Cause A fungus, Septoria petroselini, which can be seedborne affects both cilantro and parsley. Infected seed can have reduced germination and emergence. Once this fungus is introduced to a field, it survives on non-decomposed plant debris and produces conidia under a wide temperature range (50°F to 86°F). Spread occurs under moist conditions (wind-driven rain, dew drips, irrigation) and by mechanical means (tools or workers). The pathogen may overwinter as diseased plants, which survive mild winters.
Symptoms Leaf spots start as small, grayish-brown, somewhat angular spots; small oval spots may be present on petioles. Tiny dark specks may be present on the upper leaf surface as pycnidia develop.
Cultural control
- Use Septoria-free seed or hot-water treated seed (127°F for 30 minutes).
- Rotate with unrelated crops, avoid umbelliferous plants. Four (4) year rotations are recommended for diseased fields; 2 to 3 years for nondiseased fields.
- Incorporate crop debris to speed up decomposition.
- Minimize leaf wetness periods.
Chemical control
- Badge SC (Group M1) at 0.75 to 1.5 pints/A on 10-day intervals. Preharvest interval is 0 days. 24-hr reentry for greenhouse use; 48-hr reentry for all other applications.
- Dithane F45 Rainshield (Group M3) at 1.5 quart/A or Roper DF Rainshield at 2 lb/A on 7- to 10-day intervals is labeled for Alternaria leaf spot in seed crops only and can be used for controlling Septoria leafspot in Oregon only. For Oregon seed crops only (SLN OR-130003, SLN OR-090016). 24-hr reentry.
- Fontelis (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 3 days. 12-hr reentry.
- Topaz (Group 3) at 3 to 4 fl oz/A on 14-day intervals is labeled for Alternaria leaf spot and can be used in Oregon for Septoria leaf spot. Preharvest interval is 14 days. 12-hr reentry.
References Howard, R. J., Garland, J.A., and Seaman, W.L. 1994. Diseases and Pests of Vegetable Crops in Canada: an Illustrated Compendium. Ottawa, Ontario: Entomological Society of Canada.
UMass Extension. Hot water treatment of seeds. At https://Ag.umass.edu/vegetable/news/hot-water-treatment-of-seeds (accessed 5 Dec 2023).