Late blight is a fungal disease that affects tomatoes and potatoes. It is caused by the fungus Phytophthora infestans, which overwinters in infected potato tubers and weed seeds. Late blight causes blue-gray spots on leaves that turn brown and cause leaves to drop, and irregular brown spots on fruit. It spreads rapidly in cool, wet weather. To control late blight, remove infected plant material, and use organic or chemical fungicides weekly and after rain. The best prevention methods include crop rotation, planting resistant varieties, improving drainage, watering at soil level, mulching, staking plants, and removing plants after harvest.

2. Tomato Blight: How to Identify and Treat Late
Blight in Tomatoes
Tomato blight, in its different forms, is a disease that attacks a plant’s leaves,a stems,
and even fruit.
Late blight (one form of tomato blight) is caused by a
fungus, Phytophthora infestans, which also affects potatoes. The
fungus was responsible for the Irish potato famine of 1845.
It over-winters in infected potato tubers and perennial weeds (such as
nightshade.) As the tubers and perennial seeds germinate during a new growing
season, the fungus spreads to surrounding plants.

3. What does late blight look like?
•It affects both leaves and fruit
•Leaves develop blue-gray spots
which turn brown
•Leaves eventually drop
•Fruit develops irregular brown,
greasy spots which can affect the
entire tomato
•Spots on both leaves and fruit may
develop a white, cottony ring of mold
•Late blight can overtake an entire
plant quickly (within a week) if

4. When does late blight
affect plants?
•It often strikes in cool, wet weather
•It’s most prevalent in mid- to late-
season
How do you control and treat late
blight?
The best control measure for tomato
blight is prevention (see below).
•Remove and destroy infected leaves
(be sure to wash your hands
afterwards).

5. •Organic fungicides. Treat
organically with copper spray, which
you can purchase online, at the
hardware store, or home
improvement center. Follow label
directions. You can apply until the
leaves are dripping, once a week and
after each rain. Or you can treat it
organically with a biofungicide
like Serenade. Follow label
instructions.
•Chemical fungicides. Some
gardeners prefer chemical fungicides,
the best of which for tomatoes
is chlorothalonil (sold as Fungonil,
Daconil, or under other brand names.

6. How do you prevent late
blight?
•Rotate crops. The tomato blight
(late blight) fungus can remain in the
soil for several years. Be especially
careful not to plant tomatoes in an
area where potatoes were cultivated
the year before.
•Plant disease-resistant hybrids to strengthen your plant’s chances of
being blight-free.
•Plant tomatoes in a raised bed to improve drainage and prevent diseases from
spreading.
•Give tomato plants extra space (more than 24 inches) to let air to move
among leaves and keep them dry.
•Water the soil – not the plants – to prevent
splashing. Avoid overhead watering.

7. •Mulch with black plastic or landscape fabric to
prevent fungus from spreading up onto leaves.
•Stake tomato plants for better circulation.
•Remove and destroy affected plants at the end of
the season.