Cause Stemphylium vesicarium (teleomorph = Pleospora herbarum), a fungus that overwinters in infected debris from the previous season's crop. Small, black, spherical fruiting bodies (pseudothecia) form in late fall and winter on asparagus fern material.
Symptoms Elliptical, slightly sunken spots 0.03 to 0.06 inch across and up to 0.125 inch long develop on spears. Spots often occur on only one side of the spear. Spots are purple or are purple or brown in the center with a purple margin. Fern lesions are tan to brown with dark purple margins and in length 0.125 to 0.5 inch in length. Needles drop when infections are severe.
- Remove, bury, or destroy crop debris before spring harvest.
- Bravo Ultrex (Group M5) at 1.8 to 3.6 lb/A in 25 to 50 gal of water/A on 14- to 28-day intervals. Begin applications after final harvest of spears. Do not apply within 190 days of harvest the following season. 12-hr reentry.
- Echo 720 (Group M5) at 2 to 4 pints/A on 2- to 4-wk intervals. Preharvest interval is 190 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.
- Dexter Max (Group M3 + 11) at 2 to 2.2 lb/A on 7- to 14-day intervals. Preharvest interval is 180 days. 24-hr reentry.
- Dexter Xcel (Group M3 + 11 + 3) at 64 fl oz/A on 7- to 14-day intervals. Preharvest interval is 180 days. 24-hr reentry.
- Flint Extra at 3 to 3.8 fl oz/A on 14-day intervals. Preharvest interval is 180 days. 12-hr reentry.
- Quadris Flowable at 6 to 15.5 fl oz/A on 7- to 14-day intervals. Do not apply within 100 days of harvest. 4-hr reentry.