What problems did the collapse of the american economy cause in other countries ?

Do you mean during the time in Great Depression? if so, is due to the state of US. It was the largest producer and creditor in the world after WWI. It lent money to Britain and France to help them recover their economies and helped Germany pay its war indemnity. In addition, many other countries depended on the US as a major market for their products. As a result, US played a leading role in the economic development of the world.
Sadly, someone shared a rumour of stock shares that there would soon be a recession. US was in panic and many sold their shares leading to the decrease in economy, which affected every other country, especially European countries and Germany, whom had to pay the indemnity and pay the loan.

In the midwest because it has more moderate climate than the north or south west which makes farming far easier.