Cause Alternaria radicina and potentially Alternaria dauci have been reported on parsnip in the US and Canada. 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. Lesions can increase and coalesce, killing the leaves. Older leaves are more susceptible to Alternaria than younger leaves. Plants can defoliate.
Cultural control
- Rotate with unrelated crops, avoid umbelliferous plants. Four (4) year rotations are recommended for diseased fields; 2 years for nondiseased fields.
- Remove and destroy infected plants.
- Incorporate crop debris to speed up decomposition.
- Minimize leaf wetness periods.
- Avoid overhead irrigation.
- Increase plant spacing.
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 seed crops only. 24-hr reentry.
- Fontelis (Group 7) at 16 to 30 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 0 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