Cause Alternaria petroselini and potentially other species have been reported on parsley in the US. Alternaria species can be seedborne. Once this fungus is introduced to a field, it survives on non-decomposed plant debris. Disease is promoted by wet leaves and high relative humidity. Spread occurs with wind, splashing water (wind-driven rain, dew drips, irrigation), and by mechanical means (tools or workers).
Symptoms Leaf spots start as small, round brown-black spots with white centers and sometimes yellowish margins. Lesions can increase and coalesce, killing the leaves. The youngest and oldest leaves are most susceptible. Plants can defoliate and rot on the crown and upper taproot.
Cultural control
- Use Alternaria-free seed or hot-water treated seed (122°F for 20 to 30 minutes; 127°F for 10 min).
- Rotate with unrelated crops, avoid umbelliferous plants. Four (4) year rotations are recommended for diseased fields; 2 to 3 years for nondiseased fields.
- Remove and destroy infected plants.
- Incorporate crop debris to speed up decomposition.
- Avoid overhead irrigation.
- Minimize leaf wetness periods. Increase plant spacing. Improve air circulation inside greenhouses.
Chemical control
- Dithane F45 Rainshield (SLN OR-240011, SLN WA-220008) (Group M3) at 1.5 quarts/A or Roper DF Rainshield (SLN WA-1300003) at 2 lb/A on 7- to 10-day intervals is labeled for Alternaria leaf spot for Oregon and Washington parsley seed crops only. 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. Preharvest interval is 14 days. 12-hr reentry.
Biological control
- Stargus at 0.5 to 4 quarts/A on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
References McGovern, R.J. 2023. Diseases of Parsley. In: Handbook of Vegetable and Herb Diseases, Handbook of Plant Disease Management, W. H. Elmer et al. (eds.), Springer Nature Switzerland. https://doi.org/10.1007/978-3-030-35512-8_45-1
UMass Extension. Hot water treatment of seeds. At https://ag.umass.edu/vegetable/news/hot-water-treatment-of-seeds (accessed 22 Dec 2024).