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Ministry of Agriculture, Food and Rural Affairs

Phytophthora

Phytopthora Decaying Spear
Click to enlarge.

Beginner

Scientific Names
Phytophthora asparagi

Identification

  • Phytophthora infections cause soft, water-soaked lesions to develop on emerging shoots at, or slightly below, the soil line.
  • These lesions elongate and turn light brown, causing the infected shoot to collapse and shrivel.
  • The infected side of the spear ceases to grow, causing bending and hooking.

Often Confused With
Fusarium Crown Rot

Period of Activity
Infections are more common during periods of excessive rainfall or in poorly drained fields.

Scouting Notes
Monitor fields during harvest and early fern development for signs of infection at the base of the spears. Infections often occur in low-lying areas or poorly drained soils. Even on coarse, sandy loam soils, drainage is often impacted by heavier clay subsoil layers or soil compaction.

Thresholds
None established.

Advanced

Scientific Names
Phytophthora asparagi

Identification
Phytophthora infections cause soft, water-soaked lesions to develop on emerging shoots at, or slightly below, the soil line. These lesions elongate, turn light brown, collapse and shrivel. The infected side of the spear ceases to grow, causing bending and hooking. Infected crown roots develop brown flesh and may become hollow. Symptoms are not always readily apparent on infected crown roots.

Phytophthora infections seriously impact crown vigour and reduce carbohydrate reserves, impacting the long-term productivity of infected fields.

Often Confused With
Fusarium Crown Rot

Biology
Phytophthora is a soil borne pathogen. It survives as oospores in infected fields for long periods of time. This disease also produces zoospores which are mobile in water and easily spread from plant-to-plant in wet growing conditions.

Laboratory studies indicate that the pathogen requires a wound (sand blasting, rain splashing, insect damage) for infection. The optimum temperatures for infection are 10-12 C. Phytophthora may also be spread by transplanting infected crowns.

Infected asparagus fields in Michigan were originally identified as Phytophthora megasperma. However, further studies have indicated that the species, while morphologically similar to P. megasperma, is specific to asparagus and has been named Phytophthora asparagi.

This disease is particularly challenging to isolate in a laboratory setting, making confirmation of a field diagnosis difficult. Selective media and specific incubation methods are recommended as described by Saude and Hausbeck, 2005. Plant Disease. 89:1011.

Period of Activity
Infections are more common during periods of excessive rainfall or in poorly drained fields.

Scouting Notes
Monitor fields during harvest and early fern development for signs of infection at the base of the spears. Infections often occur in low-lying areas or poorly drained soils. Even on coarse, sandy loam soils, drainage is often impacted by heavier clay subsoil layers or soil compaction.

Thresholds
None established.

Management Notes

  • Avoid planting asparagus in poorly drained soils, or fields with a clay sub-soil.
  • Compaction on sandy-loam soils may reduce drainage and increase the likelihood of infection.
  • Manage the crop to reduce stress and promote vigourous crown growth.
  • Avoid depleting the crown's carbohydrate reserves through pre-mature harvest or over-harvesting young plantings.