Potato (Solanum tuberosum) - Early Blight
Cause Alternaria alternata, a fungus that overwinters as viable mycelium and spores in infected crop residue, and is more commonly found now than prior to 2000. Warm temperatures and moist conditions as well as plant injury promotes disease. Severe outbreaks have been observed after hailstorm injury in potato growing in the Treasure Valley.
Symptoms Small, brown, round spots appear on leaflets and may have very faint concentric rings. Leaf spots coalesce across large veins into brown blotches with dark brown margins. Whole leaves may turn necrotic and hang dead on plants. Leaves in the center of plants are the most susceptible. Elongated stems cankers can develop under high disease pressure. Small, deep black pits can develop on tubers when infected during harvest.
- Rotate out of potatoes for two years.
- Time irrigation to allow foliage to dry rapidly after irrigation.
- Rake and burn vines before harvest.
- Harvest only when tubers are fully mature and handle tubers carefully to avoid bruising.
- Store tubers under conditions that promote rapid suberization.
- Chlorothalonil products (Group M5) such as:
- Bonide Fung-onil is available for home gardens. H
- Bravo Ultrex at 0.7 to 1.36 lb/A or Bravo Weather Stik at 0.75 to 1.5 pints/A on 7- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
- Echo 720 at 0.75 to 1.5 pints/A on 5- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
- Echo 90 DF at 0.625 to 1.25 lb/A on 5- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
- Ridomil Gold Bravo SC (Group 4 + M5) at 2.5 pints/A, no later than row closure. Do not use within 14 days of harvest. 48-hr reentry.
- Luna Tranquility (Group 7 + 9) at 8 to 12 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) applications before alternating to a fungicide in different FRAC groups (non-Group 7 and non-Group 9). Preharvest interval is 7 days. 12-hr reentry.
- Mancozeb products (Group M3) such as Dithane F-45 Rainshield, Gavel 75 DF or Ridomil Gold MZ (Group 4 + M3) ; see label for rates and intervals. Do not apply within 14 days of harvest. 24-hr reentry for Dithane F-45 Rainshield and 48-hr reentry for Gavel 75 DF and Ridomil Gold MZ.
- Miravis Prime (Group 7 + 12) at 9.2 to 11.4 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 14 days. 12-hr reentry.
- Provysol (Group 3) at 3 to 5 fl oz/A on 7- to 14-day intervals. Preharvest interval is 7 days. 12-hr reentry.
- Veltyma (Group 3 + 11) at 5 to 10 fl oz/A on minimum interval of 14 days. Preharvest interval is 7 days. 12-hr reentry.
Biological control Efficacy in the Pacific Northwest is unknown.
- Double Nickel LC at 0.5 to 6 quarts/A on 3- to 10-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- Serenade Opti at 14 to 20 oz/A on 7- to 10-day intervals. Applications can be made up to harvest. Can rotate with other fungicides. 4-hr reentry. O