Antagonism

Many reports have shown that the growth-promoting effect of PGPF is due to their ability to suppress harmful microorganisms in the soil. It is generally accepted that hyperparasitism, antibiosis, and competition are all involved in the antagonistic activities of PGPF. The mechanisms of disease suppression by PGPF isolated from zoysiagrass are shown in Table 2. The isolates of PGPF did not show hyperparasitism to other fungi. In some cases involving Trichoderma there was a relation between antibiotic activity and disease suppression, however in most cases, disease suppression was closely related with the ability to compete for infection courts or nutrient on the surface of the plant root.

The production of antagonistic substances is thought to be one of the mechanisms of protection provided by ecto-mycorrhizal fungi. As an example of this, antibacterial activities have been demonstrated for Paxillus involutus and Hebeloma crustuliniforme in pure culture (Marx 1973) and for Cenococcum graniforme in mycorrhizal symbiosis (Krywolap et al. 1964). The antibiotic effect of mycorrhizal fungi was attributed to the production of organic acids as demonstrated for P. involutus (Duchesne et al. 1989). Olsson et al. (1996) demonstrated that presence of the ecto-mycorrhizal mycelium decreased bacterial activities as

Table 2 Mechanisms of disease suppression against pathogenic fungi and bacteria by plant growth-promoting fungi (PGPF) isolated from zoysiagrass

Pathogenic fungi

PGPF

Hyperparasitism

Antibiosis

Competition

Induced resistance

R. solani

T. harzianum

— *

+

+

NT**

Phoma sp.

+

NT

F. equiseti

+

NT

P. irregulare

T. harzianum

+

+

NT

Phoma sp.

+

NT

F. equiseti

+

NT

S. rolfsii

T. harzianum

+

+

NT

Phoma sp.

+

NT

F. equiseti

+

NT

F. oxysporum f. sp. cucumerinum

T. harzianum

+

+

Phoma sp.

+

+

F. equiseti

+

C. orbiculare

T. harzianum

+

+

Phoma sp.

+

F. equiseti

+

P. simplicissimum

+

P. syringae pv. lachrimans

T. harzianum

+

+

Phoma sp.

+

F. equiseti

+

P. simplicissimum

+

* + /— : Effective/not effective; **NT: Not tested.

* + /— : Effective/not effective; **NT: Not tested.

measured by using the thymidine incorporation technique. Zak (1964) suggested that ectomycorrhizal fungi may: (a) utilize surplus carbohydrates in the root thereby reducing the amount of nutrients stimulatory to pathogens, (b) provide a physical barrier, i.e., the fungal mantle, to penetration by pathogen, (c) secrete antibiotics inhibitory to pathogens, and (d) support along with the root, a protective microbial rhizosphere population. Marx (1969) suggested that inhibitors produced by symbiotically infected host cortical cells may also have a function as inhibitors of the infection and spread of pathogen in ectomycorrhizal roots.

Competition for colonization sites, direct antibiosis, nutritional aspects, and plant defense reaction have all been considered as possible mechanisms in disease suppression by AM fungi (Azcon-Aguilar and Barea 1996). However, these mechanisms are still poorly understood.

Antifungal activities are thought to be involved in mechanisms of disease suppression by endophytic fungi. The endophytic fungi Neotyphodium coenophialum and N. lolii have been shown to form an inhibition zone under dual culture with the pathogenic fungi, Colletotrichum graminicola, Rhizoctonia cerealis, R. zeae, etc. (Siegel and Latch 1991). These results suggest that these endophytic fungi produce antifungal substances. Volatile compounds were collected from both endophyte-infected and endophyte-free tall fescue, and the sheath of endophyte-infected plants was found to produce high levels of 1-octen-3-ol, a characteristic fungal toxic volatile compound derived from lipid peroxidation in fungi, which was absent in endophyte-free plants (Yue et al. 2001). Hydroxamate siderophore synthesis by P. fortinii, a typical dark septate fungal endophyte, was reported by Bartholdy et al. (2001). Iron is an essential micronutrient for almost every organism and siderophore synthesis by P. fortinii may be expected to play a key role in iron nutrition for the plant, resulting in a lack of available iron for pathogens.

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