Preventing Late Blight on Tomatoes and Potatoes

Late blight is spread by a fungal infection that produces millions of spores during wet seasons and quickly infects healthy plants.  Not only can spores survive in infected tubers and soil, once active, spores can travel great distances and infect neighboring areas 5-10 miles away.  Late blight was responsible for the Irish potato famine in the 1840’s and is detrimental to entire crops.  Late blight generally emerges in August when weather is hot and humid and can destroy plants in days.  Rain can gather and carry spores for miles away.  Once the infection is evident, very little will save the plant, but you can prevent the spread to other plants.

1.  Check plant daily.  Dark brown or gray lesions on the leaves, stems, joints and new 

growth are common.  Leaf browning, white powdery mold or lesions on fruit are all 

signs of blight.

2.  Avoid overhead watering; water plants at the base not on the leaves.

3.  Remove infected plant immediately and put in the garbage - do not compost or bury 

plants because blight can continue to survive and spread.

4.  Spray healthy plants with fungicide every 5-7 days (read the label).  Active ingredients 

include maneb, mancozeb, chlorothalonil or fixed copper.  Apply liberally.

5.  If a wet season, spraying early may prevent blight all together.  

6.  Use mulch or landscape fabric as water splashing from the ground onto plants may set 

blight into motion.

7.  Plant tomatoes at least 2 feet apart to allow for air circulation.

8.  Rotate crops to other areas from year to year especially tomatoes and potatoes.  

PRODUCTS

Bonide Copper Fungicide - Apply in flower bed as soon as plants are established.  Repeat at 4-14 days intervals throughout the growing season (available in liquid and dust).

Bonide Garden Dust - to control insects and blights.

Bonide Tomato and Vegetable 3 in 2 - fungicide, insecticide, miticide.

Organicide K Neem Oil - apply weekly (organic option)

Late blight on Potatoes:  On potato tubers, late blight appears as a shallow, coppery-brown dry

rot that spreads irregularly from the surface through the outer 1/2 inch of tissue.  Lesions on the surface can be brown, dry and sunken.

Peppers and eggplant can also be infected with late blight.