I understand why permaculture is good but what makes monoculture bad and more susceptible to disease and pesticides?

Monoculture is more susceptible to disease due to the ease with which most disease can spread in a closed area. When the virus/bacteria evolves in normal conditions, it is used to finding small areas of natural growing plants that it can infect. When a lot of the same plants are grown near each other, the virus has no resistance from the plants due to time and space, and it can infect the corn quickly. Same concept applies to the pests that destroy crops. Pesticides can be used and standardized in monoculture, allowing for a degree of pest and disease control.

Monoculture means that the if one variety of crop gets a virus that it can not defend itself against, it can easily and quickly spread to all the other crops of that variety. This is enhanced by the increase in farming intensity in space and season. In the world we now use only five main types of wheat as opposed to over twenty that used to exist. Due to monoculture if one strain gets a disease the potential is that we loose one fifth of the worlds supply of wheat. As we become reliant on certain fertlizers and pesticides Super bugs and viruses are becoming an issue. Monoculture enhances this risk because they are constantly used and bugs and viruses become resistant to them. This is also an issue with GE crops.

Monoculture increases the evapotranspiration and reaps the nutrients from the soil and does not replace them. Fertilizers are now required to fullfil the gap in nutrients that are no longer present.

Through rotational cropping and interplanting ie rice and beans (beans can grow up the rice also), soil nutrient can be increased and pest and diseases can be repelled potentially without a total reliance on pesticides.