Cause Alternaria species. These fungi survive in residues of affected crops including carrot, parsley, celery, and parsnip; they also may be seedborne. Spores move by wind and water.
Symptoms Seedlings and older leaves are more susceptible. Yellow spots develop and centers may be coated with sooty black spore masses.
Cultural control
- Rotate crops.
- Use clean seed. Seed not known to be clean may be treated in water at 122°F (50°C) for 25 to 30 min. Treat a small sample of each seed lot, and check the germination after treatment to ensure seed will survive treatment.
- Bury or remove crop residues.
Chemical control
- Dithane F45 Rainshield (Group M3) at 1.5 quart/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-220008, SLN OR-090016). 24-hr reentry.
- Roper DF Rainshield (Group M3) at 2 lb/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-1300003, SLN OR-130003). 24-hr reentry.
- Switch 62.5WG (Group 9 + 12) at 11 to 14 oz./A on 7- to 10-day intervals. Do not apply for than two (2) sequential applications. Do not apply within 7 days of harvest. 12-hr reentry.