Early Blight

Problem type: Disease

Name of problem: Early blight

Plant name(s): Potato, tomato

Symptoms / Characteristics:
Lower leaves are the first to become symptomatic, exhibiting small (<2 mm), dark, circular lesions with papery texture. As the disease progresses, the lesions expand until the leaf veins delimit them, giving them an angular appearance as opposed to circular. Lesions may eventually form on upper leaves, stems and flowers (in tomato). Raised and depressed areas of dead tissue often cause lesions to take the form of concentric rings. Undefined areas of yellow, chlorotic tissue typically surround the lesions. Infected potato leaves tend to remain on the vine, whereas severely infected tomato leaves tend to fall.

Potato tubers can, in rare cases, exhibit dark, depressed lesions with elevated purplish edges. Beneath the skin, a brownish, cork-like rot may penetrate approximately 2 cm into the infected tuber. A similar symptom is associated with late blight infection. Infected tomato fruit may initially exhibit dark spots near the fruit stalk or beside wounds or fractures. As the disease progresses, these spots develop into black, depressed lesions that exhibit the characteristic concentric ring pattern.

Early blight is a fungal disease caused by Alternaria solani. This fungus is also known to attack pepper and eggplant, though such infections are rare. Although fluctuating wet and dry conditions yield the most severe outbreaks of early blight, prolonged periods of high moisture or humidity facilitate the initial infection process. Injury and stress render host plants extremely susceptible to infection.

Control / Preventions:
The fungus survives on infected potato tubers and crop debris and in spore-infested soil. Remove and destroy all infected plant debris in the fall, after harvest. Plant only disease-free seeds and transplants. Ensure adequate spacing between plants and rows, as close contact between plants creates a high-humidity microclimate that favors the development of the disease. Furthermore, in a high density planting, lower leaves will begin to senesce, rendering the plants extremely susceptible to infection. Keep the plants as healthy and stress-free as possible by ensuring adequate nutrition and implementing a regimented watering program. Avoid over-watering and restrict watering activity to early in the day to allow adequate leaf drying before evening dew sets. Avoid irrigating at night. Tuber resistance increases with maturity and fully mature tubers will resist infection unless wounding has occurred. Therefore, harvest only mature tubers and minimize injury during the harvest process. The fungal spores tend to concentrate near the soil surface and, if wounding has occurred, even a mature tuber will become infected when pulled through spore-infested soil. Harvesting a few days after vine killing further enhances tuber resistance. Although some varieties are more resistant than others, there are no existing varieties with complete immunity. A foliar fungicide may be applied as soon as symptoms are detected. Repeated applications may be warranted if favorable conditions for the disease (warm and humid) persist. Always use a crop rotation that includes non-host crops in order to reduce disease incidence