Commercial growers have a number of fungicides that if applied early and on a regular basis, can reduce the spread of Late Blight. They would choose not to spray if they could, but this destructive disease does not give them any other option. Homeowners do have a few products that are registered for use and the common name of chlorothalonil should appear on the product label. Even here, these products are only effective if used before the disease appears and should be reapplied every 5-7 days if wet weather persists. For organic growers the options are very limited. Copper fungicides can be used, but they are not very effective.
If spraying any type of fungicide, remember that these materials only protect healthy tissue-infected leaves cannot be saved.
Good coverage of all the foliage is critical, and repeat applications are needed to protect new growth from infection. Always read the pesticide label and follow the instructions carefully.