Many thanks for your reply JD
3 x 600w HPS @ aprox 18" from bulb to plant tops, whilst the plants nearest to the lights are the worst affected leaves on the edges of the canopy 3ft away are still having the same problems.
These plants are 6 weeks and 5 days from flip, the very first signs of this started at around 4 weeks the same as it always does. I have another room on a different system and it has just started showing the same symptoms. they are different strains and some (Sherbalato) are affected worse than others some also have Taco leaves but this may be due to hot spots under the Adjusta-wing hoods.
I feed GH 3 part ratio more or less all 3 equal until finish of stretch then adjust 3B 2M 1G followed by 3B 2M 0G then 2B 1M 0G. I was running 123 321 but changed with just the same results. Only other additives are Botanicare cal/mag @ just over 1/2 strength and Canna Boost (recent addition) 2ml per litre. My background tap water ec is 0.2. Feeding at around 1.0 ec first week in flower raising to 1.2ec, these numbers are inclusive of my background ec so maybe I should be knocking this off. PH corrected to around 6 every morning dropping as low as 5.5 before corrected.
This is how it starts
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Then this
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Then this all crinkled and dead, dry as old sticks
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Curling leaves on plants less affected
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Room temps lights on
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Lights off all be it my other room but it is the same.
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OK. After reading the whole thread, and looking close....
I have several issues you should address.
Nizza is correct! Control the res or buckets to under 70 but not below 65 F.
pH... STOP allowing that pH to drop to 55 - AS BEST YOU CAN... This is better controlled by setting the pH to 6.2 or 6.3 in the mourning.
I know many who like low pH in hydro BUT, it is easy for it to lead to issues...just like yours. ESPECIALLY SINCE THE pH SHOULD BE A CPL OF TENTHS HIGHER IN BLOOM! Your way to low for blooming cannabis in hydro.
Rule of thumb in hydro applications for pH values... 5.7 to 6.3 The common quality book scale is 5.8 to 6.2.....
Part of the above issue is likely a habit of not dumping and starting with fresh nutrient mix properly. The other side of that coin is how much in concentration your adding back to "top off" the tank or buckets...
Waste build-up from the plant effects the ability of the mix in the bucket to work properly..
Lets take a look at a simple solution to that. "The 33% add back rule"
This is an old school, original hydro rule that still works well today!
Fill your res/bucket with your 100% feed solution. Note how much in volume you filled it with.
Now as that res/bucket reduces in solution, you add back to top off,,,correct? Keeping track of how much your adding back..
You should be adding back a 33% concentration of that feed solution and not 100%...
To make a 33% solution: Simply take a gallon of the 100% solution and add two (2) more gallons of plain water to it. You now have a 33% solution.
Top off using that 33% solution.
Now here's the other important part. Remember I asked you to keep track of how much your adding back?
Now when the amount you would add back comes to equal the amount you filled the res/bucket with the first time.
DUMP AND REFILL WITH 100% SOLUTION and restart the whole topping off with 33% all over again.
Do this in veg AND bloom!
Lastly: Mixing
Add each chemical after adding an amount of water.
EXAMPLE: Your making 3 gallons of 100% mix.
Add 4 cups of water to your mixing container.
Add the first chem by pouring the next 4 cup scoop over the measuring cup with the chem in it.
Rinse the measuring cup (I use a 60ml mini cup)
Now do the same for the each chemical you add.
ALWAYS ADD THE CAL/MAG LAST.
Now pH your solution and use it.
I get the distinct feeling your not dumping and refilling enough and are suffering from hydro waste build-up..
This sure won't hurt you, so give it a big try out.
Good luck!