Q. I am worried about another outbreak of late blight, after 2009’s epidemic. Can I do anything to prevent tomato disease?
Q. What are those giant green caterpillars on my tomato plants?
Q. My tomatoes flowered but didn’t set fruit. What’s up?
Q. My tomato plants are sick or the fruit is disfigured (or both). Help!

Q. I am worried about another outbreak of late blight, after 2009’s epidemic. Can I do anything to prevent tomato disease?

A. Some surprising late-blight facts and tactics to try:

  • Assuming 2009’s late blight was the same strain normally seen, it can only overwinter on live plant tissues, not dead, in a cold zone like the Northeast, which would mean just one potential host: potato tubers overlooked during harvest. Dig and destroy (trash) all overwintered potatoes as they resprout.
  • Volunteer tomatoes that sprout in the garden or compost can be the carrier of various other diseases (though not late blight). Always remove volunteer tomatoes as they appear, to prevent other tomato afflictions.
  • Seed that was saved is not a potential transmitter of late blight, which cannot get inside a seed or survive on it outer surface. (Some other tomato diseases do taint seed.)
  • Other fungal diseases are soil-borne and do overwinter even in a cold zone like mine. A three-year rotation sequence is the best control—not replanting the area with tomatoes and potatoes and their relatives for that time (though peppers and eggplant are less readily infected).
  • Some such diseases (not late blight) can even contaminate cages and stakes and tools, which should be disinfected with a 1:9 bleach:water solution. First wipe or brush off any caked-on soil; soak the implements for 10 minutes.
  • Resistant tomato varieties such as ‘Mountain Magic,’ ‘Plum Regal’ and ‘Legend’ can offer some protection from late blight, says the University of Massachusetts Extension on their great late-blight fact sheet. Investigate appropriate resistant varieties to offer some protection against other diseases; this chart of disease resistance by tomato variety may help.
  • Tomato grafting, as we have discussed—using a more vigorous rootstock to support less-robust growing heirlooms in particular—is another possible tactic, but not as easy as merely sowing seed.
  • At plant-shopping time, buy local (trucked-in seedlings sold at big-box stores were implicated in the spread of the 2009 late blight outbreak) and carefully check seedlings for any spots on stems or leaves. Even better, grow your own.

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Q. What are those giant green caterpillars on my tomato plants?

A. Be vigilant about watching for tomato hornworms and their telltale droppings or first signs of their chewing damage. More often than not in my garden I get their cousin the tobacco hornworm, which eats tomato foliage but also Nicotiana and other Solanaceous plants. How to tell the two apart. The droppings of either are easier to see than the green caterpillars, who normally start their eating at the tops of plants. Pick them off and destroy them (unless you have enough tomato or flowering tobacco plants to share with these larval forms of native hawkmoths).
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Q. My tomatoes flowered but didn’t set fruit. What’s up?

A. Sometimes, despite all the love you give them, tomatoes fail to set fruit. Assuming you did not give the plant too much Nitrogen, it may be weather-related: Nighttime temperatures that remain above 70 or temperatures below 50ish interfere with pollination. Fruit set can also be hampered by over-feeding with Nitrogen or by irregular watering.

Hot, dry conditions at blossom time prevents proper pollination and can causes buds or tiny fruit to drop. If it’s early enough, hopefully a next round of flowers appears during more favorable weather. I know some gardeners who hose down their plants if the weather is inhospitable, hoping to encourage fruit set.
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Q. My tomato plants are sick or the fruit is disfigured (or both). Help!

A. There are so many potential tomato problems it’s a wonder we ever ripen a red fruit…but we do. Here’s the barest minimum of explanation to why some tomato problems occur, followed by links to expert diagnostic sites that may help you get even more specific.

Rot on the bottom: The bottom of a tomato fruit is the blossom end, where the blossom used to be before the fruit expanded. Blossom-end rot can appear as leathery and sunken, or be watery-looking; the end is discolored, and dark.  The cause: not enough calcium, caused by water stress.  Some gardeners work lime or calcium into the beds as a preventive measure, but does it help? Here’s what I learned in 2012 when I had BER on some early fruits.

Green on the top: On the top or stem end, problems such as “greenback” can occur when areas around the stem remain hard and green, unripened. Too much sunshine can sometimes be the culprit there.

Seeing spots? Various fungal diseases, cankers, viruses and bacterial conditions can show up as spots on tomato skin, whether red or green.  If your tomatoes get anthracnose (sunken round spots that then go dark in the middle, sort of a bull’s-eye effect), alternaria canker (also known as blight, with numerous sunken gray-brown marks on fruit, both green and red, and lesions on plants, too), or black mold and ghost spot (watery spots with dark centers), crop rotation might have helped prevent it, and is a must next year.

A three-year cycle is ideal; two is good; skipping a year helps with some conditions and not others, but is better then no rotation.

Tomato foliage can also experience all manner of spotting, and many such afflictions are symptoms of the same cankers, blights, fungi and viruses above. Sometimes leaves start to fall off after yellowing, moving up the plant, other times it’s top-down. Bacterial wilt is a top-down deal; fusarium and verticillium begin at the bottom of the plant.

Tom Stearns of High Mowing Organic Seed says rotation won’t do the whole trick, though, since certain fungal pathogens, such as septoria and early blight, reside in the soils of the Northeast, mid-Atlantic and upper Midwest in particular. That means that no matter how much you move your tomatoes around year to year–how much you rotate your crop–you can’t completely avoid these troubles.

A barrier of clean mulch applied at planting time can reduce some spores that splash up from the soil onto plants. Stearns uses black plastic, and stakes and prunes his tomatoes (always removing the lower leaves that would otherwise act as a “ladder” for spores). Get his whole management program–he calls it “tomato hygiene.” Heirloom tomato expert Amy Goldman is using a new (reusable) material, ground cloth, the stuff of greenhouse floors and nursery pathways.

Cracks in the fruit often develop when soil moisture is uneven, and lots of moisture becomes available suddenly after not enough. Fruit swells faster than the skin can expand.  Too much Nitrogen can bring on cracking, as can some fungal diseases and even merely plant genetics (large-fruited beefsteak types are said to be more susceptible). More on that and other tomato issues with “Epic Tomato” author Craig LeHoullier, plus his top tomatoes.

Other damage to fruit can come from hail, from sucking insects, maybe from fire and brimstone, huh? It really is endless. So much so that there’s a whole annual conference called the Tomato Disease Workshop for breeders, farmers, and other professionals to try to work on problems together. One recent year’s event, the 25th edition, coincided with the third international version and took place in Italy. Think pomodoro! (Apparently tomato troubles are a global preoccupation, not just domestic.)

My favorite diagnostic sites: The University of Maryland’s simple pdf, as well as Texas A&M and also Cornell University, each has a great, photo-driven diagnostic tool that you will want to bookmark. Rutgers has a great PDF on fungal diseases of tomatoes. And Cornell’s chart of the disease resistance of various tomatoes was one of the most extensive I could find.

There’s even an app! Recently, data and photos from Cornell-based plant pathologist Dr. Meg McGrath’s laboratory research, along with findings from expert colleagues around the nation including University of Minnesota, Purdue, UMass-Amherst and more, made its way into a new app called Tomato MD that can help gardeners narrow down what’s going wrong, and learn what to do. (My whole interview on growing tomatoes successfully with Dr. McGrath.)
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  1. Abbie says:

    Wow, Margaret, I don’t even grow tomatoes. I’ve got enough problems with what I’m just trying to get a grip on and keep alive here. But what a lot of wonderful information in your blog!

  2. Brian G. says:

    I heard you say on your podcast today that you would put out your tomatoes this weekend. I am afeared! Even though the days are heating up the nights are still going down to the 50’s. Is it really safe?

    1. Margaret says:

      @Brian G.: I have gotten away with it most every Memorial Day weekend (which is often a week earlier than this year) for just about forever (which is how old I am; forever). I think the 50s are OK–but if you are not impatient, and your plants are not getting too big for their pots, you can wait another week or two for certain. I have planted as late as July 4th weekend one year (having nothing to do with frost) and still had tomatoes.

      Welcome, Abbie. Tomato planting time comes this week or next here in my area, so I thought people might need a reminder post about tomato troubles. I never go a year without some odd thing happening, but always manage to get a crop anyhow. See you soon again, I hope.

    1. Margaret says:

      Welcome, Eleanor. Your are the sweetest tomato commenter ever…so I think we have an even barter here. :) Isn’t that Cornell info great? Thank goodness for them. Hope to see you soon again.

  3. Brian G. says:

    The plants are more impatient than me (they are getting huge, glandular cases) but i think I will wait an extra week anyway to be safe. You, old? Never. You are just a slip of a girl:-)

  4. Dave says:

    wow , still concerned about cold nights up yonder, tomatoes have been in the ground here in TN since the middle of april, should have my first one ripening soon!! Will let you know how it tastes!!

    Great info on the problems with tomatoes, the websites are huge in their information. thanks for letting us know about them.

    1. Margaret says:

      Welcome, Dave. You are making me jealous. :) Glad the resources are helpful; I spend a lot of time trying to track down places that I think we can all rely on, and I am always glad to share. Hot here this week, feeling more like July…so maybe I should have jumped the gun and planted my tomatoes out, too. See you soon.

  5. Chris McLaughlin says:

    I loved the information here and am obsessed with checking my darling heirloom tomato plants for various diseases as I grew almost all of them from seed this year. Of course, like any other gardener worth their salt, I abhor the thought of anything munching my delicious crop. That said, I just had to show you what tomato horn worms turn into and see if you and your readers are as fascinated as I was.

    http://www.examiner.com/x-2981-SF-Wildlife-Examiner~y2009m3d7-Sphinx-moths-wear-a-hummingbird-disguise

    1. Margaret says:

      Hey, Chris. I am crazy about (and usually visited by many) sphinx moths. They are my friends on the nights of high summer, attracted as they are to the light I have on an electric eye outside the door/pathway here. I always think they are birds for a split second…and then I remember. :)

  6. Chris McLaughlin says:

    It was so fun when I wrote this last year because I had people emailing me and saying , “So, that’s what I saw!”, LOL. They had seen these little guys and thought they were hummers at first and upon closer inspection – they were stumped.

    Nature is cool like that.

    1. Margaret says:

      Thanks, Naseer. Not sure exactly what I did there, or when I did it. A lot of the great (especially older) pdfs I find are things I have to put in my DB because (as you know) they often don’t have working links to them after universities change their sites and so on, so I wonder if I dreamed that was one that was in my WP Media Library. :) Good thing you have my back.

  7. Brett says:

    I have a cherry tomato plant with some yellowing leaves. It is setting a lot of fruit, and they seem to be growing pretty well, yet every day I seem to be cutting off affected branches and leaves. We’ve had lows in the mid-70s and highs in the mid to upper 90s, so I’ve given them a bit more shade and been watering daily. Oh yeah, they are in containers. Is this a fungal thing?

    1. Margaret says:

      Welcome, Brett. There are so many reasons that tomatoes can get yellowing of the foliage, it practically takes a plant pathologist to be certain what’s up in a particular case. The Cornell U. diagnostic site, for instance, shows you many of them…scroll down this page. Whether the yellow starts fromt he bottom up or top down, whether there are specks or other marking on the leaves, too….it’s all part of the puzzle.

      You may find this diagnostic tool from the U. of Maryland even easier to use.

  8. Jackie says:

    I love this site. So much information. I tried planting tomatoes in pots in past years without much success (I didn’t take proper care of them). This year I actually have a small garden. I enjoy watching my plants grow. And I like the idea of everything being organic, no pesticides or chemicals for my garden. I live in Texas, my tomatoes are starting to ripen!

    1. Margaret says:

      Welcome, Jackie. Congratulations on having a real garden — wonderful! And I agree: the thought of raising food in particular without chemicals is very exhilarating. Hope to see you again here soon.

  9. Janeen says:

    Hi,
    I live in Iowa, and I think we finally have Spring, but my tomatoes have been moved from windowsill to windowsill, to a spot in front of a window in my garage for about 6 weeks while we’ve dealt with a little extra winter. They’re starting to get leggy and a bit yellow. I try not to overwater. Do I just plant them anyway and hope for the best, or chuck it in and buy new plants?

    1. Margaret says:

      Hi, Janeen. Hard to say without seeing them…but definitely bury them deep if they are leggy and feed them (organic/all-natural not chemicals). Or start over. Again, without seeing, so hard to say…

  10. Josh says:

    Great site! I ws wondering… I’m growing in containers on a drip system in NYC. I purchased some heirloom tomato plants (about 8″ high) this year and put them in their containers around the middle-end of May. Was that a bad idea? Did I hurt the plants by planting them so early? They had some leaf curl that happened early on, then straightened itself out, only to curl again a month later. Now, as happens every other year, the leaves have one of the many possible yellowing leaf problems. In addition, every year I have spots on the stems that look like wet bark. Do you think any of these problems are due to my early planting?

    1. Margaret says:

      Welcome, Josh. Not too early in NYC at that time. As for leaf curl, or leaf roll, or leaf curl, assuming they are rolling upward, it’s a physiologic distortion (not a disease) that sometimes develops with cool, rainy weather. The foliage gets leathery, too. Not a big worry, just the plant reacting to the weather so to speak — again, assuming that’s what you are experiencing.

      Yellow leaves can be so many things…and as for the spots on stems, that’s the part I don’t like. Start here with this diagnostic tool and click you way through the photos/text of the various parts and see if you can ID what’s up.

  11. Beverly says:

    Regarding tomato hornworms, also known as sphinx moth larvae…When they are discovered with rows of white, torpedo like appendages sticking off their back, leave these be. Their days are numbered.

    Those white forms are wasp eggs which, if allowed to hatch on the hornworm will parasitize it, kill it, become adult wasps themselves, fly off and dispatch another hornworm. And so on….

    IPM at work.

    I saw this in person last summer in my own backyard, zone 6 southeastern PA. Prior to that, I had only seen photos in books. I’m a believer now.

  12. I’d seen something on TV recently about putting aluminum foil around the base of the stem and flaring it out along the ground to deter those worms. Hubby has planted his tomatoes in pots on the deck so hasn’t encountered those issues…perhaps another option to be considered…?

  13. June Millette Fisher says:

    Margaret,
    Tons of tomatoes, no complaints and not too bad looking. Is there a way to freeze them while I pkg up sauce, salsa, etc? They are way ahead of me.
    Thanks,
    June

  14. Dave Pidcock says:

    For those that are suffering with “BLOSOOM END ROT””, your problem is solved. I’m 75 years old, and have grown tomatoes for at least 45 years. Often (Up to about 7 years ago) I too would have beautiful tomatoes only to find blosoom end rot. A friend who also is a tomato grower gave me this tip, and ever since I have NEVER had this problem. When planting your tomato plants, in the hole you have prepared, put two double hands full of compost and 1 handsful of BONE MEAL – Mix this together, then place the plant in the middle. (By handsful I mean put both hands together) Then about half way thru the growing season, sprinkle about 2 tablespoonfuls of BONE MEAL around the plant. Trust me, this will definitely solve your problem. The tomato plant desperately needs the calcium. Obviously you must have a regular watering schedule. “Happy tomatoing”… :-)

  15. Elizabeth says:

    Wow! That article by Dave really brought back memories of my Grandfather who was an avid gardener. He, too, was years ahead of his time, in knowing what best to do for his plants. Wish I had paid more close attention—like Dave, he had a wealth of knowledge about tomato plants. Thanks, Dave, for that informative posting.

  16. Maura says:

    In southern Westchester County we are expecting temperatures into the 90s tomorrow and Thursday, and then we will drop back into the 70s. My tomatoes are gorgeous but they are GREEN (excepting the cherry tomatoes). Should I root prune this week during the heat, or will that overstress the plants?

  17. Melissa says:

    LOTS of septoria here in Chicago this year — I blame our incredibly rainy June (>7.64″ of rain, we usually get around 4″). I only planted locally obtained tomatoes in Earthboxes this year, and used fresh soil covers, but didn’t sterilize my containers and the septoria is raging. I’ve been removing leaves as I see disease, but there doesn’t seem to be much more I can do. I will dispose all soil from these (six) boxes this year, sterilize them and my homemade wire cages and see what next year brings. My plants seem to produce reasonably well even with lots of leaf loss from septoria, so hoping I have a decent (although small with only 6 plants this year) crop this year. Resting my 80 square feet of raised beds from all nightshades for at least this year. I also have a pretty nasty case of anthracnose on my crab apple and my huge sugar maple, (and apple scab on the crab). It’s FUNGAL CITY here in Chicago! Any thoughts on using fungicides in the small home garden? I never have, but it seems to get worse every year here. I will have my crab treated next Spring — it’s really suffering and it’s a very big, important tree to my landscape.

  18. Diana says:

    Banner year for acorns last year = chipmunk population boom this year = chipmunks eating tomatoes as soon as they blush pink = gardener rage

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