Asparagus Diseases - Purple Spot (Stemphylium)

Scientific Name

Pleospora herbarium (anamorph = Stemphylium vesicarium)

Purple spot is a significant disease of asparagus. Infections occurring during harvest reduce the marketability of the crop (figure 1). Severe summer infections result in the premature defoliation of the fern, which can reduce the following year's yields by up to 52%.

Identification

Tan-to-brown sunken, elliptical lesions with purple edges appear on infected spears, stems and fern (figure 2). As the disease spreads the lesions expand and merge together killing the affected tissue and eventually causing wide-spread defoliation (figure 3). Small black spores (pseudothecia) may be visible in the centre of the lesion, particularly on the previous season's crop residue.

Initial infections commonly occur on the bottom 30 cm (12 inches) of the stem, moving upwards onto the fern as the season progresses.

Figure 1. Unmarketable spear

Figure 2. Elliptical lesion with a purple border on asparagus stems

Figure 3. Expanded lesions on mature asparagus fern

Biology

The sexual stage of this disease is known as Pleospora herbarium. P. herbarium produces pseudothecia that overwinter in the asparagus crop residue. In the spring, the pseudothecia release ascospores which are the primary source of infection in the emerging asparagus crop. After the initial infection, the asexual state of the fungi (stemphylium vesicarium) begins to release conidia. The conidia are responsible for continuing infection periods throughout the growing season.

The temperature range for stemphylium is 0-30 C; however the disease reaches its peak activity from 15-25 C. Infections during harvest often occur as a result of microscopic wounds caused by wind damage or sand blasting. The stomata also act as a point of entry for this pathogen. High humidity and prolonged periods of leaf wetness promote disease development. Infection can occur in as little as 3-24 hours of leaf wetness.

Period of Activity

Purple spot is most prevalent during cool, wet conditions. It is often active during the late spring and again in the late-summer. Hot, dry conditions in July and August will slow the progression of this disease, however levels of infection can increase dramatically as the temperatures cool in early fall.

Scouting Notes

Carefully inspect 10 groups of 10 plants across the field. Look for signs of lesions at the base of the plants as they begin to develop fern. Continue monitoring throughout the growing season, especially if cooler night-time temperatures lead to heavy dew fall and prolonged periods of leaf wetness.

Thresholds

None established. Apply registered fungicides after harvest during fern establishment. Do not wait for symptoms to appear on the fern and in the upper canopy before initiating a spray program.

The TOM-CAST system, originally developed for the Ontario processing tomato industry, can successfully be used to predict stemphylium outbreaks in asparagus.

Management Notes

• Reduced tillage and the use of cover crops and windbreaks can help reduce the occurrence of sand blasting.
• Maintain a 7-21-day preventative fungicide program. Use the shorter spray interval during periods of cool, wet weather or prolonged heavy dewfall.
• No-till fields may host higher levels of stemphylium inoculum. However, tillage damages the crown, impacting the long term productivity of the field. Tillage also increases wind erosion and subsequent sand-blasting. The benefits of zero-till systems outweigh the associated stemphylium risks.
• Second and third year asparagus plantings often act as a source of secondary innoculum, infecting the main commercial crop. Begin scouting these fields for signs of infection immediately after harvest.