Posted: 11/05/2012 at 09:34
There seems to be a bit of confusion between blight (an airborne fungal problem) and virus or bacteria (which are soil-based).
Blight, in terms of toms, has taken on a generic meaning. The term gets applied to any or all of the fungal problems that manifest mainly on tomato leaves. There are Blights - two of them, Early and Late - but also half a dozen other fungal problems that can resemble the Blights. Septoria Leaf Spot is only one of them.
Fungal spores are airborne. They're everywhere. You can't stop them if they're around. And once they've settled on the leaves and the disease has manifested, you can't cure it. The best you can do is take preventive measures against the spores.
The fungal spores' best friends are dampness, humidity and a lack of air circulation. Adequate space between plants and judicious trimming of excess foliage will help with air circulation.
Trimming the lower branches in order to maintain 10" to 12" between the lowest branch and the soil will help guard against reinfecting the plant during watering. Fungal spores fall from the leaves to the soil, watering can splash them back up onto the leaves, and the gap offers some protection.
Spraying is another means of prevention. Copper sprays have been around forever. Like the other preventive sprays, they coat the leaves, providing a barrier between the spores and the leaf surface. You have to spray both sides of the leaves, for obvious reasons, and respray after rain. Copper is classified, technically, as organic - because it's naturally-occurring - but it's still a metal. And it will kill foraging insects.
Other spraying options include chlorothalonyl-based products developed for tomato and other similar plants. They're chemical, but harmless to foraging insects. I've used them and found them the most effective preventive spray. But they're hard to come by for the domestic market.
As I said, once the fungal disease has manifested, there's no cure. If you catch it early enough, remove the affected leaves and destroy them. And bear in mind that you can transfer the spores from affected to unaffected leaves with your hands.
Of the Blights, Late Blight is the worst. It can and will kill a plant within weeks. Early Blight will eventually kill a plant, but, unless the infestation is severe, the plant will usually survive to the end of the season. Late Blight will also affect the fruit, unlike Early Blight, which doesn't usually, or at least not until late in the season. Nor do most of the other fungal diseases.
Fungal spores can and will live on on top of the soil. You don't need to remove the soil. Buried under a couple of inches of fresh soil, the spores remain harmlesss. Unless you stir them up again.
<a title="Early Blight" href="http://www.tomatodirt.com/tomato-blight-early.html" target="_blank">Here's some info on Early Blight, probably the most common fungal problem in the domestic tomato patch.</a>
<a title="Late Blight" href="http://www.tomatodirt.com/tomato-blight-late.html" target="_blank">Ditto on Late Blight, the most destructive of the Blights, but uncommon in the domestic tomato patch.</a>
<a title="Septoria Leaf Spot" href="http://www.tomatodirt.com/septoria-leaf-spot.html" target="_blank">Ditto on Septoria Leaf Spot, often confused with Early Blight, and just about equally common in the domestic tomato patch.</a>
These days you'll see hybrid tomato plants advertised as Blight-resistant. It doesn't mean they won't be affected by Blight - or the other fungal diseases - it just means they've been bred to battle on a bit longer.
Tomato virus and bacterial diseases are a different matter. They can be seed-borne or soil-borne. They're also less common in the domestic veg garden than fungal diseases because they ne