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This spring I've talked to numerous gardeners in the Puget Sound area who have given up on growing tomatoes. Each year their tomatoes start off fine. Then in August or September, just when the crop is starting to size up, the plants get brown spots on the leaves and stems. The green fruit rots before it ripens and the plant collapses as though hit by a premature frost.
Late blight, the fungus disease causing this disappointing scenario, used to visit us one year out of five or six. Now its annual reoccurrence is depressingly routine in many gardens, my own included. Despite its name, the disease is also getting earlier in recent years. I have seen it as early as the end of June in community gardens, where the disease pressure is very high.
Not growing tomatoes at all is certainly one way to avoid the problem. It's a little like throwing out the baby with the bath water though. What you really need to throw out is your tomato cages and your old methods of growing tomatoes.
Before I get ahead of myself, let me give you a bit of background. Late blight disease spores are nearly everywhere in our region. They infect potatoes as well as tomatoes. (This disease was responsible for the Irish Potato Famine in the 1840's.) Some cultivars claim some resistance. Among tomatoes varieties you might like to try include 'Juliet,' 'Santa' and 'Legend'.
'Juliet,' like 'Santa,' grows in grape-like clusters, but is slightly larger at about an ounce. Both are oval and both are sweet and flavorful. 'Legend' is a new, large (4-5 inches!) cultivar with late blight resistance. Curiously, the Territorial Seed catalog doesn't say a word about its flavor and, since I'm growing it for the first time, I can't enlighten you either yet. Its breeder, Dr. Jim Braggett at Oregon State University, has a track record of selecting for flavor as well as earliness and other desirable characteristics, so I'm expecting good things. Keep in mind that these are merely resistant, not immune by any means. My 'Juliet' last year got late blight, but it resisted better than 'Early Girl,' 'Fantastic,' 'Sun Gold' and 'Carnival' - the other cultivars I grew.
The Achilles' heel of late blight is that it requires free moisture on the plant in order to infect. Your job is to grow the plant in a way that will keep it as dry as possible. That means irrigating at the base of the plant, not by overhead sprinkling. The best method is drip hoses or emitters under mulch. Unfortunately, when it rains, the water comes from above. Imagine a big tomato plant in a traditional cage about an hour after the rain stopped. If you plunged your arm down inside the cage among the foliage, it would come up wet, wouldn't it? That's why we can't use cages any longer.
There are a couple of alternatives. The first is to build a cover over the tomatoes. You can construct a pup tent of clear plastic, a very tall cold frame or cloche or some sort of plastic mini-greenhouse. Considering that doing proper rotations requires that you move your tomato plot each year, this is a lot of work. If you can construct something portable, you're in business. I've never gone to the trouble to build a tomato cover, but every three years I use the bed on the south side of my house for tomatoes. The wide eaves shelter the plants fairly well, if I keep them flat against the wall.
A second method of growing tomatoes is to stake each plant and grow them as single vines. As the plant grows, pinch out every side shoot that starts developing where the leaf meets the stem. As the plant gets taller, you may want to leave a leaf or two on some of the side shoots. Just remember that those leaves will also develop shoots that must be removed. Eventually, you will have a tall, columnar plant that will dry very quickly after a rain.
A plant grown this way will produce less fruit, but it is often earlier and larger. You can plant more plants per unit area, which will make up somewhat for the fewer fruits per plant. Just don't crowd them in too closely or you defeat your purpose. Allow at least a foot between the leaves of fully-grown plants for proper ventilation.
I usually use a third method, which I call espalier tomatoes. Like an espaliered tree, you train its branches to grow flat along a trellis. As with the staked method, you do lots of pinching out of side branches. Here you can leave three or four branches and train them up the trellis. Any growth that grows away from the trellis, to the front or back, gets lopped off. With all the foliage fanned out only one layer deep, the plants dry very quickly after a rain.
The bottom leaves that touch the soil surface will stay wet, so they should be removed. Late in the season, you can also start pinching out the flowers and pea-sized fruit that have no reasonable chance of maturing. I have seen gardeners prune off lots of foliage to expose the fruit to sun believing that this will hasten ripening. In fact, the fruit doesn't need direct sun and can be sun-scalded by this practice. The plant needs its leaves to produce the sugars that will be transported to the fruit to ripen it. Excess leaf removal can actually delay ripening.
These growing methods should suppress the disease sufficiently to allow you
to harvest a crop of tomatoes. Late in the season, I am usually handpicking
leaves with spots to get a few more weeks of production. If you choose to use
a fungicide, copper compounds do a fair job of preventing the disease. You must
apply it before infection and often enough to cover new growth as it emerges.
Copper has a low toxicity and is not very soluble in water, so it is relatively
safe for you and for the environment.
Hortsense: Managing plant problems with Integrated Pest Management
