Potato (Solanum tuberosum)-Black Dot

By K. Frost and C. M. Ocamb

Cause Colletotrichum coccodes, a fungus with a wide host range and present in many potato-growing regions.

Symptoms The fungus forms many minute, black sclerotia on senescencing potato tissue (including roots, stolons, tubers, and stems), especially toward the end of the growing season, hence the disease name, black dot. Leaves of affected plants may progressively yellow and wilt. Stem tissues turns yellow, brown, and dies; then sclerotia appear at the base up to several inches above soil level. Other symptoms, such as dark, necrotic, sunken lesions on stems, petioles, and veins, may occur also. Lesions may advance, causing the affected stem to die back or completely wilt. C. coccodes infection is commonly observed at the stolon end of the tuber and lesions appear brown, with a poorly defined margin. Tuber infection negatively impacts the surface appearance and can be confused with silver scurf.

Cultural control

  • Do not plant seed potatoes from fields with a history of high incidence of black dot.
  • Crop rotation longer than three years may help reduce the inoculum level in the soil.
  • Carefully manage water and fertilization to avoid crop stress.

Chemical control

  • Bravo Ultrex (Group M5) 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.
  • Blocker 4F (Group 14) at 3 to 5 pints/A on 7- to 10-day intervals. Foliar applications of Blocker 4F cannot be made if Blocker or other PCNB products were used in-furrow at planting; a maximum of two (2) foliar applications of Blocker 4F/A/year. Do not apply within 28 days of harvest. For use in Oregon, Washington, and Idaho only 12-hr reentry.
  • Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
    • Evito 480 SC at 0.16 to 0.24 fl oz/1000 row feet. Preharvest interval is 7 days. 12-hr reentry.
    • Quadris Flowable at 6 to 15.5 fl oz/A on 7- to 14-day intervals or at 0.4 to 0.8 fl oz/1000 row feet applied in a 7-inches or less band either in-furrow at planting or as a directed spray at cultivation. Preharvest interval is 14 days. 4-hr reentry.
  • Vertisan (Group 7) at 14 to 24 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 (non-Group 7). Preharvest interval is 7 days. 12-hr reentry.
  • Premixes of fungicides are available and offer ease for fungicide resistance management. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action; for other fungicide groups apply no more than two (2) sequential applications alternating with another fungicide with a different mode of action.
    • Amistar Top (Group 11 + 3) at 8 to 14 fl oz/A on a 7- to 14-day interval. Preharvest interval is 14 days. 12-hr reentry.
    • Dexter Max (Group M3 + 11) at 1.6 to 2.1 lb/A on 5- to 10-day intervals. Preharvest interval is 14 days. 24-hr reentry.
    • Elatus (Group 11 + 7) at 0.34 to 0.5 oz/1000 row feet applied in a 7-inches or less band either in-furrow at planting. Preharvest interval is 14 days. 12-hr reentry.
    • Miravis Prime (Group 7 + 12) at 9.2 to 11.4 fl oz/A on 7- to 14-day intervals for disease suppression. Preharvest interval is 14 days. 12-hr reentry.
    • Orondis Opti (Group M5 + 49) at 1.75 to 2.5 pints/A on 5- to 14-day intervals. Preharvest interval is 14 days. 12-hr reentry.
    • Quadris Opti (Group 11 + M5) at 1.6 pints/A or Quadris Top (Group 11 + 3) at 8 to 14 fl oz/A on 7- to 14-day intervals. Preharvest interval is 14 days. 12-hr reentry.
    • Quadris Ridomil Gold (Group 4 + 11) at 0.82 fl oz/1000 row feet applied in-furrow using 3 to 15 gal water/A.
    • Revus Top (Group 3 + 40) at 5.5 to 7 fl oz/A on 7- to 10-day intervals. Preharvest interval is 14 days. 12-hr reentry.