MINI-REVIEW Concepts and applications of foliar spray for microbial inoculants Claudia Preininger 1 & Ursula Sauer 1 & Ana Bejarano 1 & Teresa Berninger 1 Received: 9 February 2018 /Revised: 12 June 2018 /Accepted: 13 June 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Damages of the (agro)ecosystem by extensive use of chemical fertilizers and pesticides, the global dying of bee populations possibly linked to pesticide spraying, and stricter regulations for pesticide use together with successful use of microbials in IPM programs are pushing on the development and commercialization of new microbial products and a large and growing biostimulants and biocontrol market. This review focuses on microbial inoculants including bacteria, fungi, and viruses used as biostimulant or biocontrol agent for foliar application and covers all important steps from inoculant development to successful field application. Topics presented comprise typical spraying equipment including the importance of the spraying process and relating effects, furthermore formulation development including classification and adjuvants, and thirdly regulatory aspects as currently applied or under discussion. Microbial inoculants for foliar spray reported in scientific literature are summarized and contrasted with selected commercial products. Special attention is given to factors most important in microbial spray: (a) type of active ingredient (bacteria, fungi, viruses), (b) mode of action (ingestion, contact, competition), (c) interaction with the plant leaf surface, (d) droplet size in terms of microbe concentration and leaf coverage, and (e) environmental conditions during spraying. Finally, we want to emphasize that timely administration is of utmost importance for successful spraying and maximum efficacy. This might be supported by weather stations and disease/pest models as an important step towards precision farming. Keywords Spraying equipment . Formulation . Regulatory aspects . Biocontrol . Plant growth promotion Introduction Microbials such as bacteria, fungi, or viruses are applied onto crops and either have a direct impact on the target pest or stimulate the endogenous defense system of plants to protect themselves against the pests and diseases. Ten years ago, over 149 microbial strains were registered for agricultural products (Copping, 2009). Since then, the number of strains and micro- bial products has been steadily increasing, mainly due to the following reasons: regulators are urging for sustainable plant protection and reduced pesticide use; food companies and consumers are demanding more and more zero-residue food; big agrochemical companies are showing increasing interest for biological alternatives as a new business opportunity; some weeds, pests, and disease organisms have become resistant to agrochemicals; and successful implementation of integrated pest management (IPM) and boost of the organic sector driven by consumers and food companies. Depending on the type of disease or pest and the mode of action, microbials are typically applied as seed treatment, root dip, soil amendment, or foliar spray. Using foliar spray, the plants can be treated throughout the whole season, at certain growth stages to promote plant growth and improve plant resilience or at distinct weather conditions to fight plant dis- eases/pests. Microbial sprays can be also applied more frequently de- pending on the disease or pest pressure and the effectiveness of spraying. Successful spraying is typically achieved by time- ly administration of an efficient microbial formulation with proper spraying equipment. Herein, we will review recent scientific literature on micro- bial inoculants for foliar application and compare with respec- tive commercial microbial products for plant growth promo- tion and biocontrol. All literature cited in the review was col- lected with a clear focus on foliar spray. Papers on microbial inoculants not clearly addressing foliar application, the spraying process, and/or spray formulation development were * Claudia Preininger claudia.preininger@ait.ac.at 1 Center for Health & Bioresources, AIT Austrian Institute of Technology GmbH, Konrad-Lorenz-Strasse 24, 3430 Tulln, Austria Applied Microbiology and Biotechnology https://doi.org/10.1007/s00253-018-9173-4