Extensions are academic or research programs, typically found in agriculture schools like Ohio State, that are focused on agricultural and horticultural education. Extensions produce easy-to-read information about lots of different crops and plants. They are not peer-reviewed in the same way that many research articles in academic journals are, but they are still authoritative and contain quality information.
Extensions are typically cited as web pages. Check out the guide on APA for information about citing websites, or ask a librarian!
Since extension publications are typically put out by agricultural schools, an easy way to find them is by using a search engine like Google and limiting to websites that have a ".edu" domain suffix. This can be easily done by adding site:.edu to the end of your search.
My recommendation is to structure your search like this:
[name of your crop] extension site:.edu
For example, if I'm interested in looking for extension publications about apples, I would search for this in Google:
apple extension site:.edu
This will only bring back search results that are hosted by a university or educational institution, so you can be fairly confident that, while perhaps not all of your results will be extension publications, most will be and they will be easy to look through.
Here are some links to some of the big extension programs in and near Ohio, if you are not sure what crop you are interested in and just want to look around to see what's out there.