Scientific name: Alternaria solani

Early blight and brown leaf spot are endemic foliar diseases in potatoes, both caused by Alternaria species. If left uncontrolled, Alternaria diseases can cause significant yield reductions. In most potato production areas, early blight occurs annually to some degree because the inoculum is always present, so the disease develops each growing season. Early blight affects potato leaves, stems, and tubers. The disease can reduce yield, tuber size, storability of tubers, quality of fresh-market and processing tubers, and marketability of the crop. The severity of early blight depends on the degree of foliar wetness from rain, dew, or irrigation; the nutritional status of the foliage; and cultivar susceptibility. Disease severity is highest when potato plants are injured, under stress or lack proper nutrition.

Early Blight Identification

Early blight lesions can be readily diagnosed in the field by the dark concentric rings alternating with bands of light-tan tissue, which give the leaves a distinctive “bullseye” appearance. The lesions develop first on lower, older leaves. In severe infections, the foliage is completely covered by lesions and dries up.

If early blight is severe, lesions may also be found on stems and tubers. Tuber infection occurs at harvest primarily through cuts, bruises or wounds. Lesions on tubers are dark brown to black, slightly sunken, round to irregular in shape. They are often surrounded by a raised violet-blue border. The internal decayed tissue is usually dark brown, dry and corky. On stems, fleck-like brown stem spots may develop if foliar infection is severe.

Field Scouting Practices

Regular field scouting is essential for management of the disease. For best scouting practice:

  • Scouting should start before the rows close.
  • Fields should be monitored at least twice a week.
  • When walking fields, stop at as many sites as possible and check plants at random.
  • Always examine the lower part of the plants where initial infection starts.

Chemical Control

In addition to scouting, preventative foliar fungicides have proven to be effective to control early blight. Initial fungicide applications should occur prior to row closure.

Mancozeb and chlorothalonil are perhaps the most frequently used fungicides for early blight management. But they tend to provide insufficient control under high disease pressure. As a result, locally systemic and translaminar fungicides are often necessary for control, especially for crops under irrigation.

For exceptional control of early blight and brown leaf spot, in addition to protection against white mould and black dot, more and more potato farmers depend on Proline® GOLD and Propulse® fungicide. Proline GOLD and Propulse uses two effective modes of action – fluopyram (Group 7) and prothioconazole (Group 3) – to deliver outstanding disease protection and unlock yield and quality potential.

Proline GOLD and Propulse’s single application rate of 750 ml/ha (304 mL/ac.) makes it easy to use preventatively. After the first application, a second pass can be made 10-14 days later if conditions remain favourable for disease development. When disease pressure is severe, use the shorter interval. Aim for even spray coverage and maximum canopy penetration for best results.

Resistance Management

Fungicide selection and rotation is important to prevent the early blight pathogen from developing resistance. Fungicides with the same mode of action should not be applied in consecutive applications. In addition to Proline GOLD and Propulse, Scala® fungicide (Group 9) offers excellent protection against early blight and is great resistance management tool.

A chart of potato growth stages from seed to ripening period with appropriate fungicide to use at that timing for protection of early blight. From right to left: Seed-piece Treatment is first stage, with graphic of potato seed and Emesto Complete logo above. Second stage shows graphic of seed in-furrow with Velum Rise and Minuet logos above. Arrows point to these stages to indicate key black dot timings. From here, stages are numbered from 1 to 7-12 at bottom of graphic showing potato growth progress under the soil and plant growth progress above. 1 shows germination and establishment of plant, with arrow above indicating to use a Group 11 fungicide plus protectant. 2 depicts further establishment of plant and the vegetative period, with "protectant" above. 3 depicts tuber initiation period with Proline GOLD and Propulse logos above plus "protectant". 4 depicts the beginning of flowering and tuber filling starting with Scala logo and "Bravo" above. 5 shows further flowering and tuber progression with "Late blight + Protectant" above. 6 shows further progression of flowering and tubers with Scala logo and "Bravo" above. 7-12 shows ripening stage where flowers are lost and potatoes are ripening with "Protectant (adding targeted late blight or early blight fungicides as needed)."

Additional Best Management Practices

Management of early blight requires an integrated program of cultural and chemical practices to minimize sources of inoculum, crop stress, and reduce disease development. This can include the following strategies:

  • Select a late-season variety with a lower susceptibility to early blight. (Resistance is associated with plant maturity and early-maturing cultivars are more susceptible).
  • Rotate fields to non-host crops for at least three years (three to four-year crop rotation).
  • Avoid crop stress caused by drought, insect damage and over irrigation.
  • Avoid nitrogen and phosphorus deficiency.
  • Time irrigation to minimize leaf wetness duration during cloudy weather and allow sufficient time for leaves to dry prior to nightfall.
  • Rotate fungicides with different modes of action. Fungicides with the same mode of action should not be applied in consecutive applications.
  • Avoid injury and skinning during harvest.