Handbook of Vanilla Science and Technology

The isolates from Ionopsis that stimulated germination and growth of Vanilla (Figures 16.6 and 16.7) belong to Ceratobasidium Clade B, as defined by Otero et al. (2007). Clade B was also the most successful Ceratobasidium clade at stimulating germination and seedling growth of Ionopsis (Otero et al. 2004). Furthermore, it was apathogenic on sterile Dendrobium mericlones (Porras-Alfaro 2004) and on bean leaves (unpublished). Clade B is very widely distributed: isolates related to those found in mycorrhizae of I. utricularioides were isolated from the same species in Cuba, Costa Rica, Panama, and Trinidad (Otero et al. 2007). This is an enormous range considering that this clade is previously unknown and does not even have a species name. Its ubiquity suggests that Ceratobasidium Clade B will not be difficult to find where Vanilla is grown, facilitating the use of local isolates. All these data suggest that this clade is potentially useful for orchid biotechnology.

16.6 CAN MYCORRHIZAL FUNGI PROTECT VANILLA PLANTS FROM PATHOGENS?

The gravity of root rots of Vanilla caused by Fusarium and other pathogens is addressed in the chapter by Juan Hernández Hernández in this volume. One of the principal motives for the study of mycorrhizal fungi in Vanilla is the possibility that mycorrhizae can protect plants from these pathogens, as well as aid in the transition of mericloned plants and seedlings to the greenhouse and then the field. In this section we argue that although this ability has not been demonstrated, it is plausible and worth studying. Our argument is based on evidence that other types of mycorrhizal fungi can protect plants from diseases, and that Rhizoctonia fungi (i.e. Ceratobasidium) are useful for biocontrol of a variety of plant pathogens, including Fusarium.

16.6.1 Biocontrol of plant diseases using VA mycorrhizal fungi

Since most crop plants have vesicular-arbuscular mycorrhizae (VAM, also called arbuscular mycorrhizae or endomycorrhizae), this is the best-known type of mycorrhiza in terms of biocontrol. In many cases VAM have been shown to increase plants’ resistance or tolerance to diseases, but only if the symbiosis is well-established before exposure to the pathogen (AzctSn-Aguilar et al. 2002). There is no single mechanism of protection: the VAM fungus may affect the pathogen, the plant, or the rhizosphere environment (Azcon-Aguilar and Barea 1997).

VAM have been used to control Fusarium root rots in several plants. VAM fungi reduced damage in Passiflora edulis var. Flavicarpa caused by a combination of F oxysporum f. passiflorae and several nematodes (Tofino and Sanchez de Prager 1997). Co-inoculation with VAM fungi and rhizosphere bacteria controlled F oxysporum f. sp. lycopersici in tomato (Akkoprii and Demir 2005). The VAM fungus Glomus macrocarpum protected bean roots from F solani, apparently by excluding the pathogen from zones of the rhizosphere where the VAM was established (Muchovej et al. 1991). Fungicides that controlled the pathogen F oxysporum in the grass Vulpia ciliata also reduced growth of VAM fungi, resulting in no net change in plant fecundity (Newsham et al. 1995). In this case the VAM fungi had no beneficial effect on the plant in the absence of the pathogen, illustrating the complexity of microbial interactions in the rhizosphere.

16.6.2 Biocontrol of plant diseases using Ceratobasidium

Ceratobasidium isolates have been used extensively for biocontrol of a variety of plant diseases in a variety of crops. Ceratobasidium is often called “binucleate Rhizoctonia” in the older literature; hyphal cells are typically binucleate, distinguishing them from the multinucleate hyphae of Thanatephorus (Figure 16.2; Sneh et al. 1991). There are several possible modes of action: directly inhibiting growth of the pathogen, competing with the pathogen for space or nutrients, or strengthening the plant’s defense through production of growth promoters or systemic induced resistance (Cartwright and Spurr 1998; Burns and Benson 2000; González et al. 2000; Sneh et al. 2004). In orchids there is another potential mode of action that has not been explored: a mycorrhizal relationship with Ceratobasidium could improve the nutritional status of the plant, making it less susceptible to pathogens.