Handbook of Vanilla Science and Technology

In practice, isolates of Rhizoctonia are often tested for pathogenicity on seedlings of radish or cucumber before they are used for biocontrol. In one such study, 248 Rhizoctonia isolates from crop residues in Japan were tested, and 5 Ceratabasidum and one Thanatephorus isolates were found to be hypovirulent in both radish and cucumber seedlings (Villajuan et al. 1996). Similarly, a quarter of 153 Rhizoctonia isolates from soil in the United States were hypovirulent or apathogenic on cabbage, and several protected cabbage and cucumber seedlings from pathogens (Sneh and Ichielevich-Auster 1998).

The same Ceratobasidium that protects a plant from fungal pathogens may itself become mildly pathogenic when no other pathogen is present (Burns and Benson 2000). This is true for many types of microbial biocontrol agents, and is compatible with the view that mutualisms are balanced antagonisms, in which either partner may become parasitic under favorable conditions (Saikkonen et al. 1998, Schultz et al. 1999). There maybe a tradeoff between the protective function of the Ceratobasidium against pathogens and the cost of maintenance at the expense of the plant.

The difficulty in identifying Ceratobasidium, the occasional pathogenicity of some biocontrol isolates under certain conditions, and its relationship to the more pathogenic genus Thanatephorus will complicate its importation for biocontrol purposes. The plant health inspection service of any country will be reluctant to permit its importation, even if the same species or clade already exists in the country. Instead, it is more practical to find local Ceratobasidium isolates with no or minimal pathogenicity for domestic use. The isolates can be identified by DNA sequencing and the ones most closely related to the desired clade can be selected. Collaboration among universities and commercial agriculture are necessary to advance research in Vanilla fungal symbionts with potential to be used for biological control in the field.

In many areas a single clone of Vanilla is cultivated, making the crop more susceptible to any pathogen that can attack that particular genotype. Changing mycorrhizal partners may allow adaptation to changing conditions when no variation is present in the population of the Vanilla plant itself - a sort of extended phenotype, or a hologenome in the sense of Rosenberg et al. (2007).

Interactions between organisms are complex (Saikkonen et al. 1998, Schultz et al. 1999), so three-way interactions, such as those between Vanilla, Fusarium, and mycorrhizal fungi, will be still more complex. The argument that Ceratobasidium is potentially useful for biocontrol of Fusarium root rots in Vanilla is complicated by the facts that some Cerato-basidium can be plant pathogens and that the lifestyles and basic biology of Ceratobasidum are still largely unknown. However, this complexity applies to other biocontrol organisms and strategies as much as to Ceratobasidium. In this respect, we agree with Alfonso X of

Castille, the thirteenth-century Spanish king who reportedly said (cited from Perkins 1991):

“If the Lord Almighty had consulted me before embarking upon the Creation, I should have

recommended something simpler.”

REFERENCES

Agrios, G.N. (1997) Plant Pathology, 4th Edn. Harcourt Academic Press, San Diego.

Akkopru, A. and Demir, S. (2005) Biological control of Fusarium wilt in tomato caused by Fusarium oxysporum f. sp. lycopersici by AMF Glomus intraradices and some Rhizobacteria. Journal of Phytopathology, 153, 544-550.

Alconero, R. (1969) Mycorrhizal synthesis and pathology of Rhizoctonia solani in Vanilla orchid roots. Phytopathology, 59, 426-430.

AzctSn-Aguilar, C., Jaizme-Vega, M.C. and Calvet, C. (2002) The contribution of arbuscular mycorrhizal fungi to the control of soil-borne plant pathogens. In: Mycorrhizal Technology in Agriculture: From Genes to Bioproducts, Gianinazzi, S., Schuepp, H., Barea, J.M. and Haselwandter, K. (Eds), Vertag, Switzerland, pp. 187-196.

AzctSn-Aguilar, C. and Barea, J.M. (1997) Arbuscular mycorrhizas and biological control of soil-borne plant pathogens - an overview of the mechanisms involved. Mycorrhiza, 6, 457-464.

Bayman, P. (2006) Diversity, scale and variation of endophytic fungi in leaves of tropical plants. In: Microbial Ecology of Aerial Plant Surfaces. Bailey, M.J., Lilley, A.K., Timms-Wilson, T.M. and Spencer Phillips, P.T.N. (Eds), CABI Publishing, Oxford, UK, pp. 37-50.