Laidback Gardener Tip of the Day

Disease-resistant Tomatoes

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

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.

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!

2 comments on “Disease-resistant Tomatoes

  1. Pingback: First-time Gardener: Tomatoes from Seed – Laidback Gardener

  2. Pingback: Yes, Blight-Free Tomatoes are Possible! | Laidback Gardener

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