Biological Controls of Diseases
How can you make use of these tiny chemical factories?
When I started at the University of Florida in the early 1980s, we had no effective products for biological control of diseases. It was the time frame when many of our industry standard fungicides were being developed including chlorothalonil, benomyl and fosetyl-Al. This was the beginning of development of more targeted chemical fungicides which was in a way phase 2 of products for disease control. Phase 1 was the chemical revolution with the advent of broad-spectrum fumigants and fungicides (like captan and mancozeb) which allowed us to continue along on our merry way to producing huge monocultures without the very natural and predictable development of disease epidemics. We should recognize that without this revolution, there would be a lot fewer of us around due to food shortages.
The fact that by the end of the 1980s we had realized the broad-spectrum chemical approach was tough on the environment (all of it — not just us) led to development of more narrow-spectrum chemicals with fewer obvious side-effects. But the key here is obvious side effects. It takes a long time to find out what side effects may be since they are aptly named. They are side effects and we are looking at main effects of controlling a disease. We have a long list of possible bad side effects that producers must evaluate now (from fish to quail to pollinators) before a new product is labeled at EPA. The list of tests may seem oppressive to manufacturers of new products. But we can never know what to look for if we have not seen something before. We cannot know what we don’t know.
That time has come and gone for most of us and we are entering a time when true IPM will be practiced using biological, chemical, physical and cultural controls to manage diseases. Any way you look at it, growers must use every tool available to produce high-quality, sustainable crops. Our expectations for biological control products are sometimes not as high as for standard synthetic products. We have become reliant on what these narrow spectrum, highly active chemicals do in our monocultures produced in a protected greenhouse environment. How about incorporating biological control agents into effective plant production systems? Very few of us are still expecting to stop using simple synthetic chemicals entirely.
To be successful, you must agree to look at BCAs without preconceptions about what they do. One of the things I have been thinking about is chemistry as it relates to BCAs. Do you realize that even BCAs are made of chemicals — as are we — and they produce an array of chemicals? As chemical factories they affect pathogens like fungi and bacteria in many ways (many of which we are not currently aware). This complexity available only from biological sources is what I am most interested in at this point.
Using BCAs successfully is becoming a more common story within the horticultural industry (see sidebar by Roger McGaughey, head grower at Pioneer Gardens Inc. in Deerfield, Massachusetts). There are some diseases that really are not well controlled with simple chemicals including Fusarium, Cylindrocladium and the majority of bacterial pathogens. I am actually starting to think that biologicals in their complexity may be the answer to controlling these tough diseases.
It also may be our best hope for re-establishing a background of beneficial/normal flora to make sure that pathogens do not invade the biological vacuums we create when we use non-soil, man-made products to grow our plants. I don’t think going further into the “sterile” production of plants will work since they are not put into a sterile end-user environment. They can only fail to thrive if they are test-tube babies thrown into the wide world. While use of soil fumigants is on its way out, we really should try to repair some of the damage we have done to the bacteria, fungi and other microbes that create a healthy root system. Soil inoculants (hopefully mixtures) should be part of this remediation.
On the Horizon
A recent article by Ali and Norman (University of Florida plant pathologists) called biological controls a potential game changer. I could not agree more with this sentiment. The conclusions we made 30 years ago about how well biological control products worked are no longer valid. Many new products have been developed and brought to market and they work far better than some of us thought they could. In addition of that, the market for biologicals is expanding with the change in societal attitudes toward agricultural production throughout the world.
The only thing that has not changed is that disease control is still hard and the products we use must work. Biological control products are still more difficult to use than standard chemistries. This is primarily because when a living organism is employed, whether it is a fungus or bacterium, you have to be concerned with its welfare. Using a BCA for disease management means understanding the plant and its needs, the pathogen that causes the disease and finally the biological agent and its needs. Using BCAs for disease control successfully will end up making us better growers.
We are more able than ever to find, characterize and develop new BCAs. Until the last few years, we did not know immediately what we were looking at when a new BCA was found. Now we can run them through DNA analysis and tell very quickly if this is a new organism and what it is most closely related to. We also need to pursue mixtures of micro-organisms. It is unlikely that a single pure culture of a BCA will have the type of impact that a mixture will have. I foresee a time when we will be able to sample our soil that will tell us exactly what combination of microbes is needed to produce particular crops. Agriculture will hopefully become more complicated again and thus approach nature as managed by humans. There are probably many years of research and trial and error ahead of us but the process is currently under way.