Garden Mastery Tips
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June/July 2006 |
Late Blight of Tomato
 Infected tomato plant |
What is tomato late blight? Patches of brown discoloration develop on the leaf tips and edges. In damp conditions these may often be ringed with a white fungal growth. The leaf tissue is killed, and where the patches merge the whole of the affected leaflet may eventually die. The fruits first develop a brown discoloration and then start to shrink and rot. Even fruits that appeared to be healthy when picked may begin to deteriorate rapidly within a few days. Spores from the fungus Phytophthora infestans, produced on the foliage of tomato plants, are carried from plant to plant by wind and rain, and may be washed down into the soil.
Cultural Control
By taking measures to keep plants dry, late blight can be minimized. Provide ample space and prune to avoid dense foliage. This improves air circulation so foliage dries more quickly. Stake plants up off damp soil and irrigate instead of overhead watering to help keep plants dry. Also, be sure to remove and destroy dead or severely infected plants.
 Infected tomatoes |
Tomato Culture
Grow tomatoes in heavily amended compost. Provide 4 square feet per plant. Prune indeterminate plants to 2 or 3 stems and remove side shoots every few days. Trellised plants are happier. Fruit clusters should be pruned to 5 fruits on the first 2 clusters and 4 fruits thereafter. A top dressing of an inch or so of compost should be added to the soil every month. Mulch and irrigate regularly.
Chemical Control
To protect plants and fruit, spray with an appropriate fungicide. Spray plants before development of the disease when rains are anticipated. Make sure the plant name and late blight are indicated on the fungicide label. Be sure to read and follow all label directions prior to any use. PNW Plant Disease Management Handbook lists current fungicide recommendations. This information is also available at the Cooperative Extension office.
For more information see Extension Bulletin 0958
Sources
- The New Organic Grower by Eliot Coleman
- Four Season Harvest by Eliot Coleman
- Extension Bulletin 0958