Diseases of Asparagus

Agdex#:	254/635
Publication Date:	05/82
Order#:	82-045
Last Reviewed:	05/98
History:	Revision of "Diseases of Asparagus, May 1982."
Written by:	R.T. Wukasch - Department of Environmental Biology/University of Guelph

Table of Contents

1. Introduction
2. Fusarium Root and Crown Rot
   • Description
   • Control
3. Asparagus Rust
   • Description and Life Cycle
   • Damage
   • Control
4. Botrytis Blight
   • Description and Life Cycle
   • Control
5. Viruses
6. Pesticide Application
7. Related Links

Introduction

Three diseases caused by fungi affect asparagus in Ontario. Fusarium root and crown rot is caused by a soil-borne fungus, while asparagus rust and Botrytis blight are caused by fungi whose spores are primarily carried about by wind. The life cycles and characteristics of these fungi will be discussed in this Factsheet to develop an understanding of how they may be recognized and controlled.

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Fusarium Root and Crown Rot

Description

The major disease problem of asparagus is caused by two species of fungi called Fusarium. Fusarium moniliforme causes decay of storage roots, stems and crowns. This fungal species is present in all agricultural soils and infects corn, grasses, and other monocotyledonous plants as well as asparagus. Fusarium oxysporum f. sp. asparagi causes root rot and seedling blight, and may also plug the water conducting vessels, causing wilting of spears and fern.

These species of soil-dwelling fungi are very prolific, long-lived, and capable of growing as saprophytes, feeding on decaying asparagus residues and soil organic matter. They colonize old roots and crowns, invading directly through root tips or through wounds from implements, cutting tools, or insect feeding. Asparagus plants which are under stress are more susceptible to infection than those which are growing vigorously.

Affected spears may shrivel and rot in spring before or after emergence (Figure 3). Infected crowns have hollow, rotted feeder and storage roots (Figure 2). When crown and stem tissue is sliced open, a reddish-brown discoloration is visible. Symptoms on fern includes stunting, yellow to brown discoloration of one or more stalks per crown, and fewer stalks per crown (Figure 1). Affected crowns decline in vigor and die. Such damage is scattered throughout the field and increases until the stand is too sparse to harvest economically.

Figure 1. Yellow discoloration of asparagus fern infected by Fusarium.

Figure 2. Cross-section of asparagus crown infected by Fusarium. Note hollow storage roots, absence of fine feeder roots, reddish-brown discoloration of diseased crown and bud tissue. (0.3x actual size)

Figure 3. Cross-section of asparagus spear infected by Fusarium. Note the decayed cortex (pith), and the reddish-brown staining, especially in the vascular bundles (water-conducting vessels) - white arrow. (Actual size)

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Control

Because Fusarium species are present in most agricultural soils, the diseases are almost impossible to avoid. Control is therefore directed at minimizing infection early in the life of crowns, and at maintaining a vigorous, long-lived asparagus stand by careful management. Suggested management practices include:

1. Avoid replanting in land which previously grew asparagus. Fusarium builds up to extremely high levels during the long period of asparagus culture in the field, and survives many years in that soil even after the crop is removed.
2. Soil fumigation with a suitable material will reduce infection in crown nurseries or direct-seeded asparagus.
3. Use treated seed. Untreated seed should be disinfested with 1 part sodium hypochlorite bleach in 5 parts water for two hours. Rinse seed in fresh water and dry before planting.
4. Use only vigorous, one year old crowns and encourage proper handling procedures for transplanting. Weak crowns are highly susceptible to infection.
5. Minimize stress in young and established plantings. Stresses which weaken asparagus crowns and thus promote Fusarium diseases are:
   • overextending the harvest season, or picking before crowns are well established;
   • insect and disease damage;
   • weed competition;
   • mechanical wounding from tillage equipment, cutting knives, etc.;
   • poor soil drainage;
   • acidic soil pH;
   • drought;
   • injury from misapplication of pesticides, fertilizers, and soil ammendments;
   • low fertility; and
   • soil compaction.
6. Use resistant varieties. At present, no varieties recommended in Ontario are resistant. However, lines and varieties being developed in the University of Guelph breeding programme are being screened for Fusarium resistance.

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Asparagus Rust

Description and Life Cycle

Puccinia asparagi, which causes asparagus rust, has a complicated life cycle consisting of several stages, all of which occur on asparagus. Some members of the onion family, such as cooking onions and chives, are also susceptible. There are no alternate hosts such as is common with other Puccinia rusts on wheat and oats.

The fungus overwinters as teliospores on asparagus debris (Figure 8). Teliospores germinate in spring, producing small, basidiospores which are blown onto emerging spears and cause infection. Later, from April until July, small upraised, light-green, oval lesions (patches) called aecia occur on the lower portion of the infected fern stalks (Figure 6). As the aecial lesions turn creamy orange in color, aeciospores are released and re-infect asparagus fern during several hours of continuous leaf wetness. Twelve to fourteen days after re-infection, upraised tan blisters called uredia appear on asparagus stalks and foliage. Uredia break open to expose masses of rusty-colored spores called uredia spores, after which the disease is named (Figure 7). Urediospores repeatedly re-infect asparagus from June until September. Warm weather with heavy dew, fog, or light rainfall enhances rust development. Late in summer, telia develop, producing black teliospores, completing the yearly life cycle (Figure 8).

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Damage

The fungus develops in the tissue of asparagus fern and drains the plant of vital nutrients. The foliage then dries out and falls prematurely, further reducing the production and storage of food for the following crop. Successive years of infection by rust will weaken asparagus crowns.

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Control

1. Cultural control involves breaking the continuity of the rust life cycle. Inoculum from overwintered teliospores in fern residue may be spread from wild or neighboring asparagus, uncut spears, young or seedling plantations, or volunteer seedlings in the asparagus crop. It is important to:
   • destroy wild and volunteer asparagus;
   • clean cut all fields during harvest; and
   • locate nurseries and young plantings remotely from established fields if possible.
2. Use resistant asparagus varieties. Viking varieties have some degree of tolerance to rust, but will become infected under conditions of favorable environment and abundant inoculum. Breeding and variety trials at the University of Guelph are including rust resistance as an important criterion in the development of new varieties.
3. Control with fungicides is presently limited to the chemical zineb. Spraying should begin when the aecial or initial uredial stages are evident on the fern - in May on young plantings, in late June in harvested plantings. Because zineb is a protectant fungicide, adequate coverage before rust appears and throughout the season is essential to achieve control.

Figure 4. Botrytis lesions on asparagus fern branches. Note the tufts of fruiting bodies produced by the fungus in these lesions. (4x actual size)

Figure 5. Extensive blighting of the lower fern canopy caused by Botrytis.

Figure 6. Aecial lesion of asparagus rust. Note oval to elliptical shaped, upraised lesion. (Actual size)

Figure 7. Uredial lesions of asparagus rust. Note tan blisters which have opened, exposing the rusty colored spore mass. (0.3x actual size)

Figure 8. Telial lesions of asparagus rust. Note black teliospores on overwintering asparagus fern. (Actual size)

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Botrytis Blight

Description and Life Cycle

This disease, caused by the fungus Botrytis cinerea, occurs during summer, causing browning of the lower fern canopy (Figure 5). Botrytis progresses most rapidly during hot, moist weather when the fern does not dry adequately. This fungus attacks many other crops, such as strawberries and other small fruit, potatoes, beans, and other vegetables, and many ornamentals.

The disease begins on senescing (dying) flowers or injured fern. Botrytis spores are spread by wind and rain within the dense fern canopy. Individual lesions are tan with dark brown borders, often surrounded by a fellow halo (Figure 4). When wet weather persists, newly-emerged spears may be completely blighted, turning brown or black, and often covered with grey, fuzzy, spore-bearing fungal growth.

Control

Zineb will also give some control of Botrytis when used regularly for rust control.

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Viruses

Though several viruses have been identified in asparagus in Europe and Western USA, they have not yet been reported in Ontario.

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Pesticide Application

As fern growth becomes dense, more water and higher rates of pesticides must be used to obtain adequate spray coverage. Good coverage is essential, especially for protective fungicides like zineb. Drop nozzles are recommended. Up-to-date pesticide recommendations are available for commercial growers in OMAFRA Publication 363, Vegetable Production Recommendations. Homeowners are referred to OMAFRA Publication 64, Insect and Disease Control in the Home Garden.

In conclusion, as a perennial crop, asparagus is a long-term investment. It will pay to monitor pests regularly throughout the harvest and growing seasons. Only then can problems be detected and controlled before they cause long-term economic damage.

Related Links

• Publication 363, Vegetable Production Recommendations
  

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