Late blight is a disease that affects mainly tomato and potato plants. Once its spores take hold, it can spread rapidly. Late blight can and will spread from potatoes to tomatoes. 
Look for signs of this fungal disease in late July, August, and into September. Keeping an eye on weather conditions can also benefit the home gardener. As cool, wet weather encourages late blight development. There is no cure for late blight, but there are a few preventative steps you can take to reduce the risk of late blight in your garden.
Plant resistant varieties. Keep in mind that a handful of varieties are resistant to late blight, but not totally immune. They may be slower to get and spread the disease, but they’re not fail safe.
Keep the disease out of your garden. When late blight has been a problem in your area during the growing season, you should not save potatoes to replant. However they’re fine to eat. Late blight does not affect humans or the flavor of the potatoes.
Plant only certified seed and seed potatoes. Late blight spores can winter in potatoes that were infected last year. So if you saved infected seed potatoes, you could be reintroducing the problem into your garden. Do not save them to replant and do not dispose of them in a compost pile.
Keep a close eye on your plants, to catch problems early. If you’re having a rainy summer and late blight has been reported in your area, you can protect your plants by using a fungicide spray. Fungicides will help prevent late blight infection, but they will not cure plants that are already infected. Prevention is the key.
For organic control, Blue Seal recommends using Bonide Copper Fungicide. This organic solution has fixed copper as an active ingredient and is specifically formulated to treat fungal infestations on tomatoes and potatoes. Remember to always follow the label directions. More is not better.