Late blight on tomatoes (or potatoes) is caused by the organism Phytophthora infestans. Originally from Central America, late blight appeared in the northeastern United States in 1843, spread widely over the next few years, and was carried to Belgium and Ireland, where it wreaked havoc. Yes, this is the same organism largely responsible for the great starvation in Ireland in the mid-1840s. You may know it by another name-potato late blight. It may come as a surprise to learn that late blight is one of the most notorious plant pathogens in history. It is important to know that symptoms can be very difficult to distinguish from several other tomato diseases, especially Septoria leaf spot. ![]() If you are quick to observe early blight, removing affected lower foliage and protecting upper foliage with a fungicide can help reduce further spread. Infected lesions soon produce spores of their own, capable of spreading farther by wind and rain. When fruit is infected, black, sunken lesions form next to the calyx (top side). When stem lesions appear near the soil line on young plants, the canker may encircle the whole stem, causing the entire plant to wilt and die. Lesions also develop on petioles and stems, forming concentric rings if they become large enough. These gray to brown lesions commonly form a bull’s-eye pattern of rings as they expand, a highly characteristic symptom. Spots grow up to half an inch in diameter and often merge as leaves gradually yellow and die. Symptoms include circular brown lesions that start very small with a surrounding yellow ring called a chlorotic halo. Since early blight requires a long period of wetness, high humidity plays a key role as well. The disease thrives in warm summer temperatures, spreading best in the 75° to 85☏ range. If the plant surface remains wet for 5 to 10 hours, spores germinate to infect the plant. The spores, called conida, spread to lower stems and foliage by water splash, either from rain or irrigation. Other solanaceous plants, including eggplant and nightshade weeds, may also show symptoms.Īlternaria spores overwinter on infected plant debris left in the soil, or they arrive in the garden on infected seeds. ![]() The former is more virulent on tomatoes, while the latter also causes early blight on potatoes. The disease is caused by two species of fungi, Alternaria tomatophila and Alternaria solani. Usually appearing each summer wherever tomatoes are grown, early blight is quite common. ![]() There are actually two very different diseases: early blight and late blight of tomato. Adding to the confusion is that there is no single pathogen known as tomato blight. Simply put, tomato blight becomes the catch-all term for myriad pathogens and disorders that affect tomatoes. ![]() On the one hand, I am grateful that many people are quick to point in the direction of a plant disease and its management, but too often these conversations end in more confusion. What usually follows is a circling of sentiments: “Is this blight?” “Definitely blight.” “Better spray it for blight.” “Maybe tomato blight?” And my personal favorite: “It might be blight!” As growers monitor their tomato plants, they may notice scattered yellow spotting, a graying stem lesion, or a few branches wilting suspiciously. Each year as spring warms into summer, verdant vegetables hold the attention of many an eager gardener.
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