Yes, Blight-Free Tomatoes are Possible!

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20160306A

Tomates showing tell-tale symptoms of late blight.

New strains of late blight of the tomato (Phytophthora infestans) appeared out of nowhere a few years ago, taking both seed suppliers and gardeners by surprise. This is an old disease, the one that caused the Irish potato famine in the late 1840s (other strains of the same disease attack potatoes), and we all thought that the disease was pretty much under control, since most modern potato and tomato varieties are resistant to the original strains. But the new strains that have appeared, notably US-22 and US-23, are much more virulent than the older strains and particularly harmful to tomatoes. In fact, in climates with fairly humid summers, where late blight is most prevalent, late blight of the tomato is now that plant’s most devastating disease.

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Late-blight lesions on tomato leaves.

You can recognize late blight by its symptoms. First, it shows up in late summer (it’s not called “late blight” for nothing!). At first, brown spots appear on the lower leaves and grow quickly in size. White cottony growths may appear under the affected leaves… if the air is humid. The disease rises successively upwards, affecting leaf after leaf. Often stems also turn brown. Worse, just when the fruit is almost ripe, soft brown or black depressions form on it and it begins to rot. Soon it is only good for the trash.

Resistant Varieties

There are now however tomatoes with genetic resistance to late blight. Don’t panic: these are not OGMs. Natural resistance to the new strains of the disease has been found in certain tomatoes, notably wild cherry tomatoes, and has been bred into garden varieties by the same old-fashioned methods our ancestors used to create heirloom tomatoes like ‘Brandywine’ (which is terribly susceptible to late blight, by the way). Just buy and sow varieties that are resistant to the disease, and follow normal tomato cultural directives (grow them in full sun, practice crop rotation, leave space for aeration, water the roots, not the leaves, etc.) and you ought to be able to get a bumper crop of tomatoes!

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‘Defiant’. Photo: Johnny’s Selected Seeds.

  1. ‘Berry’
  2. ‘Brandywise’
  3. ‘Cherry Bomb’
  4. ‘Clou OP’
  5. ‘Cloudy Day’
  6. ‘Damsel’
  7. ‘Defiant PhR’
  8. ‘Defiant’
  9. ‘Fandango’
  10. ‘Fantasio
  11. ‘Fantastico’
  12. ‘Ferline
  13. ‘Foronti’
  14. ‘Golden Currant’
  15. ‘Golden Sweet
  16. ‘Invincible’
  17. ‘Iron Lady’
  18. ‘Jasper’

    ‘Fantastico’. Photo: All-America Selections

  19. ‘JTO-545’
  20. ‘Latah’
  21. ‘Legend’
  22. ‘Lemon Drop’
  23. ‘Lizzano’
  24. ‘Losetto’
  25. ‘Manalucie’
  26. ‘Manyel’
  27. ‘Matt’s Wild Cherry’
  28. ‘Merlice’
  29. ‘Mountain Fresh Plus’
  30. ‘Mountain Magic’
  31. ‘Mountain Merit’
  32. ‘Mountain Supreme’
  33. ‘Mr. Stripey’ (‘Tigrella’)
  34. ‘Oh Happy Day’
  35. ‘Old Brooks’
  36. ‘Plum Regal’

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    ‘Matt’s Wild Cherry’. Photo: Johnny’s Selected Seeds.

  37. ‘Pruden’s Purple’
  38. ‘Quadro’
  39. ‘Red Alert’
  40. ‘Red Pearl’
  41. ‘Resi’
  42. ‘Romello’
  43. ‘Rote Murmel’
  44. ‘Rote Zora’
  45. ‘Santa’
  46. ‘Stellar’
  47. ‘Striped Stuffer’
  48. ‘Sweetheart of the Patio’
  49. ‘Tommy Toe’
  50. ‘Tropic’
  51. ‘Sweetheart of the Patio’
  52. ‘Wapsipinicon Peach’

Sow tomatoes indoors 6 to 8 weeks before the last frost date. That date tends to be in mid to late May in most regions (it’s about June 15th in mine!).

Or Buy Disease-Resistant Plants

If you’re not into growing tomatoes from seed, look for tomato plants resistant to late blight in local nurseries. Things have improved considerably since the early 2010s, when the average garden center employee didn’t seem to know late blight even was. There are now late-blight resistant tomatoes being sold in garden centers in most areas. And if there is a local specialist in tomato plants, the kind of place that always offers many varieties, they’ll almost certainly offer several resistant tomato plants.

Bring Your List

I suggest you print this list or put it on your smart phone. You’ll want to have with you the next time you shop for tomato seeds or plants.

For more information on tomato diseases, read Disease-Resistant Tomatoes.

Disease-resistant Tomatoes

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20150619B

The 3rd plant is suffering from fusarium wilt.

Tomato diseases are so confusing! First, there are so many of them, plus their symptoms tend to be similar… and they cause gardeners so much trouble! But sometimes you can avoid them by choosing a disease-resistant tomato… and for that, all you really need to do is to be able to decipher the codes that follow the plants name. And even if you’ve forgotten exactly what each code means, at least you’ll know, if you buy a plant followed by a string of letters, such as VFNT, that the plant offers resistance to at least some diseases.

Tomato Disease Resistant Codes

Here is a table that shows most of the codes commonly used for tomato diseases. The letters indicate that the cultivars is resistant to the disease cited.

V                             Verticillium wilt
F                              Fusarium wilt race 1
FF                           Fusarium wilt races 1 and 2
FFF                         Fusarium wilt races 1, 2 and 3
A                             Alternaria
 (early blight)
N                             Nematodes
Ph, PHR or LB      Late blight
St                            Stemphylium
T, TMV or ToMV  Tobacco mosaic virus

So tomato varieties with a long line of letters following their name, like ‘Big Beef’ VFFNTA or ‘Celebrity’ VFFNT, are excellent choices for gardeners who have had disease problems in the past. Before buying a tomato plant, always ask the supplier let you know its disease resistances.

Do note, however, that disease resistance doesn’t mean total immunity. Especially if the summer is cool and wet, or if the soil is contaminated with spores, even a resistant plant may well get the disease, but if so, its resistance delays the start of the infestation and slows its progress, often allowing a reasonable harvest.

Late Blight: a Special Case

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A soft brown depression on tomato fruits is a sign of late blight.

New, more virulent strains of late blight (Phytophthora infestans), a tomato and potato disease, the one that caused the Irish potato famine in the 1840s, suddenly arrived in our gardens just a few years ago, taking seed companies and gardeners by surprise, because the disease had largely been quiet for over 100 years. At first, no tomato seemed resistant to these new strains. Especially active in areas with humid summers like in Northeastern North America, late blight has quickly become the most devastating disease of tomato in many gardens. Moreover, unlike other tomato diseases, which tend to weaken the plant and reduce the harvest, but often let the plant continue to produce, late blight can kill the plant outright or destroy any fruit it produces. And as its spores are spread rapidly by wind rather than remaining in the soil where tomatoes were planted in previous years, the case with most other tomato diseases, crop rotation alone is not enough to prevent it.

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First symptoms of late blight.

You can recognize tomato late blight by its symptoms. First, towards the end of the summer (it’s not called “late blight” for nothing!), brown blotches appear on the lower leaves and quickly grow and spread. On the underside of the leaf, the blotches may be covered in whitish mold. The disease then starts to move up the plant as do so many other tomato diseases, affecting leaf after leaf. Often stems turn brown as well. Worse, just when the fruit is nearly ripe, a soft, black depression forms on it, leaving it only good for the trash.

Fortunately, late blight resistant tomatoes have been found and newer resistant varieties are being launched yearly. This select group includes ‘Defiant’, ‘Fantastico’, ‘Golden Currant’, ‘Iron Lady’, ‘Jasper’, ‘Juliet’, ‘Lemon Drop’, ‘Matt’s Wild Cherry’, ‘Magic Mountain’, ‘ Mountain Merit ‘, ‘ Mr. Stripey’ (‘Tigrella ‘), ‘Plum Regal’,’Pruden’s Purple’, ‘Quadro’, ‘Resi’, ‘Rote Murmel’, ‘Rote Zora’ and ‘Wapsipinicon Peach’. If you have had problems in the past with late blight, you’d do well to use one of these resistant varieties for your summer garden.

Oddly, seedsmen seem reluctant to decide on the right code to use for this disease – I’ve seen pH, PHR and LB – and equally reluctant to use any code at all. So you rarely seen Ph, PHR or LB following the name of a tomato cultivar name even when it is late blight resistant. And that’s a shame, because it would be a very useful bit of information indeed!

So watch what you plant and, especially, look for disease resistant varieties in the future: they can make the difference between practically no harvest at all and a bumper crop!