Hi ya!

You might try asking this on the main tomato forum where we are fortunate to have resident PhDs in the life sciences, as well as other knowledgeable folks. :-)

From what I understand, chlorothalonil does not work to kill disease. Rather, it's effectiveness lies in preventing fungus spores from attaching to plant tissue, and it cannot kill existing infection. So resistance would not be an issue, as chlorothalonil works as a very effective preventive barrier. [Chlorothalonil has no effect on bacteria.]

For bacterial diseases, the home gardener would use a copper-based product, which has some efficacy when treating bacterial disease in tomatoes. Copper has *some* effect against fungus diseases too, and many folks will alternate chlorothalonil and copper when disease pressures are high.

I don't know if copper is a bactericide or merely a preventative barrier. But it has been used for more than a century with some measure of success, so it would appear that its action is not one that would invoke an adaptation from bacteria. But I'm not sure.

Both of these products should be applied once a week and again after rains. Considering the rainy summer we had last year, that could have been a full time job. :-)

As to your brother's predicament, I would assume what his plants got hit with last year was "late blight", which is the same disease that caused the Irish potato famine in the mid-1800s. It's a very aggressive fungus and can kill a large, fully mature tomato plant within a week. Once infected, it's a done deal. It will spread to other nearby tomato and potato plants virtually overnight. By the time the disease is visible, it's too late.

The vast majority of us lost our tomato (and potato) crops to late blight last year. This was also true for some commercial growers who didn't get to bring out the heavy professional artillery in time.

In our part of the country late blight is rare, but infected plants were brought up from the south in plants that are sold in most big box stores. The spores then spread quickly, via wind and rain.

In the East, the summer was rainy and gloomy -- perfect weather for development of the fungus spores -- so we didn't have a fighting chance, unless we quit our jobs and reapplied preventative fungicides after each and every rain.

Commercial growers/farmers have access to various systemic fungicides that are not available to the home gardeners and the organic growers, so they were less vulnerable.

Only the most dedicated home growers, combining chlorothalonil and copper treatments frequently, were able to keep late blight at bay long enough to harvest some fruit.

Otoh, early blight, also a fungus disease, can make a plant very ugly, and result in defoliation and eventually destroy the fruit, but it's rarely fatal to a plant. However, if you catch it early, remove infected foliage, and use chlorothalonil to prevent spread to other leaves and stems, the plant can recover and continue to produce fruit. Not so with late blight.

Truly, though, if you wish more information, go to the main tomato growers forum and search for chlorothalonil.

Cheers.