Cause A fungus, Septoria petroselini, which can be seedborne. Infected seed can have reduced germination and emergence. The disease is wide spread on parsley. Avoid direct skin contact with infected plants because infected plants produce high levels of photo-carcinogenic compounds, which can result in rashes and blisters. 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.
- Use Septoria-free seed or hot-water treated seed (122°F for 30 minutes).
- Rotate with unrelated crops, avoid umbelliferous plants (carrot, celery, celeriac, dill, fennel, parsnip, etc.). Four (4) year rotations are recommended for diseased fields; 2 to 3 years for non-diseased fields.
- Incorporate crop debris to speed up decomposition.
- Minimize leaf wetness periods.
- Plant highly resistant varieties, 'Early Sugar' and 'Hamburg Thick Rooted' or resistant varieties, 'Plain Dark Green Italian' and 'Improved Market Gardener'. Highly susceptible varieties like 'Sherwood Decorator' and 'Banquet' should be avoided.
- Dithane DF Rainshield or Roper DF Rainshield (Group M3) at 2 lb/A on 7- to 10-day intervals is labeled for Alternaria leaf spot in seed crops only and will aid in controlling Septoria leafspot. For Oregon and Washington seed crops only (SLN OR-130003, SLN OR-020030, WA-020028) 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.
- Merivon Xemium (Group 7 + 11) at 4 to 11 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 11). Except for spinach, first test on a small portion of crop for phytotoxicity. Preharvest interval is 1 day. 12-hr reentry.
- Topaz (Group 3) at 3 to 4 fl oz/A on 14-day intervals. Preharvest interval is 14 days. 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.
- Quadris Flowable at 6 to 15.5 fl oz/A on 7- to 14-day intervals. May be applied the day of harvest. 4-hr reentry.
Reference 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 20 Dec 2017).