Early blight is a disease which infects tomatoes and potatoes, as well as other crops including okra and eggplant. The disease is usually caused by the fungus Alternaria solani, though the closely related Alternaria tomatophila is a common cause of early blight in tomatoes.
Early blight can be a devastating disease, as it affects every part of the plant, including the leaves, stem, and fruit, causing spots and wilting, and often resulting in leaves and fruit dropping off prematurely.
Early blight is easily identified by its bullseye-patterned spots, as well as the general pervasiveness of symptoms throughout infected plants.
The first symptoms of early blight usually appear on the oldest, lowest leaves. Spots don’t form all at once, instead starting as small pinpoint lesions near leaf centers. Over time, increasingly large rings of tissue around the lesions turn brown or black, forming a ridged spot often described as resembling a bullseye, but which can also be characterized as looking like tree rings. These spots may be surrounded by yellow leaf tissue.
As spots grow, more spots form nearby. Eventually, infected leaves turn yellow, develop fissures, and die. Dead leaves may remain attached to the plant, or drop to the ground where they can serve as a vector for the disease. In potatoes, early blight also causes extensive premature defoliation.
As the disease progresses, spots appear on younger leaves increasingly higher up the plant. Eventually, the disease spreads to the stems, where it causes brown, sunken spots. On seedlings, these infections can circle the stem (“collar rot”), causing the seedling to wither and die. On older, more robust plants, these points of infection are usually oval or irregular in shape, eventually forming dark concentric rings like those found on the leaves.
Potato tubers and tomato fruit may be infected at any stage of development, producing spots which are dark and sunken. Infected tissue becomes leathery and produces larger versions of the ridged bullseye spots found on leaves. In tomatoes, the disfigured fruit may survive, or drop prematurely. With potatoes, tubers which may look only lightly infected when harvested may become significantly more disfigured while in storage.
The damage to tomatoes and potatoes is not merely aesthetic. The tissues beneath surface lesions darken and harden. This damage makes it even more difficult to sell visibly affected crops. In addition, infection spreads into seeds and eyes, meaning that the next generation of plants start life already infected.
Early blight is caused by Alternaria solani and A. tomatophilia, which prosper in warm, damp conditions.
Alternaria solani—and its tomato-favoring cousin A. tomatophilia—is a soil-dwelling fungus found just about anywhere. The success of this pathogen is thanks in large part to its ability to happily live in and feast upon the thousands of wild and cultivated plants in the family Solanaceae, the nightshades, which in addition to tomatoes and potatoes include peppers, eggplants, paprika, tobacco, eggplants, horse nettles, and a variety of ornamentals and weeds.
In cooler climates, A. solani spores overwinter in infected plant debris. In more mild climes, the pathogen can survive in crops and weeds. When conditions warm in the spring, to where temperatures reach highs in the mid-80s Fahrenheit for at least half an hour or so, dormant spores germinate. These same temperatures are also enough to reactive active infections which have gone into dormancy.
In addition to warm temperatures, A. solani also need a great deal of moisture. Hence the disease is more frequently seen in humid climates, or in fields where poor drainage allows for standing water and waterlogged soils. However, consistent warmth and moisture isn’t necessary. The disease is capable of suspending growth and reproduction during dry, cool periods, and resuming when conditions improve.
A. solani has multiple means of infecting plants:
- Through already infected seed
- Migrating to low-hanging leaves when they contact the soil, or when infected soil is splashed onto leaf surfaces
- In potatoes, wounds in tuber surfaces allow for the pathogen to migrate directly from the soil to the plant
- Through cuts and scrapes in the surfaces of tomato fruit
The pathogen completes reproduction cycles rapidly, with spore production occurring frequently. This allows for the disease to spread rapidly through passive means such as wind and water, as well as by being carried through fields by workers and contaminated equipment.
Control of early blight requires a multi-prong treatment approach: sanitation, drainage, and proper nutrition.
Many in the agricultural community currently advocate the use of early blight-resistant plant cultivars. However, these plants are not immune to early blight, but merely resistant. In addition, we have personally observed that many disease-resistant cultivars become vulnerable to infection when grown in less than optimal conditions. If your growing conditions are such that you frequently see outbreaks of early blight, odds are that any cultivars you plant, regardless of their advertised resistances or immunities, will ultimately succumb to the combination of excessive moisture, less than optimal nutrition, and high levels of disease inoculum in the field.
This is why controlling early blight requires the grower to exert greater control over the conditions in the field.
It’s critical to ensure that soils do not remain soaked for long periods of time, and that plant surfaces can dry out quickly. Make any changes to irrigation systems necessary to allow for proper drainage. In addition, pruning low-lying leaves has dual benefits: it allows for better air circulation and dryer conditions, while also making it more difficult for infected soils to splash up onto leaf surfaces. Irrigate in the morning to allow for rapid and complete drying of leaf surfaces. Avoid doing any work while conditions are wet, as it is easier to spread the disease when soils and leaf surfaces are damp.
Remove any plants you believe to be infected. When removing infecting plants, ensure that all plant debris is removed. Tilling under debris is also acceptable, but only if complete decay can be ensured prior to the next growing season. Incompletely decayed plant matter can harbor A. solani for years.
If possible, rotate out tomatoes, potatoes, and other vulnerable nightshade species for at least two years. But crop rotation is only an effective approach if you are also careful to remove and destroy any weeds which can harbor the disease, such as horse nettle.
The use of any of a number of common preventative fungicides, such as mancozeb and chlorothalonil, can help to prevent outbreaks during periods of vulnerability. But these fungicides must be applied every one to two weeks to maintain resistance. Copper sprays are also effective at reducing and slowing the incidence and spread of early blight. But fungicides are only a complement to other efforts, and will not prevent early blight if conditions are otherwise ripe for infection.
As always, proper nutrition is key. Because seedlings are particularly vulnerable to succumbing to the disease due to collar rot, early applications of Fusion 360 Soil and Iota 0-0-1 are advisable. These products help to ensure more vigorous growth early on, and better immune response to infections. Plant vigor can be maintained with foliar applications of products like Foliar FG-31. Lastly, because A. solani infection often occurs through leaf, fruit, and tuber surfaces, ensuring the integrity of plant tissues is key. Applications of Integrity Calcium can bolster the rigidity and strength of plant tissues, making it more difficult for pathogens like A. solani to gain a toehold.