Diseases Of Nematodes

Volume II

George O Poinar

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9781315892375
• eBook ISBN: 9781351088374
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

The present work deals with the diseases of nematodes. Although the term disease implies a pathological condition brought about by an infectious agent, a broader concept is used here.

Chapter 1

Fungi Attacking Vermiform Nematodes

N. F. Gray

TABLE OF CONTENTS

I. Introduction

II. The Nematophagous Habit

A. Endoparasites

1. Encysting Spores

2. Adhesive Conidia

3. Ingested Conidia

B. Predators

1. Unmodified Adhesive Hyphae

2. Adhesive Branches

3. Adhesive Nets

4. Adhesive Knobs

5. Nonconstricting Rings

6. Constricting Rings

III. Identification

A. Taxonomy

IV. Isolation

V. Physiological Considerations

A. Nutrition

B. Host/Prey Specificity

C. Chemotaxis

D. Antibiotics

E. Trap Induction

VI. Ecology

A. Distribution

B. Habitat

C. Factors Affecting Distribution

D. Spatial Distribution

1. Vertical Disbtribution

2. Horizontal Distribution

References

I. Introduction

A wide and diverse range of fungi which feed on nematodes occur in the soil. This is not surprising when one considers the long co-evolution of nematodes and soil fungi which has inevitably occurred in the close confines of the soil habitat. So predaceous and parasitic relationships have evolved amongst most of the major groups of soil fungi from the Phycomycetes to the Basidomycetes.¹ Collectively they are known as nematophagous or nematode-destroying fungi, with the term nematode-trapping fungi used to describe predators only. They are natural enemies of nematodes and have developed very sophisticated strategies for either infecting or capturing these small animals. Nematophagous fungi fall into two broad groups, those which parasitize nematodes (endoparasites) with small conidia or zoospores, or those which capture nematodes (predators) using modified hyphal traps. The fate of the nematode will be the same whether infected by an endoparasite or captured by a predator, with fungal hyphae developing within the nematode and the body contents utilized by the fungus. In nature, nematophagous fungi help to recycle carbon, nitrogen, and other important elements from the often considerable biomass of soil nematodes which are feeding on the microbial decomposers. Due to the frequency at which nematophagous fungi are isolated, it is tempting to surmise that their role of nutrient recycling in the soil is a major one, although it has never been quantified. For man however, the ability of the group to capture and destroy nematodes has presented the attractive possibility of harnessing the fungi as a biological control for this most damaging agricultural pest.

The first nematophagous fungi to be isolated and described was Arthrobotrys oligospora by Fresenius in 1852.² He was unaware of its predatory habit which was first observed 36 years later by Zopf.³ So the study of nematophagous fungi is now almost a century old. However, virtually nothing was known of these fungi until Charles Drechsler, working in Beltsville, Maryland, began his systematic study of the group. His vast contribution to our knowledge of these fungi, especially in the isolation and detailed description of new species between 1933 to 1975, and the work of others most notably Duddington in London (1950 to 1972), has provided the solid basis for research enjoyed today. At present over 150 species of nematophagous fungi are known, with new species reported each year as isolation techniques improve and new habitats are examined. The group has been extensively reviewed with a beautifully illustrated monograph produced by Barron.⁴ More recent reviews have been prepared by Mankau,¹ Barron,⁵,⁶ Peloille,⁷ and Lysek and Nordbring-Hertz.⁸

II. The Nematophagous Habit

Nematophagous fungi can be categorized as either endoparasitic or predatory in habit. Endoparasites do not produce extensive mycelium but exist in the environment as conidia, which infect nematodes by either adhering to the surface of the prey or being ingested. The conidia rapidly germinate and invade the entire nematode with assimilative hyphae absorbing all the body contents. The life cycle is completed by the fungus breaking through the body wall of the host to form reproductive structures such as conidiophores which support conidia, or evacuation tubes from which zoospores are released. In contrast, predatory fungi produce extensive hyphal systems in the environment and produce trapping devices at intervals along each hypha. The traps capture nematodes either mechanically or by adhesion, allowing the fungus to rapidly penetrate the prey and digest the body contents.

The classification of species as either endozoic or predatory is usually quite straight forward, however some species lie between the two modes. For example, some species produce free-swimming, flagellate spores (zoospores) which are probably chemically attracted to nematodes by body excretions. The zoospores swim towards the nematodes and encyst on to their surface then penetrate and kill the nematode as a normal endoparasite. Because such species actively pursue, catch, and kill nematodes, Barron⁴ has suggested that they could be classed as predators, but in terms of morphology and life-cycle they are included with the endoparasites. The genus Nematoctonus forms a bridge between the two groups with some species predatory and others endoparasitic in habit. The predatory species capture nematodes by means of adhesive knobs growing directly from the hyphae, while adhesive knobs are never found on the hyphae of the endoparasitic species of the genus, rather nematodes are captured by adhesive knobs that form on the conidia only.

A. Endoparasites

In the soil, endozoic parasites of nematodes exist mainly as conidia. Most endoparasitic nematophagous fungi are obligate parasites, spending their entire vegetative lives within the infected host. There is no extensive hyphal development from infected nematodes, with only evacuation tubes, or conidiophores and conidia, produced externally. Infection is achieved by three distinct strategies. In the higher fungi, conidia are passive and either ingested, lodging in the digestive tract where they subsequently germinate, or are adhesive, and become attached to the surface of the nematode and penetrate through its cuticle. Some species of the lower fungi are more aggressive as parasites and produce flagellated spores which are attracted to potential hosts and encyst directly on to their cuticle. In general terms, the conidia of endoparasites are an order of magnitude smaller than the conidia of predators, with adhesive spores usually <4 µm in diameter. The reason for this is the conidia of endoparasites do not require large food resources in order to produce extensive vegetative hyphae, just enough energy to allow them to germinate and penetrate the host.

Endoparasites are found in a number of taxonomic classes which can be loosely categorized into three groups: Group I, Encysting species of the Chytridiomycetes (Catenaria) and Oomycetes (Myzocytium); Group II, Deuteromycetes producing adhesive conidia (Verticillium, Cephalosporium, Meria) and ingested conidia (Harposporium); and Group III, Basidiomycetes forming adhesive conidia (Nematoctonus). In a survey of Irish nematophagous fungi Gray⁹ measured the frequency of occurrence of endoparasites isolated from a range of soils. He found that encysting (Group I) species represented 21.4% of isolations, Deuteromycetes (Group II) 67.1%, and Basidiomycetes (Group III) 11.4%. Adhesive Deuteromycetes were more frequently isolated (45.7%) than ingested conidia (21.4%).

1. Encysting Spores

The endoparasitic species of Chytridiomycetes and Oomycetes have similar methods of infecting nematodes, with flagellated zoospores produced in hyphal bodies (zoosporangia) and released into the environment via evacuation tubes. The zoospores have only a short time in which to find a suitable host and do this by following the chemical gradient formed by exudates released from the body orifices of nematodes. On reaching the nematode the zoospore encysts close to an orifice, i.e., the anus, vulva, or buccal cavity. Infection is achieved either by germ tubes which pass into the nematode through an orifice or by direct penetration through the animal’s cuticle. Inside the host an infection thallus is formed which absorbs the body contents, and on reaching maturity sections of the thallus swell to form discrete zoosporangia in which zoospores are produced. The life cycle is completed when the zoospores are released into the soil environment via the evacuation tubes which connect each zoosporangium to outside the host’s body. While this method is used by Catenaria anguillulae and many species ...

Table of contents

• Cover
• Title Page
• Copyright Page
• Introduction
• Editors
• Contributors
• Dedication
• Table Of Contents, Volume II
• Parasites and Predators
• 1 Fungi Attacking Vermiform Nematodes
• 2 Fungi Colonizing Cysts and Eggs
• 3 Infection Events in the Fungus-Nematode System
• 4 Invertebrate Predators
• 5 Biological Control of Plant-Parasitic Nematodes
• Index