Why did wealthy plantation owners fear the potential combination of poor whites and african americans? how did the planters counter that threat?

The number of African-American slaves was rapidly increasing and combined with the poor white people they started to outnumber the white rich majority. If they ever decided to unite and to overthrow the institutions of power, they could make their own government where there would be no place for those who were white and rich plantation owners.

They countered this through various laws. For example, they forbid African-Americans from arming themselves even if they were free, or they forbid them from mixing up with local native population. If there would be miscegenation, the child would be a slave and live with African-Americans even though their fathers would be those same white rich people. They got the support of the poor white people because they would amnesty those who would rebel against them.