When it comes to battling plant diseases in greenhouses and nurseries, chemicals remain the clear weapon of choice among greenhouse growers, though the emergence of fungicide-resistant strains of pathogens and the potential adverse effects of pesticides on the environment are driving more integrated pest management (IPM) efforts that include low-risk resources such as biological control agents, or biologicals. While biologicals are widely used for insect control, this article will focus on the use of biologicals for controlling plant diseases caused by fungi and bacteria.
It’s true that only 15 percent of greenhouse growers use biologicals in their IPM programs, which shows there is great potential for deployment of biologicals. There are several reasons why biologicals aren’t popular in managing plant diseases. These include inconsistent season-to-season and crop-to-crop results, below par control efficiency compared to chemicals, and slower action. Just like plant pathogens, biologicals are living organisms requiring specific environmental conditions to thrive and produce the desired disease-suppression effects. If crops are grown in temperatures and soil characteristics that are different from those required by biologicals, it is highly likely that biologicals will fail to work or would produce inconsistent results. Furthermore, biologicals are not pesticides and they will not achieve 100 percent control of plant diseases; they are not intended for disease control.
So what exactly is their value when it comes to IPM strategies?
Simply put, biologicals are effective preventive weapons. They need to be incorporated into the whole production system, starting with planting seeds or cutting, by mixing them with soil mixes. They also need to be applied at regular intervals to soil during the growth season to maintain healthy population in soils. They will assist in suppressing diseases, when used in combination with other means of control, such as sanitation and chemicals.
It is important to make sure that the chemicals used are compatible with biologicals. This information can be found on the labels of each biological product. Just like plant pathogens, they also need to establish in soils and attain a healthy population before they can work. Therefore, growers need a great deal of patience and persistence.
Biologicals reduce plant diseases via several mechanisms. First, since biologicals are living microorganisms, they can infect plant pathogens directly and interfere with their disease-causing ability. They also produce antibiotics, compete for nutrients and/or induced systemic resistance in crops, suggesting that biologicals use a combination of different mechanisms for suppressing plant diseases. In greenhouses and most nurseries, potting media consist of peat moss, vermiculite/perlite, and other components. These media are, in most cases, not supplied with indigenous soils. Therefore they are excluding the beneficial micro flora, including mixtures of biocontrol agents, which reside in natural soils. Supplementing artificial potting soil with commercially available biological formulation can help restore some of the benefits associated with beneficial microbes.
|Click the image above to view Table 1 with examples of commonly used biologicals in greenhouses.|
After disease resistance varieties, biologicals are perhaps the next most important natural suppression method of plant diseases. There are many benefits associated with biologicals including reduction in the number of fungicide sprays, and reduced selection pressure on fungal pathogens, resulting in delayed emergence of fungicide resistance in fungal pathogens. Reduced fungicide sprays in turn mean less exposure of sprayers and other personnel to pesticides. In addition, end-user consumers will be protected from exposure to fungicides. Biologicals are present in the environment and are routinely isolated from various agricultural soils. According to Biopesticides Registration Action Documents (EPA, office of pesticides programs), their risk to non target organisms including humans, animals, birds, fish and insects is expected to be minimal. The restricted entry interval (REI, time required before reentering a greenhouse when sprayed) of these biologicals is very short, allowing for quick reentry of workers with little disruption to the normal operations of nurseries. By increasing their use, the use of fungicides will be reduced and as a consequence health risks. Risks to the environment that are associated with chemical fungicides will also be reduced.
The most extensively studied biologicals for plant diseases are the fungal species of Trichoderma, Pythium oligandrum and Glomus intraradices, and several rhizobacteria such as Pseudomonas fluorescens, P. putida, Streptomyces lydicus, and Bacillus subtilis, which are commercialized in different formulations. Some examples of commercial formulations of these biologicals are provided in Table 1.
Mention of the trade names does not mean that other formulations will not work.
Growers are encouraged to work with knowledgeable representatives of different biologicals companies. Biologicals are registered by the Environmental Protection Agency and have labels, which describe important information about their application rates, target diseases, and precautions. Several experimental species are also promising candidates for potential deployment in IPM programs.
These include binucleate Rhizoctonia, Piriformospora indica, Penicillium spp., and Phoma spp.
Gul Shad Ali and David Norman are plant pathologists at the Institute of Food and Agricultural Sciences at the University of Florida.