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.