In her article, Ms. Haspel points out that USDA’s Risk Management Agency (RMA) administers a crop-neutral crop insurance program. This program provides subsidies for farmers’ premiums and insurers’ costs, largely regardless of the crop, in order to provide a degree of financial stability to our crop-production system. Under the RMA’s program, producers of commodity crops (like corn, soy, wheat, cotton) have the opportunity to buy affordable crop insurance. But here is the important point: producers of specialty crops (like fruits and vegetables, the foods we Americans should be eating more of) have the same opportunity.
This is a perfect moment to point out that I am not an agricultural economist, nor do I claim any expertise in either crop insurance or in crop subsidies. So, I am not going to argue for or against the policy directly. Instead, I discuss whether a planting diversity affects disease pressure.
As a plant pathologist with decades of studying plant disease epidemiology and management, I can affirm Ms. Haspel’s point that diversity in farming systems is usually (though not necessarily always) a good thing from the standpoint of plant disease management. If particular federal policies favor particular commodity crops over other cropping options, such policies could potentially increase the risk of crop disease pressure.
Temporal diversity and disease
Like host plant resistance, crop rotation is a fundamentally important disease-control measure. Rotating to another crop provides the opportunity to put a pathogen into “hunker-down” mode. Deprive it of food (its host) and it must either survive or perish. Wait long enough, and nearly all the pathogen propagules in a field can be expected to perish. Of course, how long that wait is depends on the pathogen and the site conditions. But the core principle is clear: rotation starves the pathogen while growing a suitable non-host crop. And there is more. Certain soilborne pathogens not only thrive when their host is cultivated year after year; some can become more aggressive, by adapting to their long-term food source.
In practice, rotation has its limits, not the least of which are economic (such as limited access to markets for rotation crops, or the cost of having suitable equipment for producing and harvesting diverse crops). From a disease standpoint, some pathogens have a wide host range, so choosing a marketable rotation crop that is a nonhost can sometimes be challenging. Other pathogens can be indifferent to rotation. For example, some rust fungi don’t overwinter locally but blow in from distant fields in other states. Last year’s crop rotation has no effect on where the wind blows. And sometimes the unexpected happens: in limited cases, continuously growing the same crop has been shown to foster a buildup of natural biocontrol in certain fields. The best example of this is “take-all decline” in wheat. On the whole, however, crop rotation—crop diversity through time—is a good thing from the standpoint of disease management.
Spatial diversity and disease
In addition to temporal diversity, there is spatial diversity. Vast plantings of a single crop species offer agronomic advantages, especially those relating to economies of scale, and I do want to acknowledge that this confers benefits to us as consumers, including lower food prices. However, such farming systems also provide an opportunity for rapid disease progress caused by airborne and vector-borne pathogens. As a general principle, greater spatial diversity (diversity of crop species, or even of cultivars) can be expected to reduce disease pressure.
Spatial diversity can exist on a landscape scale, such as a patchwork of fields growing different crop species. Alternatively, it can be on the most modest of scales. For example, polyculture is the concurrent cultivation of more than one crop in the same space (Figure 1). Another practical example: one could easily increase spatial genetic diversity by mixing the seed of several varieties of a grain species, presuming similar days to flowering and to maturity. Multilines and other practices can also increase spatial diversity.
|Figure 1. Polyculture of maize and cucumber in central America. Polyculture sometimes reduces disease pressure but also presents practical challenges and may increase labor requirements.|
There are several reasons why spatial diversity on a small scale contributes to disease control. Here is one mechanism: if a plant is infected, and some of its neighbors are resistant to infection, many of the spores produced by the fungus will be wasted (from the standpoint of the fungus), because they will land on the resistant host.
As with rotation, there are challenges to the practical implementation of spatial diversity. For example, while polyculture commonly offers biological benefits, it often can be expected to create challenges to mechanization. Higher labor costs can often be expected, even if a labor pool is available. And in some cases, polyculture can increase pressure from certain diseases. Multilines, while seemingly a good idea, have rarely been used on a commercial scale, probably because of breeding challenges. These examples remind us that rarely are there simple answers on the path to sustainability. But as a general principle, increased genetic diversity in space will often contribute to disease management.
Haspel’s point is that federal policies should not attempt to pick “winners” among the various crops, but that they should provide needed support for crop insurance, regardless of the crop. If such an approach increases cropping diversity in time and in space, it certainly can be endorsed from the standpoint of crop disease management. And that certainly is consistent with the goals of food-system sustainability.
 One thing that complicates a discussion on planting diversity is what is meant by the word, monoculture. For me, “monoculture” refers to the growing of a single crop in a field (=spatial genetic uniformity). However, some people use the term “monoculture” to mean growing the same crop in the same field for two or more years in succession (=spatial and temporal genetic uniformity). I think the latter use of the term is potentially confusing, but either way, the important point is to be clear on what an author means. By the former definition, monoculture has been practiced in many agroecosystems all over the world, even before the 20th-century industrialization of agriculture. Think of a scenic European vineyard or olive grove, for example. Take small grains (wheat, barley, etc.), as another example: I am unaware of any documented examples—present-day or in the past—of cultivation of small grains in deliberate polyculture.