Handbook of Vanilla Science and Technology

Apathogenic and hypovirulent strains of Ceratobasidium are common in the soil, representing 10 to 30% of total Rhizoctonia isolates in a study in Israel (Ichielevich-Auster et al. 1985) and almost half the isolates in a study in New Zealand (Sneh et al. 2004). Both these studies found that some isolates were highly effective at protecting seedlings of a variety of crops from damping-off diseases. Both studies suggested systemic induced resistance as the mechanism of protection.

Particularly relevant to prospects for Vanilla are studies that used Ceratobasidium to control Fusarium diseases. Fusarium oxysporum causes vascular wilts in spinach and tomato (F. oxysporum f. sp. spinaciae and f. sp. lycopersici, respectively). In both crops, prior inoculation with a hypovirulent Ceratobasidium significantly reduced disease severity (Muslim et al. 2003a,b). Inoculum levels of the pathogens were also reduced and germination of Fusarium propagules was delayed. A newly described species from Spain, C. albasitensis, protected a wide range of crop plants from pathogens, including F. solani (Gonzaalez et al. 2000). Ceratobasidium has also been used successfully for control of Thanatephorus pathogens in a variety of crops, including melons, tomato, lettuce, radish, chillies, potato, beans, and sugar beets (Gonzaalez et al. 2006).

In some cases effective control of pathogens has been achieved by joint inoculation with Ceratobasidium and bacteria. A combination of Ceratobasidium and Burkholderia cepacia controlled R. solani root rot in poinsettia, especially when B. cepacia was inoculated first (Hwang and Benson 2002).

It is not known if these Ceratobasidium isolates from soil can form mycorrhizae with orchids, or if Ceratobasidium isolates from orchids can protect other types of plants from pathogens. Unfortunately, it is difficult to compare fungi in different studies, because few studies have identified Ceratobasidium isolates to the species level and few have provided DNA sequence data that would allow phylogenetic analysis.

Effective, low-cost formulations for applying Ceratobasidium for biocontrol in the field have been demonstrated. For example, grain colonized with the fungus can be used to inoculate the soil around the crop (Cartwright and Spurr 1998). Most of the studies cited above contend that Ceratobasidium must be inoculated before exposure of the plant to the pathogen.

16.6.3 Are Rhizoctonia strains used for biocontrol also potential pathogens?

Before using Rhizoctonia isolates for biocontrol of root rots in Vanilla, we must ask whether these isolates have the potential to become pathogenic on Vanilla or other nearby plants. This question is difficult because lifestyles of orchid mycorrhizal fungi are not well understood. Do the orchid fungi primarily make their living as plant pathogens, saprotrophs, or mycorrhizal symbionts of orchids or other plants? No one knows. However, the entire genome of R. solani AG-3, a potato pathogen, has recently been sequenced and is currently being annotated (Cubeta et al. 2008) and will surely provide some insight into these questions.

Some Rhizoctonia groups are clearly more pathogenic than others. Thanatephorus includes serious pathogens, most notably R. solani, which devastates many crops worldwide (Agrios 1997). Orchid mycorrhizal fungi may also be pathogens. Thanatephorus isolates from the orchid Pterostylis acuminata were moderately pathogenic on lettuce and severely pathogenic on cauliflower and radish (Carling et al. 1999). An R. solani isolate capable of forming mycorrhizae on Vanilla planifolia and V. phaeantha also formed pathogenic lesions on the same roots (Alconero 1969). These isolates are too pathogenic to be used for biocontrol of root rots in Vanilla; the cure may turn out to be more harmful than the disease. Some Ceratobasidium isolates are also pathogenic. Examples in which Ceratobasidium have caused root rots include bean, pea, and soy roots in China (AG-A, Yang et al. 2005), Smallanthus sonchifolius in Brazil (AG-G, Fenille et al. 2005), and pine and spruce in Norway and Finland (Hietala et al. 2001).