I assume you live somewhere where tomatoes can be growing this time of year? Indication of your growing zone or location is a big help when answering questions.

These plants are in containers, correct? How big are the containers? Why did you not use soil-less potting mix as is recommended for containers? Potting soil and top soil, when used in containers, compacts and causes drainage problems and root rot so that is likely the cause of your problem. Over-watering is another cause especially when a poorly draining mix is used.

But "I water twice a week" tells me you are on a fixed schedule of watering - whether the plants need it or not? Container plants don't tolerate fixed schedules since their water needs vary greatly from day to day depending on the weather, container size, and type of mix used. Some days they require multiple waterings and other days you will need to skip it all together. So you will want to re-evaluate that approach to watering only when the plants need it, when the soil is dry 4-6" down in the pot.

Also feeding is vitally important to container plants. Since the nutrients leach out every time you water they must be replaced regularly to keep the plants healthy. So what and how often are you feeding your plants.

Lastly, what variety are we talking about? Indeterminate varieties will require different care from determinate plants.

As you can see there are a number of variables that can contribute to your problems so can you provide us with more details please.=?

Dave

SimonJ, you didn't indicate what part of the country you live in. I have been growing tomatoes in an open garden, in containers on my porch, and in a greenhouse environment for years in Upstate NY. For the last two years both my open garden and porch container tomatoes have experienced the 'late blight' symptoms very similar to what you describe, while my greenhouse tomatoes have been unaffected.

After two consecutive years, local universities etc. have determined that this is a variation of the famous 'potato blight' of 150 years ago ... which has now becoming a more or less regional problem in the Northeast. And it doesn't appear to be confined to tomatoes either, having wreaked havoc on my squash, pumpkins etc. at right around the time that the fruit are really starting to develop nicely.

As Digdirt indicated, one of the best things you can do to minimize the effects is to invest in a soil moisture meter and water based on actual need not a time schedule. The 'late blight' is apparently encouraged by excess watering. Next, you can try using an organic fungicide like Serenade ... but you have to use it before the 'late blight' symptoms start appearing. Mulching the soil at the base of your tomato plants, both the containers and in the open garden, appeared to slow down the 'late blight' for me last year to the point of getting a good initial tomato harvest ... but beyond the middle of august only my greenhouse tomatoes were still healthy and still producing.

Also, with two years of experience, a new variety of tomato has been developed and tested that is supposedly very resistant to 'late blight'. It's called 'Legend' ... and I'll be trying it for the first time this year in my open garden and porch containers.