"Decrease on" or "Decrease"
Below sentence comes from VOA, I don't understand why she says "decrease on that." Can I say "decrease that"? I have never seen "decrease on."
I saw on the news and social media that that there was a blood shortage everywhere. And I've been a donor in the past and know that my blood is particularly valuable, I have O negative blood. And so, I've really felt this was a time for me to come out. We have quite a dedicated mobile donor base. However, we have had to decrease on that.
Decrease may take a prepositional phrase complement when used as a noun, but never as a verb. The verb form may be either intransitive or mono-transitive.
noun + on
Property owners within these special service areas will see a decrease on their property taxes because the interest rate on the bank loan is lower than the average interest rate of roughly 4 percent that village previously charged those taxpayers. (Chicago Tribune 16-09-12)
verb (intransitive)
This past fall, enrollment in Virginia public schools dropped for the first time since 1984 as birth rates decrease. (Virginian-Pilot 19-02-03)
verb (transitive)
Thus, if you're using milk, you can decrease the likelihood of curdling by stirring in 2 tablespoons of potato starch dissolved in an ounce or two of water. (Chicago Tribune 19-04-28)
So, to answer your question, when using decrease as a transitive verb, we may say decrease that. In the text given, decrease is used as a verb and hence the on following it is likely a mistake.
We have quite a dedicated mobile donor base.However, we have had to decrease on that."
The decrease is most likely on the mobile base. They have had to decrease the amount of, or the dedication of, or the activity of the mobile donor base. It does not sound quite like correct English. It is an example of someone writing out what someone is saying. And what they are saying is not as careful as it might be. They might have said, "We have had to restrict that." or "We have had to discontinue that." Your suggestion of decrease that is quite correct.