Paecilomyces

Paecilomyces is a nematophagous fungus which kills (harmful) nematodes by pathogenesis, or causing disease in the nematodes. Therefore the fungus can be used as a bio-nematicide to control nematodes by applying it to soil. Nematodes are crop pests.

Paecilomyces lilacinus
Paecilomyces lilacinus (Thom) Samson is a common fungus which has frequently been isolated from plant-parasitic nematode eggs and from soil in many parts of the world. It has been isolated from a wide range of habitats including cultivated and uncultivated soils, forests, grassland, deserts, estuarine sediments and sewage sludge as well as from nematode eggs and occasionally from females of root-knot and cyst nematodes. In addition, it has frequently been isolated from the rhizosphere of many crops. The species can grow at a wide range of temperatures – from 8°C to 38°C for a few isolates, with optimal growth in the range 26–30°C. It also has a wide pH tolerance and can grow on a variety of substrates (Samson, 1974; Domsch et al, 1980).

Taxonomy of Paecilomyces
P. lilacinus is classified with the Fungi Imperfecti or Deuteromycetes, fungi for which perfect (i.e. sexually reproducing) states have rarely been found. The last major revision of the genus Paecilomyces was in 1974 by Samson. Paecilomyces lilacinus is classified in the section Isarioidea, for which perfect states have not been found. Many isolates of P. lilacinus have been identified from around the world and it is accepted that variation exists within the species. P. lilacinus forms a dense mycelium which gives rise to conidiophores. These bear phialides from the ends of which spores are formed in long chains. Spores germinate when suitable moisture and nutrients are available. Colonies on malt agar grow rather fast, attaining a diameter of 5–7 cm within 14 days at 25°C, consisting of a basal felt with a floccose overgrowth of aerial mycelium; at first white, but when sporulating changing to vinaceous near Light Vinaceous Drab and Light Brownish Drab. Reverse sometimes uncoloured but usually in vinaceous shades near Daphne Red to Verona Purple. Vegetative hyphae smooth-walled, hyaline, 2.5–4.0 m wide. Conidiophores arising from submerged hyphae, 400–600 m in length, or arising from aerial hyphae and half as long. Phialides consisting of a swollen basal part, tapering into a thin distinct neck. Conidia in divergent chains, ellipsoid to fusiform, smooth walled to slightly roughened. Chlamydospores absent (Samson, 1974).

The use of P. lilacinus to control plant-parasitic nematodes
Lysek (1966) first observed P. lilacinus in association with nematode eggs and the fungus was subsequently found parasitising eggs of Meloidogyne incognita in Peru (Jatala et al, 1979). It has now been isolated from many cyst and root-knot nematodes and from soil in many locations (Stirling, 1991; Stirling and West, 1991). Jatala conducted several successful field trials using P. lilacinus against pest nematodes in Peru. He then sent his Peruvian isolate to nematologists in 46 countries for testing, as part of the International Meloidogyne project, resulting in many more field trials on a range of crops in many soil types and climates (Jatala, 1986). Field trials, glasshouse trials and in vitro testing of P. lilacinus continues and more isolates have been collected from soil, nematodes and occasionally from insects. Isolates vary in their pathogenicity to plant-parasitic nematodes. Some isolates are aggressive parasites while other, though morphologically indistinguishable, are less or non-pathogenic. Sometimes isolates which looked promising in vitro or in glasshouse trials have failed to provide control in the field (e.g. Gomes Carneiro and Cayrol, 1991).

Enzymes
Many of the enzymes produced by P. lilacinus have been studied. Bonants et al., (1995) identified a basic serine protease from P. lilacinus with biological activity against Meloidogyne hapla eggs. One strain of P. lilacinus has been shown to produce proteases and a chitinase (Khan et al 2006).Enzymes could therefore break down the egg shell to enable a narrow infection peg to push through.

Egg infection
Before infecting a nematode egg, P. lilacinus flattens against the egg surface and becomes closely appressed to it. P. lilacinus produces simple appressoria anywhere on the nematode egg shell either after a few hyphae grow along the egg surface, or after a network of hyphae form on the egg. The presence of appressoria appears to indicate that the egg is, or is about to be, infected. In either case, the appressorium appears the same, as a simple swelling at the end of a hypha, closely appressed to the eggshell. Adhesion between the appressorium and nematode egg surface must be strong enough to withstand the opposing force produced by the extending tip of a penetration hypha (Money, 1998). When the hypha has penetrated the egg, it rapidly destroys the juvenile within, before growing out of the now empty egg shell to produce conidiophores and to grow towards adjacent eggs.