Squash (Cucurbita spp.)-Septoria Leaf Spot

Cause The fungus, Septoria cucurbitacearum, can infect leaves and fruit and many cucurbits. It was found in zucchini fields in the Willamette Valley during 2004. The fungus survives on host debris for more than a single year as mycelium. Pycnidia are produced on infested debris and then form conidia, which are moved by splashing or wind-blown rain. Disease is favored by cool, wet weather in the spring or late summer.

Symptoms Lesions on leaves start as small, dark, and water-soaked under moist conditions but lighter, nearly white, and often circular under dry conditions. Small black specks (pycnidia) may form on older lesions. Leaf lesions are common on winter squash and pumpkin but are not common on summer squash. Fruit lesions appear as small, white "pimple-like" spots on fruit.

Cultural control

  • Deeply plow under plant debris soon after harvest if possible.
  • Rotate crops out of cucurbits for a minimum of two years.

Chemical control Fungicide sprays are not usually economical in the Willamette Valley as the disease is usually not severe.

  • Aprovia Top (Group 7 + 3) at 10.5 to 13.5 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. May be applied the day of harvest. 12-hr reentry.
  • Inspire Super (Group 3 + 9) at 16 to 20 fl oz/A on 7- to 10-day intervals with no more than (2) sequential applications. Preharvest interval is 7 days. 12-hr reentry.
  • Mettle 125 ME (Group 3) at 8 fl oz/A on 7- to 10-day intervals. Application can be made the day of harvest. 12-hr reentry.
  • Miravis Prime (Group 7 + 12) at 9.2 to 11.4 fl oz/A on 7- to 14- day intervals. Do not apply more than two (2) sequential applications. Preharvest interval is 1 day. 12-hr reentry.

Reference Zitter, T.A., Hopkins, D.L., and Thomas, C.E. 1996. Compendium of Cucurbit Diseases St. Paul, MN: APS Press.