Potato late blight is a destructive plant disease that severely impacts potato and tomato crops. It is caused by Phytophthora infestans, an organism that spreads rapidly and can quickly devastate entire fields.
What is Potato Late Blight?
Potato late blight is caused by Phytophthora infestans, an oomycete, which is a type of water mold, not a true fungus. This pathogen thrives in cool, wet conditions, with night temperatures between 50 to 60°F and day temperatures from 60 to 70°F being most favorable. Free water from rain, dew, or overhead irrigation is necessary for infection to occur. The disease can spread rapidly through a crop once established. The pathogen overwinters in infected potato tubers left in the ground or in cull piles, serving as a primary source of inoculum for the next season.
Recognizing the Symptoms
Early identification of late blight is important for managing its spread. On leaves, initial symptoms appear as small, dark, water-soaked spots that quickly enlarge. These lesions often develop a fuzzy, white mold growth on the underside of leaves, particularly in humid conditions. The spots soon turn brown or black, and the affected foliage can shrivel and collapse rapidly, giving the appearance of frost damage.
The disease also affects stems, causing dark, firm lesions that can girdle the plant and lead to its collapse. On potato tubers, irregular, reddish-brown, and sunken lesions develop on the skin. When cut, these infected areas reveal a rusty-brown, dry, corky rot that can extend deep into the tuber flesh.
Preventing Late Blight
Preventing late blight outbreaks requires proactive measures. Choosing potato varieties known for their resistance to late blight can significantly reduce infection risk. Crop rotation, where potatoes and tomatoes are not planted in the same location year after year, helps break the disease cycle.
Proper sanitation includes removing volunteer potato plants that sprout from unharvested tubers and promptly disposing of all infected plant debris. Adequate spacing between plants improves air circulation, helping foliage dry more quickly and reducing humid conditions that favor the pathogen. Adjusting watering practices to avoid overhead irrigation, especially late in the day, also helps keep leaves dry. Monitoring local weather conditions and blight reports can provide early warnings, allowing for timely preventive actions.
Controlling an Outbreak
When late blight is present, immediate action limits its spread. Infected plants should be promptly removed and destroyed; they should not be composted, as the pathogen can survive and spread from contaminated material. Fungicides can be used as a control measure, with both organic options, such as copper-based products, and synthetic chemicals available. Follow label instructions for application rates and safety precautions when using any fungicide.
Protecting tubers from infection is also important. Hilling up soil around the base of the plants can create a barrier, preventing spores washed down from infected foliage from reaching and infecting the developing tubers. When harvesting, carefully inspect tubers and store only those that appear uninfected in cool, dry conditions to minimize the risk of post-harvest rot.