps you should take:

  1. Examine your tomato and potato plants thoroughly at least once a week for signs of late blight.
  2. Spray fungicides preventively and regularly and/or
  3. Be prepared to destroy your plants when late blight starts to become severe. Seal them in a plastic bag. Do not put them in the compost pile. Leave the bag ‘cooking’ in sunlight for several hours to kill plant and pathogen, then put in the trash.

Fruit can rot quickly once infected, but any part not affected is safe to eat as this pathogen does not produce a toxin.

If you want to try to control late blight with fungicides, you need to begin spraying fungicide now – even before you see symptoms – and you need to continue spraying regularly. Use a product that contains chlorothalonil. Copper is not very effective on late blight.

Petunias, which are closely related to tomatoes and potatoes, can also be infected by late blight and show similar symptoms.

Late blight is very destructive. Uncontrolled it will kill plants faster than any other disease. And it affects tomato fruit — especially green ones. Considering how early it is in the growing season, how long it will be until tomatoes peak and potatoes are at a size worth harvesting, and how much spraying you may need to do in an attempt to prevent loss to late blight, seriously consider growing more of the other vegetables instead of tomatoes and potatoes this year. Even with fungicide applied every week, there is no guarantee of success, especially if the rainy weather continues.

This year late blight has been found on tomato plants being sold at garden centers in New York and other states in the greater northeast. If you started your own tomatoes from seed, they are unlikely to be infected, at least initially. This pathogen is not thought to be able to survive in seed. If you purchased your plants at a garden center and they show signs of late blight, please contact your local office of Cornell Cooperative Extension or Cornell’s Plant Disease Diagnostic Clinic to get confirmation and tell them where you purchased the plants.

Another disease affecting gardens and farms in the Northeast is a relative newcomer — basil downy mildew. In 2008, the disease was severe on many of the region’s farms. It often went unrecognized because it was new and the major symptom – leaf yellowing – looks similar to nutrient deficiency. The downy spore-bearing structures only appear on the undersides of leaves. For more information (including how to report basil downy mildew infestations), Cornell’s Vegetable MD website.

Late blight webinar

On Thursday July 2, from 7:30 to 9:30 p.m., vegetable specialists from Rutgers University and Cornell University will be holding a webinar on late blight and other important diseases of tomato will for interested homeowners, master gardeners and extension personnel in the Northeast.

All are invited to log-on by clicking on the following link at 7:30 on Thursday evening

To participate in the webinar:

  1. Click on the above link by clicking on the hyper-link (You can join the session up to _ hour before it begins).
  2. Type your name or nickname in the box.
  3. Click login button.
  4. Follow on-screen directions.

We recommend that you visit this link to make sure that your computer is configured properly to use Elluminate software. Simply click on this link ( before the session date and follow steps 1 and 2.

For more information on the webinar, please contact Steven Komar, Sussex County Agricultural Agent, New Jersey Agricultural Experiment Station at or at 973-948-3040.


  1. Beverly Beyer says:

    Heard about this website on WOR Ralph Snodsmith. Great information and will continue to go on this website. Thank you very much.


  1. […] can find details on the Cornell University Department of Horticulture blog. Meg McGrath, Cornell plant pathologist has excellent images of the disease in her photo gallery, […]

  2. […] gardeners need to be on the lookout for Late Blight – a very destructive and very infectious disease that’s killing tomato and potato plants in […]

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