Ask OGS: Tackling Tomato Blight
Dear OGS:
We just started a farm in Haywood County. Tomatoes seem to do well here but I hear talk about tomato blight being a problem. What can I do to prevent it?
Thanks
New Organic Grower
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Dear Grower,
Blight remains one of the most significant challenges for organic tomato growers in Western North Carolina. Our region’s warm days and cool nights create ideal conditions for cultivating flavorful tomatoes. However, these same conditions also favor the development of fungal diseases. The alternating wet and dry cycles throughout the growing season exacerbate this issue further.
Understanding Early Blight
Despite their similar names, early blight and late blight are distinct diseases. Early blight, caused by the fungus Alternaria solani, is a soil-borne disease. It infects tomato plants when spores from the soil splash onto the lower leaves, especially during wet conditions. The disease then progresses upward through the plant.
In drier years, tomatoes can often outgrow early blight, minimizing yield loss. Implementing organic or plastic mulch can help prevent initial infections by reducing soil splash. Regularly removing yellowing leaves that exhibit the characteristic target-like spots can also be beneficial, though this may be labor-intensive on a larger scale.
The primary organic management strategy for early blight involves cultivating vigorous plants. Ensuring the correct soil pH, providing consistent irrigation, and applying soil amendments based on soil test results are effective methods to promote healthy plant growth. In most years, early blight is more of a nuisance than a threat to your crop’s success.
Confronting Late Blight
Late blight, caused by Phytophthora infestans, is a more formidable adversary. This airborne pathogen was responsible for the Irish potato famine and can devastate tomato crops under favorable conditions. For instance, an extended period of rainy, misty weather in August once led to the complete loss of what was anticipated to be my best tomato crop.
This experience led to a SARE-funded research project at OGS partners’ farms. Compost tea, hydrogen peroxide, and copper sulfate were tested—all of which are approved for use in organic farming. Unfortunately, none proved effective in outdoor conditions. However, copper fungicides showed excellent efficacy under cover. It’s our view that copper fungicides remain the most reliable organic solution for managing late blight.
Considerations When Using Copper Fungicides
While copper fungicides are effective, they come with certain drawbacks:
Soil Accumulation: Copper is a micronutrient at low concentrations but becomes toxic at higher levels. Repeated applications over several years can lead to soil contamination.
Preventative Use: Copper fungicides act on the surface of leaves, preventing spore germination and growth. They must be applied before infection occurs, as they are ineffective once the pathogen has penetrated the leaf tissue.
Rainfastness: Copper washes off during rain, necessitating reapplication to maintain protection.
Strategies for Managing Blight
Despite these challenges, organic tomato cultivation is not hopeless. Here are some recommendations:
Use Vigorous Plants and Mulch: To manage early blight, employ healthy transplants and apply mulch to reduce soil splash.
Implement Drip Irrigation: This minimizes leaf wetness, reducing the risk of fungal infections.
Invest in Protective Structures: Greenhouses and high tunnels can shield plants from rain, enhancing the effectiveness of copper sprays.
Limit Outdoor Plantings: Only plant as many tomatoes outdoors as you’re prepared to lose, as about half the time, crops may be affected by blight.
Apply OMRI-Listed Copper Sprays: Use approved copper fungicides weekly once late blight is reported in your area. Ensure thorough coverage, especially within dense foliage.
Use Plastic Mulch: This can intercept copper overspray, preventing soil accumulation. Be prepared to discuss this practice with your organic certifier.
Continue Spraying if Infection Occurs: Even if late blight infiltrates your covered structures, maintain weekly copper applications to protect new growth.
Looking Ahead to New Varieties
Advancements in tomato breeding offer hope. The Mountain Horticultural Crops Research and Extension Center has developed varieties like ‘Mountain Merit’ and ‘Mountain Magic’ that exhibit resistance to both early and late blight. These cultivars are the result of traditional breeding methods that align with organic production standards.
For more information on disease-resistant tomato varieties, visit the NC State Extension’s Fresh Market Tomato Breeding Program.
Happy growing!
Note: Always consult with your local extension agent or agricultural advisor before implementing new disease management strategies.
This blog post, written and copyrighted by Tom Elmore, was originally published as part of a previous Organic Growers School article series called "Ask Tom," which addressed the growing concern. Some updates may have been made by OGS staff to keep the content current.