New Agriculturist

Martin Wolfe

Wakelyns Agroforestry and Elm Farm Research Centre

Martin Wolfe, Wakelyns Agroforestry and Elm Farm Research Centre
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Perspective

Diversity within crops restricts disease

Monoculture and disease

Monoculture of crop plants is often regarded as the norm - it's a simple and convenient way of planting, cultivating and harvesting a crop. But this also provides the perfect opportunity for those many crop pathogens that spread by airborne spores. However, if we look at the history of agriculture, deliberate and unconscious forms of mixed cropping have been common throughout most of that history. It is only recently that development of pure line varieties has allowed the growth of the monoculture concept and the industrialisation of field crops. But this has come at a considerable cost, which we are still discovering.

Faced with the cost of controlling disease in monoculture, two solutions emerged - to keep producing new varieties and new fungicides. But both of these solutions led to the Red Queen problem in Lewis Carroll's 'Through the Looking Glass': 'Now, here, you see, it takes all the running you can do, to keep in the same place'. Mixtures of appropriate varieties, however, can restrict disease and increase yield reliably, without need for fungicides.

How mixtures work

In a monoculture, if a single spore infects one leaf and produces more spores, many will have a high chance of success because they will land on identical susceptible leaves wherever they are dispersed. In a mixture, however, even with only three different varieties, there is a greater spatial separation among identical plants than in a monoculture. So, spores of a pathogen produced on one plant are less likely to be blown on to an identical plant that they can infect - and so disease spreads more slowly. The spaces between identical plants can be occupied by more resistant plants, thus providing a physical barrier to spores being blown from one susceptible plant to another.

Mixtures also generate an effective kind of vaccination process. Spores that are not able to infect a particular plant induce a resistance reaction. If a spore which would normally infect that plant then lands by chance in the area of the resistance reaction, it may die or struggle to survive.

Within the mixture, one of the varieties will be more infected than the remainder. This variety may use less space, light, water and nutrients than its neighbours - which then use those surplus resources. Such compensation is uncommon in a monoculture where all plants are equally susceptible. This is partly why mixtures may have more stable yields than monocultures over different environments.

Much of the mixture research has been carried out with cereals but, because of the fundamental nature of the phenomenon, there have been successful experiments with other crops including, for example, apple, rape, coffee, potato, peanut and soya. Similarly, because the same mechanisms work for many fungal pathogens, multiple disease restriction is possible. For example, during last season, in organic trials run by Elm Farm Research Centre (EFRC) in the UK, we noted simultaneous reductions in three diseases, powdery mildew, Septoria and leaf (brown) rust on some wheat mixtures.

Mixtures on the grand scale

There are still some open questions about the value of crop mixtures in the field. For example, if individual fields of mixtures restrict disease spread, does the effect increase as the mixture area increases?

To answer this, collaboration between Oregon State University, the International Rice Research Institute (IRRI) and Chinese scientists, led to a remarkable experiment on a grand scale. The researchers stimulated a unique co-operation in Yunnan Province, China whereby all rice fields in five townships in 1998 (812 ha) and in 10 townships in 1999 (3,342 ha) were planted to rice mixtures. The objective was to restrict the notorious fungal blast disease on certain susceptible rice varieties, which, as monocultures, would normally receive 3-8 fungicide applications to keep them free from disease. However, blast was 94% less severe when these varieties were planted in mixtures with resistant varieties, and the need for fungicidal sprays was eliminated.

This spectacular reduction in blast was constant over the whole area in both seasons and was correlated with a considerable increase in yield relative to the components grown in monoculture. Even the resistant varieties, slightly infected in the first year, were more resistant in the second, because of the 'damping' effect of the mixtures over the larger test area. This may have been an effect of the relatively complex pathogen populations growing on the crop mixtures causing widespread induced resistance. The small and complex populations of the pathogen surviving from one crop to the next could also have been less well-adapted to the variety mixtures than would pathogen populations surviving on one monoculture variety between seasons.

Mixtures and product quality

The rice was harvested by hand so that the different types could be separated. This is difficult for a mechanically harvested crop, which is why there has been a general reluctance to move away from one of the apparent advantages of monoculture - a uniform product. However, there can be variation in crop quality among fields of a single variety, often because of the lack of buffering against different diseases or other stresses so 'uniformity' may not be so. But mixtures can be designed both to restrict disease and to improve product quality by combining complementary characters. If component separation is required, then there are increasing technical possibilities for carrying out such separation, post-harvest. Particularly for on-farm use, there is often no need for separation and therefore every reason to use variety mixtures. Ideally, the varieties that we use should be bred for good performance in mixtures.

Diversity at different levels

And we can go further. Beyond variety mixtures, there is a huge potential for development of species mixtures and of inter-cropping. At the most complex level, we need to further develop agroforestry, integrating trees with crop and animal production. As we expand through increasing levels of diversity, it is important to ensure that each level adds something further to help crop productivity and stability. In other words, we should ensure that we make the best use of natural diversity to carry out as many essential processes of agricultural production as possible. This needs to be stressed most for the further development of organic agriculture but is equally relevant for other systems.

For further information: Zhu, Y. et al. Nature 406, 718-722 (2000) (see also Diversity for combating disease)

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