How do I know if I have Late Blight on my tomato plants?
The symptoms begin as watery grey or brown lesions on the leaves that quickly spread to the entire leaf. Dark brown stem lesions develop quickly and both lesions have white fungal growth during moist conditions. Late Blight develops rapidly, plants can die within days of infection. Usually Late Blight occurs midway up the plant with both leaves and stems affected. Check the internet for photos and visit www.hort.cals.cornell.edu for more information.
Are there other diseases that look like Late Blight?
There are other fungal diseases that cause spotting of the leaves. Early Blight is another disease of tomatoes that causes leaf spots. Early Blight lesions are small dark spots that have concentric rings in them like a target. Early Blight occurs on the older lower foliage first and does not move as quickly as Late Blight. Another disease that can occur on tomatoes is Septoria Leaf Spot. Septoria produces small tan leaf spots that are more numerous than Early blight and do not have the target appearance, Septoria starts on the older foliage and tends to occur later as the plants set fruit.
What can I do to prevent this disease from infecting my tomato plants?
Preventative fungicide sprays at 5-7 day intervals during wet weather can prevent the disease. If you are growing organically, copper-based fungicides used preventatively have shown to be effective. The key is prevention - once it has occurred, fungicides are not effective. The spores can travel miles by wind and can infect wherever they land.
What do I do if I see Late Blight on my plants?
Remove the infected plants, place them in a plastic bag and dispose of them in the garbage. Do not put them in your compost pile. If you are sure that you have Late Blight, remove all tomato and potato plants, they will eventually be diseased. Remove the stakes and ties and dry them out in the hot sun.
Why is Late Blight a problem this year?
The disease was able to spread throughout the East Coast for three reasons. The initial source of the diseased plants were distributed over a large area to many retail outlets. The weather during distribution was perfect for disease development with high humidity and wet conditions. The spores are airborne and can spread miles by the wind making it highly contagious.
Will it come back next year and does it affect seed?
Fortunately the disease does not persist in the soil over the winter without live tissue to feed from. If you remove all tomato and potato tissue from your soil, you should be able to grow tomatoes in the same area. It is recommended if you have had the disease to use preventative fungicides such as copper-based organic fungicides to keep your plants protected. The disease does not affect seed so a good clean seed source is safe to use. Due to the problem we had this year which was unusual in its scale, next year growers will be using preventative fungicides to ward off the disease.