Some plant diseases are checked during the fallow periods in the bush fallowing system of agriculture because the

In the bush fallowing system of agriculture, farmers rotate the use of land between cultivation and fallow periods. During the fallow period, the land is left uncultivated and allowed to rest for some time. This fallow period allows the land to regenerate, and as a result, some plant diseases are checked. The humus accumulates during the fallow period, which increases soil fertility, and can suppress some soil-borne pathogens. Additionally, the disease organisms lose their hosts during the fallow period, and some pathogens die off. Therefore, the answer is "disease organisms lose their hosts and die."