Early and Late Blight on Tomatoes

Early blight is an air and water dispursed fungus! It’s got the beautiful name of Alternaria, and under the microscope it is a truly beautiful-looking fungus. Yet these small little fungi can cause so much destruction on plant cells. Late blight also develops on potatoes during long wet weather spells; this fungus was responsible for the Irish potato famine of 1845.

Warm conditions with high moisture promotes early blight. The spores are
dispersed by wind and by splashing water . With little wind you can see how the disease spreads from one plant to the next where infected leaves of separate plants touch each other.

Late blight is caused by Phytophthora. This fungus can destroy an entire
tomato or potato crop within a week. Controlling late blight is difficult once the disease is established. Remove and destroy diseased plant parts as they appear. Apply a fungicide such as chlorothalonil (Daconil 2787) or mancozeb as a preventative at the first sign of disease or when conditions are favorable for disease development.

To prevent late blight in tomatoes keep strictly to these cultural
practices:

1. Don’t water overhead.
2. Don’t water in the evening.
3. Give your plants plenty of space.
4. Don’t work around your plants when they are wet.
5. Don’t plant tomatoes in the same place where tomatoes, potatoes, peppers or eggplants were grown last year if they were diseased.
6. Clean up all debris.
7. Prune out diseased branches promptly and destroy.
8. Keep weeds at a minimum.
9. Plant resistant varieties when available.

Sadly we have had some very unstable weather in our area the past few weeks with lots of thunder showers almost every afternoon with high humidity and temperatures during the day. What initiated the disease on my otherwise very healthy tomatoes was two seperate unexpected hail storms shortly after each other. This caused bruising on the tomato leaves and by the next morning the first lesions started appearing (this whilst spraying preventatively – trying to be prepared for the worst (last year I lost my whole tomato crop within 3-5 days after a similar hail storm!). Now going through a repeat of last year my tomatoes are standing up a lot better by I have to go and remove new lesions on leaves every day to try and save as much as I can.

A personal observation is that the tomatoes receiving full sun all day show more resistance to the disease than the ones closer to my wall where they only get sun slightly later.

Hein Van Kralingen, Plant Pathologist, Durban, South Africa