4. From lab bench to field trials

Hundreds of BCAs have been tested against different types and strains of mycotoxigenic fungi in vitro. However, not all of them were effective against mycotoxigenic fungi under field conditions. For instance, Johansson et al. selected 164 bacterial isolates out of 600 for a field experiment to control F. culmorum infection in wheat and three strains of Fluorescent pseudomonads and a species of Pantoea gave a high level of control and consistent results [159].

In general, the difference in BCAs performance from in vivo condition to field conditions might be related to the influence of other factors present in the field such as meteorological parameters, soil characteristics, nutrient availability, microbial community which may affect the efficacy of the screened BCAs. Other important parameters which are not present in in vivo studies include the way of delivery of the BCAs to the host (spray or direct inoculation), form of delivery (conidial or spore suspension/with or without carrier), application time (during seeding or flowering) and application route (to the soil or directly to the seed) to ensure the interaction of BCAs against the pathogen. Examples for the available BCAs in the market include AF36 and Afla-Guard® which are commercial BCAs for pre-harvest application to control aflatoxin contamination in the United States [62], Polyversum®, a recent authorized commercial product in France (Pythium oligandrum strain ATCC 38472) to be used against Alternaria spp., Fusarium spp., and other plant pathogens, and Plant ShieldTM which is the registered product for T. harzianum 22.

It is crucial to test all the application related parameters in the field as these parameters may give significantly variable results which are not usually followed in many of the performed field trials against mycotoxigenic caused diseases. For example, point inoculation of Streptomyces sp. BN1 was not effective to control FHB in wheat while spraying of bacterial spores during wheat flowering gives better results [175]. Successful formulation of C. rosea ACM941 guaranteed its efficacy to control FHB in corn, soybean and wheat under filed conditions [176], while most of the field trials used a conidial or spore suspension of the BCAs which may give variable and inconsistent results. Ear inoculation with B. amyloliquefaciens and Enterobacter hormaechei exhibit highly changeable results while treatment of seeds showed more stable results for managing F. verticillioides infection and toxin content in maize [150]. On the other hand, B. subtilis strains SB01, SB04, SB23, and SB24 were performing better to control root rot disease when they were applied to soil than treatment of soybean seeds [145]. Omitting one or more of the above parameters may lead to misevaluation of the selected BCAs.

In some cases, a mixture of two more BCAs maybe advisable in the field for a better disease control in case they have a synergistic effect. For example, mixture of L. plantarum SLG17 and B. amyloliquefaciens FLN13 showed more efficacy in controlling FHB in wheat durum [131].

Although the field trials are exhausting and time consuming, it should consider the application way, application time, effective dose and the best formula in order to precisely evaluate the performance of the selected BCAs and thereafter ensure an effective control of the mycotoxigenic fungal infection and their mycotoxins.

An important obstacle in the commercialization of BCAs is legislation. Current legislations in Europe classify BCAs as Plant Protection Products/Pesticides and hence they must follow the according regulations of the pesticides. This entails that for each BCA the mode of action must be documented and their use should be rational [177].
