Purdue expert answers questions about late blight affecting tomatoes
Purdue University experts in the Botany and Plant Pathology Department have received several inquiries during the last several days pertaining to late blight affecting tomatoes. Dan Egel, Purdue Extension plant pathologist, answers some of the most common questions received.
1. What is late blight?
Late blight is a very contagious disease that infects tomatoes and potatoes. When conditions are cool and wet, this extremely destructive disease quickly kills foliage of both crops and rots tomato fruit and potato tubers if not managed.
2. What can I do to prevent late blight on my tomatoes?
It may not be possible to totally prevent tomatoes from getting late blight and much depends on the weather. However, the timely application of the proper fungicides may slow down the disease so that tomatoes can be produced. Next year, gardeners should carefully inspect purchased transplants for any symptoms of disease. Itís also a good practice, in general, to rotate where tomatoes are grown to a new garden plot every year.
3. Are the materials people are using to prevent late blight safe?
Growers should handle all pesticides carefully and follow all label directions. Fungicides are safe if used exactly as directed.
4. What are symptoms of late blight?
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 and after rain). Sometimes the lesion is yellow or has a water-soaked appearance. Leaf lesions begin as tiny, irregularly shaped brown spots. Also brown to blackish lesions develop on upper stems. Firm brown spots develop on tomato fruit.
5. I have problems on many crops in my garden this year. Is this a result of late blight?
Late blight only affects plants in the Solanaceae family, primarily tomatoes and potatoes. Peppers, tomatillo and eggplant also are sometimes infected; though less commonly and with less damage.
6. Is there anything else that looks like late blight on tomatoes?
In early stages it could be mistaken for other foliar diseases such as Septoria leaf spot or early blight. Late blight causes much more extensive damage as it progresses.
7. The leaves on my plants look great but when I picked a tomato off my plant, I noticed that the bottom of the fruit was all black with mold. Is this late blight?
No, this is blossom end rot, a noninfectious problem related to a lack of available calcium to the fruit. Maintaining balanced moisture levels around the plants may help alleviate this problem. More blossom end rot information is available at http://ohioline.edu/hyg-fact/3000/3117.html .
8. If my tomatoes are infected by late blight, what should I do?
Plants with extensive blighting should be removed because fungicide applications wonít be able to stop the damage. Plants can be burned, buried or covered with a plastic bag for removal. Do not put blighted plants in the compost pile. They will continue to produce spores as they decompose.
9. Can I still eat tomatoes that have been affected by late blight? Are they safe to eat?
Other than avoiding tomatoes that have lesions on them, tomatoes from plants or gardens that have been affected can be eaten.
10. Should I treat my tomato fruit from a late blight infected garden differently?
No, just wash your tomatoes prior to eating them as you always would.
11. Is there anything that I can do to treat the soil around my tomato plants?
There is no soil treatment. Thin-walled spores of the fungal-like organism that causes late blight do not survive in freezing temperatures. However, it is always a good idea to rotate tomatoes to different ground every year, if practical, as a cultural method to reduce disease and insect problems in general.
12. My tomatoes were infected by late blight, can I still use my tomato stakes next year?
Yes, but it is always a good idea to clean your stakes of crop debris and then treat your tomato stakes with a solution of 10 percent bleach or a quaternary ammonia solution (e.g., Greenshield or Physan 20). Be sure to wear gloves and eye protection when treating stakes.
13. Is there anything else I need to know about treating for late blight of tomatoes?
If you have questions about treating your garden with fungicides, consult an educator from a local Purdue Extension office or a garden center. Always read the back of any pesticide product carefully and follow the directions. Be safe!
14. Are there tomato varieties that are resistant to late blight?
Currently, there are no tomato varieties resistant to late blight.
15. If I have late blight on tomatoes, what are the chances it will infect my potatoes?
There are two types of strains of the fungus that causes late blight: Tomato aggressive strains and tomato non-aggressive strains. Tomato aggressive strains cause severe disease on both tomatoes and potatoes. Tomato non-aggressive strains infect potatoes, but cause only minor amounts of disease on tomatoes. If late blight is on tomatoes, it will likely infect potatoes. Individuals should assume that this will happen. Potato harvest should occur as soon as reasonably possible and potatoes should be carefully inspected and discarded if symptoms of infection are found.
16. When was the last time late blight was observed in Indiana?
In 1998 late blight was observed on potatoes in Knox County.
17. Is there anything I need to keep in mind or know when preparing for next yearís garden?
Homeowners should not save potatoes for next yearís seed potatoes if there has been a chance of late blight occurring in the area. Late blight can be transmitted through seed potatoes.
18. Where can I learn more about late blight?
More information about late blight is available at the Purdue Plant and Pest Diagnostic Laboratory Web site at http://www.ppdl.purdue.edu .