i cant believe no 1 even posted to help much :S
pH Fluctuation:
Both of these leaves in figure 6 and figure 7 are from the same plant. It could be over fertilization, but more likely it is due to the pH being off. Too high or too low a pH can lock up nutrients in the form of undisolvable salts and compounds, some of which are actually toxic to the plants. What then happens is the grower then tries to supplement the plants diet by adding more fertilizers, throwing off the pH even more and locking up even more nutrients. This type of problem is seen more often in soil mixes, where inconsistent mixing of the medium's components leads to "hot" spots.
http://www.420magazine.com/forums/how-grow-marijuana/71664-plant-abuse-chart.html
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so id say that its between ph and ec bad,
other option for randomly dieing leaves like this is ... aeroamiosis which is where the roots arnt gettin enough oxygen for the size of the plant so it starts to shut town sections of the plant that wil dry out and limp up and just about fall off,
but my money is on ph and ec, i never take my ec more than 1.2-1.4 theres no need realy unless she tells u so
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tell me how u do ur watering mate and tell me exactly how ya do it, from howlogn its sat to when u ph and ec it