Gardeners already know the benefit of cleaning up and getting rid of diseased plants and plant parts. These go to the curb in the landscape waste bag to be commercially composted. The typical parts of a plant that make it to the bag include spent diseased flower parts, leaves and stems from our annual flowers, herbaceous perennials, and diseased vegetable plant parts. If the problem has been foliar diseases, cleaning up the already fallen leaves should also be done. There are other parts of the home landscape that benefit from being cleaned up. The reason for letting the city or village take away the diseased plant parts is our usual compost piles or bins do not function well enough to control overwintering disease organisms.
Disease organisms rely on plant tissue to feed upon (causing the many plant diseases we see) and later for protection and a place to survive the winter. Any activity we can do to promote plant tissue decay can reduce the potential for serious disease outbreaks in the next growing season. Mulching those plant parts with the lawn mower is one way. Another would be to run the diseased parts through a shredder, but not add to the compost pile. Use that material to turn under in a bed instead. Many disease organisms cannot survive below ground and you have added organic matter in the process.
A very common needle disease on pines is Diplodia tip blight, a disease that has been very strong since the drought of 2012 as pines remain stressed. Diplodia overwinters on needles AND pine cones. Removing the fallen cones will greatly reduce the inoculum present next spring. Treatments to the needles will be much more effective in managing the Diplodia tip blight. If you have apple or flowering crabapples in the landscape, leaf litter containing the overwintering structures for the Apple scab disease provides easy access to next years' tender growth at a time when it is uncomfortable for us to be out there making the necessary treatments (A foliar disease that is going to happen even with garden clean up on your apples and crabs is Cedar Apple Rust because that disease overwinters on evergreen cedar trees and blows back into your yard on the wind).Insects or insect eggs that overwinter on plant parts are also better managed with fall clean up. Disease free plant parts that have insects or their eggs usually will not survive in the compost pile, not because the compost does not heat up, but because they survive above ground naturally. Working in the yard in the fall of the year is more comfortable for gardeners than in the spring with the colder, wetter weather we usually have, the ground is drier and you get the benefit of long term disease prevention.