Biologicals are a broad category of plant protection and strengthening products that are derived from or inspired by nature. There are two key classifications: Biocontrols and Biostimulants.
Alternative products such as biological control agents (BCA), in which microorganisms with antagonistic traits are used to suppress plant diseases by various mechanisms; plant resistant inducers (PRI), which elicit resistance in the plant, and basic substances (e.g. plant extracts, lecithin) can be exploited to control plant diseases to reduce the reliance on fungicides. Alternative products are considered suitable substitutes to fungicides because they are environmentally benign and are not subject to fungicide resistance development. Several alternative products have emerged, and others have been under development in the last decades. However, the inclusion of alternative products into disease management strategies remains challenging to use due to their unpredictable field performance and difficulties with their integration into current cropping systems. At present, the documented efficacy of alternative products ranges from 20 to 60%. Moreover, their efficacy under field conditions is characterized by a huge variation, which means that their efficacy is unpredictable. Thus, the sole reliance on alternative products in crop protection is considered high-risk [9], and, to date, conventional fungicides remain essential components of disease management strategies. Therefore, for these alternative products to live up to their potential, it is essential to research and develop effective strategies for using their usage under field conditions.
In contrast to controlled conditions (e.g., greenhouse), alternative products are less effective under field conditions mainly due to
Thus, strategies that adequately address these challenges are likely to optimize the efficacy of alternative products. The high epidemic profile challenge can be addressed by integrating alternative products into IPM strategies that minimize the disease pressure. The core aim of IPM programs is to reduce the reliance on pesticides by combining various management tools that target different aspects of the target pest (or disease). Alternative products have proven more effective in preliminary studies in the field under low to moderate disease/inoculum pressure (Abuley IK, unpublished). As a result, IPM tools (e.g., host resistance, crop rotation) that reduce inoculum pressure are likely to enhance the efficacy of alternative products. The repertoire of tools (e.g., crop rotation, fertilization, host resistance) utilized in an IPM program is numerous. This project, however, does not focus on tools such as crop rotation and fertilization, as they are already integrated as part of good agronomic practices by growers. In contrast, the integration of IPM tools such as host resistance and decision support systems are poorly understood, especially with alternative products. Thus, research is required to understand how to integrate host resistance and DSS with alternative products.
Although host resistance is effective as a standalone method for managing STB, the pathogen can quickly evolve to defeat the resistance genes, thus rendering host resistance's effectiveness temporary. However, host resistance can be deployed sustainably via diversification strategies such as cultivar mixtures. Cultivar mixture, which is a practice where cultivars that differ for several characteristics, including host resistance profiles, are grown as a mixture, is an ideal method for suppressing disease epidemics as well as prolonging the effectiveness of resistance genes. The disease suppression and yield benefits of cultivar mixture in wheat are well documented. Thus, the disease reduction potential of the cultivar mixture can be exploited to optimize the efficacy of alternative products.
Host resistance has also been identified as an integral IPM tool for managing downy mildew; however, systematic classification of the resistance and its subsequent integration into IPM strategies is undone in Denmark. Thus, the successful integration of alternative products for managing downy depends on a comprehensive understanding of the varieties' resistance/tolerance to the mildew.
Alternative products on the phyllosphere usually have a short effective lifespan and are susceptible to adverse weather conditions (e.g. UV light, rainfall, temperature fluctuations). Whereas conventional fungicides last on the phyllosphere for several days (e.g., seven days), alternative products last for a few days (e.g., two days). To bring these products to their potential, improving their formulation by including adjuvants that enhance their longevity under field conditions is essential. Studies to determine the optimal formulations and the influence of adjuvants, among others, are of prime importance to enhance the efficacy of alternative products [9]. Increased survival and efficacy of Bacillus thuringiensis have been achieved under direct sunlight conditions by formulating the inclusion of UV absorbents and starch encapsulation. Other substances such as clay, lignin, green tea, yeast extract, etc. have also been proven to improve the longevity of biocontrol.
Alternative products such as BCAs use a variety of mechanisms (i.e., modes of activity) to suppress plant diseases. These modes of activity may be a direct (e.g. antibiosis, hyperparasitism) or indirect (competition for space or nutrients, induction of resistance) antagonism against the targeted pathogen. Similarly, resistance induced by plant resistant inducers (PRI) may occur by the stimulation of a range of phytohormones or compounds that may be ephemeral (i.e. lasting a few days after application) or durable (i.e. lasting for the entire growth cycle). Understanding these modes of activity is essential for developing application protocols, modeling the timing of application, and the overall success of disease control. For example, BCAs that utilize mycoparasitism against pathogens will be most effective when applied synchronously or just before the arrival of those pathogens. In contrast, BCAs that interfere indirectly (e.g., by competition or resistance induction) or by antibiosis (i.e., production of anti-fungal or antibacterial chemicals) with pathogens will be most effective when applied a few days before the arrival of spores. Understanding the mode of activity is also crucial in the formulation of compatible mixtures of BCAs, which have been proposed as an effective strategy to optimize disease control.
Timing of fungicide application via the use of forecasting models or decision support systems (DSS) has been shown to improve the efficiency of fungicides [19, 27-29]. DSSs have, thus, become an integral part of managing several diseases such as downy mildew (onions) and STB (wheat). DSSs alone can reduce fungicide usage by up to 50%, and further reduction is envisaged if such DSSs are used to optimize alternative controls to replace some or all fungicides treatments. Unfortunately, such systems have not been applied for the usage of alternative products. The use of alternative products is applied based on the calendar days, without regard to the diseases cycle. However, we see opportunities in optimizing the efficacy of alternative products via the timely application with the aid of DSSs. The current alternative products do not have a curative effect, and thus their application must be correctly timed with the arrival of the pathogen to ensure optimal efficacy. Among others, disease occurrence must depend on definable weather variables to be suitable for disease forecasting. Fortunately, the outbreak of both STB and downy mildew are dependent on weather variables such as humidity and temperatures. DSSs such as the humidity model (HM) and Crop Protection Online (CPO) have been developed and validated for timing the application of conventional fungicides for STB in Denmark with success, and thus adapting these DSSs to optimize the application of alternative products will be helpful to optimize alternative products. The HM DSS is based on hourly values for relative humidity, leaf wetness, or rain events. Thresholds for treatment recommendations are based on a continuum of a fixed number of hours with humid
conditions. The CPO DSS uses the number of days with precipitation as an indicator of risk for infection. The CPO recommends control of STB after either 4 or 5 days with rainfall higher than 1 mm per day.
Currently, there are no operational DSSs for timing neither conventional fungicide nor alternative products for downy mildew in onions in Denmark. However, DSSs (e.g., MILIONCAST, DOWNCAST) for timing fungicide application have been reported in the literature, which could be adapted for use in Denmark. Moreover, Denmark has a well-established late blight (Phytophthora infestans) DSS (BlightManager), which can be adapted for timing both fungicides and alternative products to manage downy mildews in onions. The BlightManager DSS has several sub-models, but the two key ones are the infection pressure and infection risk sub-models. These sub-models estimate the sporulation potential (infection pressure) and the risk of infection of sporangia (infection risk) based on temperature and relative humidity (or leaf wetness). Together these sub-models provides a robust metric for evaluating the risk of infection by aerial pathogens such as Peronospora destructor. The adaption of BlightManager for downy mildew could be made possible given the similarities between the pathogens (Phytophthora infestans and Peronospora destructor) for their environmental requirements. Both pathogens require high humidity (>90%) and temperatures between 5-24°C. It is noteworthy that the BlightManager is fine-tuned to use both local of field-specific weather and regional weather data, thus giving both field and region infection pressure for timing the application of fungicides.
The rationale for the choice of crops Innovate-IPM sees a clear opportunity for the utilization of alternative products
