Question
Hi.  My neighbor, an evergreen tree farmer, (Our rear
properties touch) recently knocked on all neighbors' doors
to ask that "we" spread grass clippings and not leave them
piled up. He claims that fungus grows and multiplies and
would get into the creek (about 100 yds from his tree farm  
and about 100 ft behind our property), thus infecting his
evergreen tree population and also the wild trees around
all our properties.
Is this true?
This person is of questionable character and we are
concerned his lurking around might be for another reason...
Thanks in advance.
Tom

Answer
This reminds me of "The Blind men and the Elephant" poem. Here is what I can contribute to the discussion.
Clippings are not bagged but dispersed over the lawn to improve the lawn and reduce waste going to the landfill.
Lawn clippings are layered in with other materials on a compost pile to decompose into humus. During the process, the internal temperature of the pile rises to the range where weed seeds and harmful organisms are rendered harmless.
I know of two fungi, pythium and rhizoctonia, which are harmful to grasses and also shrubs and maybe trees. Both spread when free water or wetness is present.
Reality or even truth, exists in all this. In my opinion, it is unlikely that a path can be traced from a pile of clippings, to a creek, through the creek, (up- or down- stream, you do not say) and into trees.