I had a great time presenting my research at the Oregon Mycological Society meeting on Monday. Following my presentation, I received lots of questions and the next day I was still receiving questions via email. I felt that one question in particular would be of interest to anyone reading this blog, so I decided to turn it into a post! I’ve been wrestling with exactly how to answer this question, as there are many facets to it, so I attempted to answer it in a broad sense, touching on some of the major points that I’ve read about in the literature. At the bottom, I’ve included a few selected references, so that you can read up on this too and begin forming your own opinions!
What affects the susceptibility of plants within an agricultural population, as opposed to a natural population?
There is a widely-accepted theory that an association exists between disease epidemics and monocultures (defined by the American Phytopathological Society as “the growth of the same plant species in close proximity, with few or no other types of plant present”), as compared to polycultures (multiple crops in the same space, in imitation of the diversity of natural ecosystems).
This theory, while it has some exceptions, arose from observations of the agricultural landscape.
There are 2 major factors that are thought to play a role in the differences seen in disease severity between monocultures and polycultures:
- Genetic diversity of the population
- Host population size
Let’s examine them one at a time.
#1: Genetic diversity of the population
Plant pathogens can come in many forms– fungi, nematodes, bacteria, viruses. In order for a plant pathogen to cause disease, we need 3 ingredients:
- Virulent Pathogen
- Susceptible Host
- Conducive Environment
Within the field of plant pathology, these 3 ingredients are visually displayed as the “disease triangle”.
The idea behind increasing genetic diversity is that diversity decreases the likelihood of a single pathogen genotype inciting disease on most or all plants. The more diversity, the higher the likelihood that the crop as a group has a broad range of genetic resistance to combat the single pathogen genotype. (This is greatly simplified, but communicates the general idea).
Of course, this is operating under the assumption that there is only one pathogen genotype in a given field. This assumption is often broken, however. Read King 2012 for more information (cited below).
In natural ecosystems, on the other hand, we frequently see a good deal of genetic diversity in stands of flowering plants that are not suffering from recent ecological disturbance.
#2: Host Population Size
The idea behind host population size largely relates to inoculum load (or for fungi, amount of infectious material produced such as spores). By having susceptible cultivars grouped together in a field, a pathogen that is able to infect one cultivar can reproduce and will have a relatively short distance to travel to reach the next susceptible host. For this reason, it is often recommended that growers use cultivar mixtures.
In many natural systems, we see plant species dotting the landscape– some Ribes sanguineum over here, some Aquilegia formosa over there. Each utilizing whatever space is available that can provide it with the precise conditions it needs to survive. This natural spacing of plants that occurs due to the utilization of limited resources, can create a barrier (of unoccupied space, or of other plants) that protects the individual plant from the inoculum coming from diseased neighboring plants.
Are polycultures always better?
The short answer is no. While many natural ecosystems are biodiverse, there are some that are not. For example, wild relatives of wheat often form dense stands that are comparable to the density found in cultivated fields. For more information, read Wood 1998, cited below.
Furthermore, functional diversity must be taken into account. Not all blends of plants will inherently lead to decreased disease pressure.
For example, a farmer is trying to avoid mummy berry disease and decides to increase diversity. She plants 5 different cultivars of blueberry, but they all happen to be susceptible! She may see an increase in disease.
The same can hold true even if you define diversity as meaning different crop types, as opposed to simply different cultivars.
For example, a farmer has a problem with the fungal pathogen, Sclerotinia sclerotiorum, which has a very wide host range including legumes, sunflowers, vegetables, and stone fruits. He makes the mistake of planting snap peas, tomatoes, and peppers in close proximity to each other. Again, this farmer may experience increased disease.
This is the basis of the idea of functional diversity. The plants chosen for a mixture, and thus a polyculture, should be chosen mindfully with the pathogen populations that you want to avoid in mind.
A good start is to consider where you live, the crops you’re growing, and the potential pathogens you might encounter. Resources such as the Pacific Northwest Disease Management Handbook can help you determine pathogens of concern, so you can take steps to begin adding diversity to your garden/farm if you believe the ecosystem would benefit from increased diversity.
Taking cues from nature and applying this knowledge to your home garden/farm is a great step towards maintaining a healthy agroecosystem, but just remember, not all ecosystems are equally diverse. A sun-soaked savannah may have a vastly different amount of diversity than a tropical rainforest, yet both ecosystems can be stable, productive, and resilient to disease epidemics.
And finally, a note about the complexity of nature. It’s very difficult to simply say A+B always equals C. But luckily, we have researchers and extension programs that are constantly at work trying to figure out how to produce more food in a more sustainable way. So, be sure to stay abreast of the latest research and while your at it, conduct your own! Your particular environment will have distinctive needs and it’s important to take these needs into account when making changes to your production system.
King, K. C., and C.M. Lively. 2012. Does genetic diversity limit disease spread in natural host populations? Heredity 109: 199-203.
Mundt, C. C. 2002. Use of multiline cultivars and cultivar mixtures for disease management. Annual review of phytopathology 40: 381-410.
Wood D. 1998. Ecological principles in agricultural policy: but which principles? Food Policy 23(5): 371-381.