Young asparagus fields are most susceptible to foliar diseases. Depending on the weather conditions, they will typically appear soon after the plants begin to fern out. Scout regularly, especially as harvest winds down in the immature fields. Both rust and purple spot lesions have been reported in numerous fields this week. Foliar fungicide programs are most effective if they are started at very early stages of development; before the disease gets into the upper canopy, branches and cladophyls. Both diseases start in the bottom 12-24″ of the stem.
Rust (below right) first appears as slightly raised, oval lesions; 2-5mm in length. Depending on the age of the lesion, the colour ranges from pale cream to light green or light orange. Dark orange spores appear as the disease matures. These spores disseminate in the canopy causing further infections.
Stemphylium (below left) appears as a slightly sunken, oval lesion. Very new infections are brownish purple in colour. Older ones have a light tan centre with a darker purple border. Stemphylium spores will also move quickly from the lower lesions into the new growth.
We are interested in scouting immature asparagus fields (2-3 years old) and monitoring the foliar health of these crops over the growing season. If you have a young field, and are interesting in participating in the project, call or email Elaine Roddy (519 674 1616 or firstname.lastname@example.org.