by Karen McSwain, CFSA’s Farm Services Director
Photo credit: Janine Davis, NCSU
Late blight, caused by the fungus-like pathogen Phytophthora infestans can spread rapidly under high moisture and moderate temperature (60°-80°F) conditions, meaning late blight can be a serious problem for farmers throughout the Carolinas. In fact, late blight was recently reported and confirmed in a potato sample from Carteret County earlier this month.
Managing late blight starts as soon as you see symptoms in the field, which will start with irregularly shaped gray lesions on young leaves on the top of plants. Spots can be seen on both sides of the leaves and will look like white mold on the bottom of the leaves once the pathogen has begun reproducing. Symptoms also include brown spots on plant stems and rapid loss of foliage under the optimal conditions.
Photo credit: Janine Davis, NCSU
Cultural control practices include staking tomatoes and avoiding overhead irrigation, which will reduce the level of leaf dampness by allowing for increased air flow through the plants and minimizing leaf wetness. Good sanitation is also important to reduce the spread of late blight. Now that we are in the heart of the growing season, you should scout your plants regularly and immediately remove any infected plans. ALL infected plants should be removed from the field and either burned or bagged and thrown in the garbage. DO NOT put infected plants in your compost pile as you will likely not reach temperatures needed to kill the pathogen.ng late blight starts as soon as you see symptoms in the field, which will start with irregularly shaped gray lesions on young leaves on the top of plants. Spots can be seen on both sides of the leaves and will look like white mold on the bottom of the leaves once the pathogen has begun reproducing. Symptoms also include brown spots on plant stems and rapid loss of foliage under the optimal conditions.
Organic copper fungicides have been used to control late blight; however, they need to be applied often, particularly before any rain event. If you are certified organic you may need to document that you have used preventative, mechanical, physical, and other disease management practices to control late blight and that those practices have not provided sufficient control. Depending on your certifier, you may need to update you Organic System Plan or get approval from your inspector if using copper fungicides was not included in your Organic System Plan. For a list of organically approved copper fungicides, visit the Organic Materials Review Institutes list of allowable products.
Photo credit: Cornell
If you think you might have late blight but are unsure, contact your local extension agent who will likely be able to identify it for you. North Carolina residents can send digital imagines to the NCSU Plant Disease and Insect Clinic for a free diagnosis, or send plant material for diagnosis for a fee (In-state, $20 if submitted by Cooperative Extension or NCDA&CS Regional Agronomists or $30 if submitted directly). South Carolina residents can send samples to Clemson’s Plant Problem Clinic & Nematode Assay Lab for a fee of $10 per sample.
For more information about late blight and how to control it, read Potato Late Blight Outbreak in Eastern North Carolina, written by Lina Quesada-Ocampo, NCSU Assistant Professor, Plant Pathology.