Early Blight and Late Blight Control in Tomatoes and Potatoes

Although caused by two distinctly different fungi, early blight and late blight have the same effect on tomatoes or potatoes—either one may end your garden season prematurely. I’ve dealt with both of these diseases on my tomatoes, and let me tell you, they are no joke.

• Either type of blight can appear at any time during the season, but Early Blight usually infects plants around the time of fruit set, which is late spring. Early Blight can sometimes be managed and contained, if not too severe.
• Late Blight usually appears in late summer or autumn. Unlike other fungal diseases, Late Blight is highly contagious, spreads swiftly and easily, and consumes fruit, stems, and foliage, killing the plant.

How to treat Early Blight and Late Blight

Certain fungicides are approved for treating Early Blight or Late Blight for potatoes and tomatoes in organic gardening. But even though they’re made from naturally occurring compounds, organic-approved fungicides still have a substantial soil and environmental impact and should only be used if the presence of the specific fungus that causes blight is confirmed in your area.

Most fungicides to treat Blight are a preventative, not a cure – they must be used before plants are infected. Once Late Blight sinks its teeth into your tomato or potato plants, a fungicide will have minimal control over it, but Early Blight may be controlled with proper fungicide use. Download this excellent fact sheet from Purdue University on the different fungicides available for organic gardeners.

Symptoms of Blight

Early blight

• Appears every year and can appear at any time, especially during cool, wet weather.
• Starts on the lower leaves of the plant and slowly moves up the plant as the season progresses.
• Lesions start small and grow in concentric rings.
• A mature Early Blight lesion has a bulls-eye appearance. The tissue around the early blight lesion often turns a bright yellow.
• Spores are easily carried by air currents, windblown soil, splashing rain, and irrigation water.
• Early Blight is especially aggressive when the plant canopy is large and dense.

Late blight

• Is not related to Early Blight.
• Late Blight can kill a tomato plant in just a few days.
• Moves great distances on air currents.
• Late Blight can only survive on living tissue and does not survive winter. But it can survive on an infected in-ground potato.

Buy on Amazon: The Organic Gardener’s Handbook of Natural Pest and Disease Control (Rodale Organic Gardening Books)

How to avoid Early Blight or Late Blight in your garden

Cultural controls are the most effective way of controlling blight :

• If Blight is discovered, pick off any diseased foliage, stems, or fruit and destroy it. If the fungus continues to spread, uproot and destroy the entire plant. Do not compost diseased plant plants, as the fungus easily survives the home composter.
• Prune tomatoes to allow airflow around the bottom of the plants. Blight usually starts at the bottom of the plant where foliage is slow to dry.
• Water the root zone of plants only, and water early in the day. Wet foliage encourages the development of fungi.
• Do not plant tomatoes near potatoes (they belong to the same plant family, Solanaceae, so are susceptible to the same diseases)
• Plant disease-resistant varieties. This is noted on plant tags and seed packets by the symbol AB for (Alternaria) Early Blight and LB for Late Blight.
• Practice crop rotation – don’t plant tomatoes or potatoes in the same garden bed in consecutive years, and preferably leave two years between planting.
• Late Blight requires live tissue to overwinter. While it won’t survive on your garden tools, it will survive in potato tubers, so pull any volunteer potatoes from the previous season as soon as they appear.

Resources on Early Blight and Late Blight:

• Many organic fungicides must be applied before Blight symptoms are discovered for best success. To track blight in your area, see USABlight.org.

• For excellent pics of Late Blight and Early Blight, see the Cornell University College of Agriculture website.