Nematophagous Fungi: Natural Enemies of Soil Nematodes

Overview of Nematodes and Fungi

• Nematodes are small roundworms, some of which are parasitic on plants and animals, causing significant damage to crops and livestock.

• A group of soil fungi known as nematophagous fungi act as natural enemies to these nematodes, employing various mechanisms for capturing, killing, and digesting them.

Mechanisms of Capture

• Nematodes can be captured using constricting rings or sticky branches; once captured, they are firmly gripped by the fungus.

• Different types of nematophagous fungi utilize specialized structures or spores to capture nematodes. Some employ motile zoospores while others use adhesive conidia.

Endoparasitic Fungi Behavior

• Endoparasitic fungi often live as obligate parasites in the soil. For example, Cineria anguli has zoospores that aggregate at the mouth or other openings of nematodes.

• After contact with a nematode, zoospores produce an adhesive substance that allows them to attach and penetrate the host's body.

Infection Process

• Once attached, trophic hyphae develop within the nematode to digest its contents. This process leads to the formation of zoosporangia within 10 to 12 hours post-infection.

• The release of new zoospores occurs shortly after differentiation inside the zoosporangia, allowing for further attacks on new nematodes.

Adhesive Spores and Trapping Strategies

• The genus Nemanus employs two infection strategies: producing adhesive spores like endoparasitic fungi or forming traps similar to those used by trapping fungi.

• Only one spore is needed from certain species to kill a nematode; as infection progresses, the fungus begins digesting its host.

Diverse Trapping Structures

• Various trapping structures have evolved among different fungal species. For instance, Dactylaria candida uses adhesive knobs for capturing nematodes effectively.

• Other species like Monacrosporium cyanopagum utilize adhesive branches that trap nematodes upon contact; these traps remain attached despite attempts by the host to escape.

Nematode Infection Process

Capture and Initial Infection

• The nematode is ensnared by two of the five cells in a branch, making escape impossible. The time-lapse sequence reveals vigorous cytoplasmic movement within the trap during the penetration of the cuticle.

• Vacuoles accumulate in the trap cells as the infection progresses, leading to a slowdown in the nematode's movements until its eventual death.

Development of Trophic Hyphae

• Following the nematode's death, trophic hyphae develop inside it. A new trap begins forming externally as part of this infection process.

Infection Bulb Formation

• Within two hours post-contact, an infection bulb forms, characterized by a very thin penetration tube. This bulb continues to grow for another hour and a half until it matches the width of a branch cell.

• The infection process is documented over 36 hours, showcasing futile struggles from the nematode as hyphae penetrate through its cuticle.

Digestion and Absorption