Asparagus growers should consider scouting their fields 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 overwintering sources of the inoculum, however these practices may also lead Fusarium infection. Once fernstalks 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 (M5), Folicur (tebuconazole, 3), mancozeb (M3), 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 2019 Mid-Atlantic Commercial Vegetable Production Guide. The guide is available for FREE online by following the links on the Plant and Pest Advisory website. Hardcopies of the 2019 guide can be purchased through your local county Extension office.
