Biopesticides

Disease Control

Biologicals, key components of integrated pest management (IPM) programs, are receiving much practical attention as a means to reduce the load of synthetic chemical products used to control plant diseases. In most cropping systems, biologicals should not be viewed as wholesale replacements for chemical control of plant diseases, but rather as a growing category of efficacious supplements that can be used as rotation agents to retard the onset of resistance to chemical pesticides and improve sustainability. In organic cropping systems, biologicals are valuable tools that further supplement the rich collection of cultural practices that ensure against crop loss to diseases.

Biologicals for disease control are based on several different classes of active ingredients including microbials (which are based on beneficial bacteria or fungi); botanicals (based on plant extracts); oils minerals like copper or sulfur; or viruses. Some microbial biologicals, like those based on strains of Bacillus or Trichoderma species, target seedling pathogens that attack the root system, including Rhizoctonia, Fusarium, and Pythium. Other strains help protect plants from pathogens that attack leaves and fruit.

In addition to directly controlling or displacing a plant pathogen, biologicals can induce resistance or natural defense mechanisms inherent to the plant by triggering Systemic Acquired Resistance (SAR) or Induced Systemic Resistance (ISR). These responses may include strengthening of cell walls to resist infection or the release of compound that may inhibit pathogens or deter insect feeding. Plant extracts from giant knotweed, Reynoutria sachalinensis, for example, induce ISR and inhibit development of some plant diseases. Harpin αb is a protein molecule consisting of fragments from four harpin proteins found in certain bacteria. Harpins trigger SAR in plants.

Biologicals for use against crop diseases have established themselves on a variety of crops. For example, biologicals already play an important role in controlling downy mildew diseases. Their benefits can include: a short or zero-day pre-harvest interval, a short restricted-entry interval, and the flexibility for use as a tank mix or rotational partner in an IPM program with chemical fungicides. Because some market studies estimate that as much as 20% of global fungicide sales are directed at downy mildew diseases, the integration of biofungicides into grape production has substantial benefits in terms of extending the useful life of other fungicides, especially those in the reduced-risk category.

Weed Control

Weeds reduce crop yields by competing for space, sunlight, nutrients, and water. Weeds can also serve as alternate hosts for pest insects, nematodes, and plant pathogens that may impact crop growth, yields, and quality.

Prior to the development of chemical herbicides, weeds were controlled by cultural, physical, and mechanical means. The introduction of chemical herbicides reduced the need for some crop rotations and mechanical cultivation, thereby saving growers time and money. At the same time, the widespread use of herbicides has led to new problems, including instances of groundwater contamination and resistant weed populations.

Growers seeking alternatives to chemical herbicides cite the availability of cost-effective biological solutions as one of their most significant challenges. Typically, biological weed methods have involved the use of living organisms such as insects and pathogens. Indeed, there are numerous examples of successful weed management using natural enemies: 1) control of prickly pear cacti (Opuntia spp.) in Australia by an imported moth (Cactoblastis cactorum), 2) the control of St. John’s Wort (Hypericum perforatum) on rangeland in the Western U.S. and Canada by leaf-feeding beetles (Chrysolina spp.), and 3) control of Canada thistle by a combination of insects (weevils and gall flies) and pathogens (Puccinia and Pseudomonas).

For a number of reasons, the development of biological weed control products has been relatively slow compared to the development of products for insect and disease management, especially in cultivated crops. These reasons include:

• Exhaustive testing to ensure safety to non-target plants (5-10 years)
• The host specificity requirement often dictates a multi-pronged strategy
• The length of time for the biocontrol agent to adapt to a local environment
• An unstable ecosystem in cultivated crops disrupts biocontrol agents
• Susceptibility of biocontrol agents to pesticides

More recently, biological weed control products have been developed using concentrated plant extracts or fermentation byproducts. Generally these materials are broad-spectrum in nature, and are designed to be used prior to planting or very early in the crop cycle to prevent germination of weeds in cultivated areas.

As resistance to chemical herbicides continues to develop around the world, research activities and funding toward the development of biological weed control solutions will increase. Biologicals based on such active ingredients will reduce our dependency on chemical herbicides.

Nematode Control

Nematodes are unsegmented, mostly microscopic roundworms that are ubiquitous in soils. More than 80,000 different nematodes have been described. Most species of nematodes rely on bacteria, fungi, or other microscopic organisms for food, and, therefore, are a key feature of healthy soil ecosystems. Additionally, some nematodes are beneficial to plant health by virtue of their ability to feed on plant pathogens or insect pests. Nonetheless, some nematodes parasitize plants, causing more than $50 billion in crop losses annually.

Plant-parasitic nematodes have a stylet, or mouth-spear, similar to a hypodermic needle. The stylet is used to puncture plant cells and inject digestive enzymes and other fluids. The nematode then draws plant fluids through the stylets. The most problematic nematodes are the root knot nematode, which feeds on more than 2000 species of plants including most major crops, and the cyst nematode, which is an important pest in soybeans and potatoes. More than 50% of all nematode control efforts are aimed at these two types of nematodes.

Historically, the most effective products for nematode control have been fumigants, particularly methyl bromide. With regulatory pressure to reduce or eliminate these fumigants, biologicals have begun to emerge as alternative treatments to limit nematode damage.

Nematologists have identified several bacterial and fungal products for control of soil nematodes, as well as plant extracts that display nematoxicity or reduce damage by boosting the natural defenses of crop plants. An example of a commercial biological use against plant-parasitic nematodes is the bacterium, Bacillus firmus, applied as a seed treatment to corn, cotton, grain sorghum, soybeans, and sugar beets. The bacteria live among young plant roots and create a living barrier to minimize nematode damage during the early season. Another group of bacteria that have been commercially developed for nematode management fall within the genus Pasteuria. For example, spores of Pasteuria nishizawae, which is a natural enemy of cyst nematodes, are applied to soybean and sugarbeet seed prior to planting, and the bacteria attack and feed on the cysts. Pasteuria usage has been developed to manage sting nematode in turf. Recently, nematicides based on extracts of plants like mustard have been commercially introduced. As fumigant nematicides continue to be phased out, biologicals will play an ever-increasing role in control of these important pests.

Plant Regulation & Harvest Management

A diverse group of biochemical compounds known as plant growth regulators (PGRs) is used in the production of many crops to regulate various aspects of plant growth, including flower formation, flower abscission, fruit retention, growth rate, and growth pattern. Unlike other categories of biological control products, PGRs are not intended to control or manage pests; however, these materials are regulated by the EPA and are thus considered to be biologicals.

The use of PGRs in crop production over the last few years has increased as growers become familiar with the nuances of using them effectively, and commercial products become more diverse with high-quality active ingredients and improved formulations. PGRs are typically used at very low concentrations, and the timing of their application, proper coverage, and environmental conditions can be critical to their success.

One of the oldest commercial uses for PGRs is the use of indolebutyric acid to initiate and accelerate the formation of roots in cuttings. Other common examples of management tactics that utilize PGRs are thinning fruit on fruit trees in order to reduce over-cropping (thereby increasing fruit size), and using PGRs to increase flower and fruit set. PGRs are also used as harvest aids in crops such as cotton and for managing fruit maturity during and after harvest to maintain a high level of fruit quality. In addition, some PGRs also provide the added benefit of improving crop tolerance to a variety of abiotic stresses such as temperature and drought.

Seed Treatment

Biologicals are commonly used as components of seed treatment programs. Often biological seed treatments are based on beneficial soil microbes native to the soil microbiome, the area surrounding growing roots. Often, these microbial seed treatments colonize the growing roots, thus extending their benefits and making them ideal seed treatment solutions. Seed treatments based on biologicals can deliver protection from soil diseases during the important plant processes prior to or just after seedling emergence. Biological seed treatments can protect seeds, seedlings and roots from soil insects and nematodes. Often biological seed treatments are combined with conventional seed treatments to offer growers the best protection from pests and yield enhancement.