Avoid contact of soil, hands while collecting tissue.......use specific bags to collect it ( should not be reuse)..........wipe scissors or cutters after every use........use liquid nitrogen to freeze samples suddnly after collection of tissues........i thnks this mch can help u somwht to reduce contamination

If after sample collection, you may be late for crushing then store collected samples at 4°c till use because after collection plant tissue starts secreation of phenolic compounds.

thanks for your answers, but still I am not clear how you should treat leaf before starting the process of extracting DNA. As improvise and leaves with distilled water, sodium hypochlorite 0.5% sterile distilled water and cleaned. After that, the leaves were dried in a bag with silica gel for 72 hrs in falcon tubes. Wish me luck.