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Use of cultivar mixtures to manage multiple diseases
In field conditions crops can be attacked by multiple diseases, and cultivar mixtures can be used to manage and control those multiple diseases by combining cultivars with differential disease resistance to different diseases. The basic principles are the same as those used for controlling a single disease, with the addition of the possible interactions between the diseases present in the crop.
Experimental results show that cultivar mixtures can indeed control multiple diseases, and that the level of control is associated with the already mentioned characteristics of the specific plant disease (size of genotype unit area, dispersal gradient, lesion size, pathogen generation time and degree of host specialization).
Cox et al. (2004) studied the effect of wheat mixtures for the control of tan spot (caused by Pyrenophora tritici-repentis) and leaf rust (caused by Puccinia triticina). Mixtures consisted of two varieties, one resistant to tan spot and susceptible to leaf rust, and the other vice-versa. For both diseases, severity was lower on the susceptible cultivar in the mixture as compared with monoculture. The decrease was higher in leaf rust, which is a typical windborne pathogen, highly specialized, polycyclic, and with a shallow dispersal gradient. Tan spot, by contrast, is a residue-borne disease with low number of generations per growing season, capable of infecting several species, and with a steep dispersal gradient.
In another example, Ngugi et al. (2001) used mixtures of sorghum to control anthracnose (caused by Colletotrichum sublineolum) and leaf blight (caused by Exserohilum turcicum), considered the most destructive diseases of high yielding cultivars in eastern Africa. In this case they used one sorghum cultivar susceptible to both diseases and another with good resistance to both. Mixtures were effective in controlling both diseases, delaying the time when the disease was first observed and lowering the rate of disease progress. The effect was more pronounced with leaf blight, which is a wind-dispersed disease with shallow dispersal gradient and uniformly distributed incidence. Anthracnose is splash-dispersed, which is normally associated with steep gradients.
One important practical advantage of using cultivar mixtures to control multiple diseases is the relative ease of combining varieties with resistance to different diseases compared with the development of a single variety resistant to all diseases considered. Mixtures can therefore extend the options in deployment of host resistance.
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Copyright © 2007 by The American Phytopathological Society |
