He believed in nurture above nature. Newly acquired traits would be passed on to offspring. His supporters claimed to be able to transform rye into wheat.
Stalin liked him. "Criticism of Lysenko was denounced as "bourgeois" or "fascist"." Scientific evidence was not a defense. Critics were considered elitist opponents of the wisdom of the peasants. Many opponents were fired, some imprisoned, some executed. Wikipedia says the ban on criticism of Lysenkoism was only lifted in the 60's.
I am informed that apparently substantive differences between people can be made to vanish if the right words and role models are invoked. I am informed that punishment causes crime. I am informed that math is racist and that the experiences of the disadvantaged should play a role in math. I am informed that speech is impermissible violence and violence is protected speech. I am informed that a man is not a man and a woman is not a woman. I am informed that people are harassed and fired and fined for denying these things.
So far no executions, though.
After Lysenko's rise, a similar attack was made on the "idealist" and "bourgeois" theories of quantum mechanics and special relativity. Luckily for the physicists, Stalin wanted a nuclear bomb very badly. Kurchatov warned that the bomb could not be made without scientists who knew these theories, and when Stalin was appraised of a meeting of Lysenko-types to challenge the scientists, he famously said "Leave them in peace. We can always shoot them later." For some reason the meeting never happened. (Lee Pondrom, The Soviet Atomic Project)