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Plant & Pest Advisory – July 23, 2025 – Andy Wyenandt
Asparagus growers should consider scouting their fields regularly during the summer months for foliar
disease development. Important pathogens that growers need to scout for on a regular basis include
Purple spot, Cercospora, and Rust. These pathogens are easily diagnosed by the characteristic
symptoms they produce on infected plants. Purple spot lesions can appear on the spears during the
harvest season and reduce quality, as well as, on fern growth later in the summer months.
Characteristic symptoms of Purple spot include small (1 to 2 mm) slightly, sunken elliptical
reddish-purple lesions on spears and ferns.Cercospora produces small tan lesions with darker
margins on ferns and stems. Damage to ferns can cause premature defoliation which will reduce
carbohydrate flow and reduce yield for the next growing season. Chopping the fern and incorporating
the debris in the fall after the fern senesces can help destroy over wintering sources of the inoculum,
however these practices may also lead Fusarium infection. Once fern stalks are full-size and/ or
disease is detected, fungicide applications of chlorothalonil (FRAC code M5) in rotation with
azoxystrobin (FRAC code 11) should be repeated every two to four weeks until frost.
Rust is another important pathogen of asparagus. Rust can easily be diagnosed in the field early in
the season by the cream-colored oval lesions (6 to 19 mm) its produces. A few weeks later these
lesions will appear reddish-brown. These reddish-brown lesions can produce spores which can cause
more infections leading to further disease development. Control of asparagus rust is extremely
important and necessary in one and two year old beds, even with rust resistant varieties. Growers
need to scout for symptom development in cutting and non-cutting beds and, if not already started,
apply fungicides if necessary. Fungicide applications of chlorothalonil (M05), Folicur (tebuconazole,
3), mancozeb (M03), or Rally (myclobutanil, 3) rotated on a 7 to 10 day schedule will help control rust.
For more information on controlling these important diseases please see the 2024/2025 Mid-Atlantic
Commercial Vegetable Production Guide.