questions: asking about control of root rot in arecanut 
 answers: apply bordeaux mixture (1%) in root zone
preapation-dissolve 100 g of copper sulphate and 100 g of quick lime each in 5 l of water separately.
if the prepared mixture is in the acidic range, it can be brought to neutral or near alkaline condition by adding some more lime solution into the mixture.  test the mixture before use for the presence of free copper, which is harmful to the plant, by dipping a well-polished knife or sickle for a few minutes. if the blade shows a reddish-brown colour add
more lime till the blade is not stained on dipping.