Agriculture showed weakness before the great depression, actually rural parts of America never recovered from the depression that followed WWI. Prices were declining and farmers were behind their mortgages at that time. In the Great Plains problems were even bigger, because farmers faced a severe drought, dust storms, and insects. When the Great Depression came, things got even worse.
Foreign Trade before the Great Depression was a small part of the US economic activity. It was concentrated mostly on farms. The World at the time did not demand American goods as much as before, because of WWI, also, European industry and agriculture were more productive after the War and because of the war, European countries decided not to buy goods from overseas and invest in their own countries.
Credit was another weakness before the Great Depression, as said before, farmers were having many troubles and many of them had their land mortgaged. Crop prices were down and they could not pay what they owed to the banks. This caused a crisis before the Great Depression, the country was full of small banks which made unwise loans and invested poorly in stock markets.