Disease-Free Tomatoes?

It’s not easy being a tomato gardener. The poor vegetable, by far the most popular grown in home vegetable gardens, has more than its fair share of disease problems and growing abundant, healthy tomatoes is becoming ever more difficult. Almost every gardener has been faced with tomato plants slowly (or quickly!) turning yellow or brown starting at the base, a classic symptom of such diseases as fusarium wilt, verticillium wilt and late blight. In fact, tomato diseases have become so common, some gardeners have simply stopped trying and have abandoned tomatoes in favor of easier crops.

Fortunately, there are several easy techniques you can apply—most notably the choice of resistant varieties—that can lessen the damage enough that you’ll be able to enjoy an excellent harvest of juicy tomatoes … without having to spend the summer spraying toxic chemicals. Here are a few suggestions:

1. Avoid Over Early Planting

Tomatoes planted when soil or air are still cool are more susceptible to disease. There are vegetables you can plant out early—spinach, beets, onions, etc. —, but tomatoes are not among them. Wait until both the soil and air have warmed up (nights reliably over 54? F/12? C) before planting them out.

Growing tomatoes in containers can make things easier, as potted plants can be brought indoors if there’s a sudden cold snap after your tomatoes are planted.

2. A Very Long Crop Rotation

Most tomato diseases overwinter in the soil as spores and are found wherever tomatoes or other plants of the Solanaceae family (peppers, potatoes and eggplants, among others) were grown recently. By changing the planting site annually, you should theoretically be able to avoid them.

For in-ground culture, the tradition has always been a 4-year rotation. The idea is that every year following the initial planting, the number of disease spores decreases. After four years, there should hopefully not be enough viable spores to affect the plants.

Unfortunately, while a 4-year rotation seems good enough for many vegetables, it’s becoming increasingly apparent that it isn’t for tomatoes. There are still live but dormant spores of some diseases up to 10 years later! Logically then, you’d need at least a 10-year crop rotation to be free of tomato diseases … and that’s almost impossible in many vegetable gardens!

Again, container gardening saves the day. Simply replace the soil in the container annually (it can be used for other plants that are not in the Solanaceae family) and thoroughly clean the pots with soap and bleach before using them again. That way you can use the same pots year after year with no decrease in production. (Personally, I no longer even bother growing tomatoes in the ground: container gardening gives me the results I want!)

3. Apply Mycorrhizal Fungi to the Roots

Studies show that tomato plants treated with mycorrhizal fungi (beneficial fungi) are more resistant to tomato diseases, especially those that penetrate tomatoes via their roots (fusarium, verticillium, etc.). Commercially available mycorrhizae can be applied when you sow the seeds or plant out seedlings or store-bought plants. You can also buy potting soil that already contains mycorrhizae: very convenient if you sow your own tomato seedlings.

4. Grow Your Tomatoes in Raised Beds

In cool, humid climates, where tomato diseases are especially prevalent, raised beds heat up and dry out faster, helping to prevent disease, most of which spread faster in cool, rainy weather.

5. Give Each Plant Room to Growl

Leave at least 2 feet (60 cm) between tomato plants to ensure good air circulation. Moving air dries the foliage more quickly after rain or heavy dew, helping to prevent disease.

6. Mulch Your Tomatoes

An organic mulch placed on the ground around the tomato plants (yes, right up to the base of the plants, despite a long-standing garden myth that insists mulch should never touch the base of the plants you grow!) will help prevent disease spores that overwintered in the ground from being carried onto the foliage by water droplets hitting the contaminated soil during heavy rains, then splashing onto the plant, their usual means of locomotion. Prefer a mulch rich in organic matter, like shredded leaves or ramial chipped wood.

7. Water the Soil, Not the Foliage

This one should be a no-brainer for any somewhat experienced home gardener, as it’s well known that leaves that remain moist are more subject to most diseases. Of course, you can’t control what Mother Nature does with her rain and dew, but why worsen the situation? When you water, try to moisten only the soil, not the foliage. Or water in the morning. That will allow the foliage to dry out faster than watering at night … and leaves that remain moist all night are highly susceptible to disease.

8. Remove Diseased Leaves

If, despite everything, your tomatoes appear to have yellowing, browning or spotted leaves (symptoms of many diseases), removing the affected leaves may slow the disease’s progress. Wear gloves and sterilize the pruning shears between cuts with rubbing alcohol to avoid transmitting diseases from one leaf to another.

9. Choose Resistant Varieties

Some tomatoes are naturally disease-resistant and are preferable when you have had disease problems in the past. For this purpose, tomato growers and seed companies use letters to indicate resistance. Here are some examples:

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 whole string of letters after their name, such as ‘Big Beef’ VFFNTA or ‘Celebrity’ VFFNT, are great choices for gardeners who have had tomato disease problems in the past. Before buying a tomato plant, always ask the supplier whether it is disease-resistant … and to which diseases.

These resistances seem to scare some gardeners, who take it as a sign that the plant has to be a GMO (genetically modified organism), but these are simply natural resistances that have been united in the same plant by good old-fashioned plant breeding: crossing resistant plants with each other. No OGM is needed!

Note that resistance to disease does not guarantee that the plant will not catch it, especially if the summer is cool and wet or if the soil is thoroughly contaminated with disease spores, but it will at least delay the onset of the infestation, as well as its progress, often allowing you a good harvest even though disease symptoms are starting to appear by early fall.

A Special Case: Tomato Late Blight

New, highly virulent strains of tomato late blight (Phytophthora infestans), once a tomato disease of relatively minor importance, emerged in the early 2010s and are now dispersed globally. This sudden upsurge took seed growers and gardeners by surprise, as initially no tomato appeared to be resistant to these new strains (US-23 and, to a lesser extent, US-22). Especially problematic in areas with cool, moist nights, tomato late blight has quickly become the most devastating tomato disease in many areas, including Canada, the north and northeastern US and central and northern Europe.

Also, unlike other diseases that tend to weaken the plant and reduce the harvest, but not necessarily kill it, late blight can kill the plant outright or destroy all the fruits. And it’s a wind-borne disease, so as its spores spread quickly and widely, rather than remaining in the soil where tomatoes were grown in previous years. Therefore, crop rotation has never been very useful in preventing late blight.

Tomato late blight is best recognized by its symptoms. First, towards the end of the summer (the reason it’s called “late blight”), brown spots appear on the lower leaves and grow quickly until the leaves are completely brown. On the underside of the leaves, the spots may be covered with whitish “fuzz.” Sometimes the top of the leaves and the fruit also show white mold. The disease moves successively upwards, affecting leaf after leaf. Often the stems also turn brown. Even worse, just when the fruit is almost ripe, one or more soft, brown or black depressions form on the fruit and from there on in, it’s only good for the trash.

Fortunately, it turned out that there were tomato varieties highly resistant to late blight and many more have since been bred. This is the case of ‘Mountain Magic’, ‘Mountain Merit’, ‘Defiant PhR’, ‘Iron Lady’, ‘Plum Regal’, ‘Juliet’, ‘Jasper’ and ‘Fantastico’. For a more complete list, go to Yes, Blight Resistant Tomatoes are Possible. If you’ve had problems in the past with this late blight, you should definitely be looking for one of these resistant varieties for your summer garden.

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Tomato diseases are definitely an annoyance, but there are now many ways around them. Put them into practice and you’ll soon be growing abundant, healthy tomatoes again!