thanks for the props! 
Here is a ready reckoner for Cannabis plant and pest problems. It has been put together by a master grower. I have edited for content and added to as needed. I hope you learn from this treatise.
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Phosphorus (P) Mobile Element and Macro Element
Benefit: Phosphorus does a lot of things for the plant. One of the most important parts of Phosphorus is: It aids in root growth and influences the vigour of the plant and is
one of the most important elements in flowering as well helps to germinate seedlings.
Phosphorus is an essential plant nutrient, and since it is needed in large amounts, it is classified as a macronutrient. Phosphorus is a MAJOR important nutrient in the plants reproductive stages. Without this element the plants will have a lot of problems blooming without proper levels of Phosphorus.
When your plants are deficient in phosphorus, this can overall reduce the size of your plants. Not enough causes slow growth and causes the plant to become weak, to little amount of Phosphorus causes slow growths in leaves that may or may not drop off. The edges all around the leaves or half of the leaves can be brownish and work its way inwards a bit causing the part of the leaves to curl up in the air a bit. Fan leaves will show dark greenish/purplish and yellowish tones along with a dullish blue colour to them. Sometimes the stems can be red, along with red petioles that can happen when having a Phosphorus deficiency. This isn’t a sure sure sign of you having one though, but can be a sign. Some strains just show the red petioles and stems from its genes.
So pretty much the overall dark green colour with a purple, red, or blue tint to the fan leaves is a good sign of a Phosphorus deficiency. Having Cold weather (below 50F/10C) can make phosphorous absorption very troublesome for plants.
Many people get a Phosphorus deficiency confused with a fungus problem because the ends of the leaves look like a fungus problem, But the damage occurs at the end of the leaves. side of the leaves and has a glass like feeling to it as if it had a ph problem. Parts affected by a phosphorus deficiency are: Older Leaves, Whole plant, Petioles.
Too much Phosphorus levels affect plant growth by suppressing the uptake of: Iron, potassium and Zinc, potentially causing deficiency symptoms of these nutrients to occur def in plants. A Zinc deficiency is most common under excessive phosphorus conditions,
As well as causing other nutrients to have absorption troubles like zinc and copper. Phosphorus fluctuates when concentrated and combined with calcium
Problems with Phosphorus being locked out by PH troubles
Cold wet soils, acid or very alkaline soils, compacted soil.
Soil
Phosphorus gets locked out of soil growing at ph levels of 4.0-5.5
Phosphorus is absorbed best in soil at a ph level of 6.0-7.5 (wouldn’t recommend having a ph of over 7.0 in soil) Anything out of the ranges listed will contribute to a Phosphorus deficiency.
Hydro and Soil less Mediums
Phosphorus gets locked out of Hydro and Soil less Mediums at ph levels of 6.0-8.5.
Phosphorus is absorbed best in Hydro and Soil less Mediums at ph levels of 4.0- 5.8. (Wouldn’t recommend having a ph over 6.5 in hydro and soil less mediums.) Best range for hydro and soil less mediums is 5.0 to 6.0. Anything out of the ranges listed will contribute to a Phosphorus Deficiency.
Solution to fixing a Phosphorus deficiency
Some deficiency during flowering is normal, but too much shouldn't be tolerated. Any chemical or organic fertilizers that have Phosphorus in them will fix a Phosphorus deficiency. If you have a phosphorus deficiency you should use any N-P-K ratio that is over 5.Again Peters all purpose 20-20-20 is a good mix. Miracle grow Tomato plant food, Miracle grow All purpose plant food (Only mixing at ½ strength when using chemical nutrients, or it will cause nutrient burn!) Other forms of phosphorus supplements are: Bone meal, which is gradual absorption, I suggest making it into a tea for faster use, where bone/blood meal is slow acting, but when made into a tea it works quicker! Fruit eating bat guano, which is fast absorption, Worm castings, which is gradual absorption, Fish meal, which is medium absorption, Soft Rock Phosphate, which is medium absorption, Jamaican or Indonesian Guano, which is fast absorption. Crabshell, which is slow absorption. Tiger Bloom , which is fast absorption.
Here is a list of things to help fix a Phosphrous Deficiency.
Chemical
Advanced nutrients Bloom (0-5-4)
Vita Bloom (0-7-5)
BC Bloom (1.1-4.4-7)
GH Flora Bloom (0-5-4)
GH Maxi Bloom (5-15-14)
GH Floranova Bloom (4-8-7)
Dyna-Gro Bloom (3-12-6)
Fox Farm Tiger Bloom (2-8-4)
Awsome Blossums
Organic
Dr. Hornby's Iguana Juice Bloom (4-3-6)
Advanced Nutrients Mother Earth Bloom (.5-1.5-2)
Fox Farm Big Bloom (.01-.3-.7)
Earth Juice Bloom (0-3-1)
Pure Blend Bloom (2.5-2-5)
Pure Blend Pro Bloom (2.5-2-5)
Buddswell (0-7-0)
Sea Island Jamaican Bat Guano (1-10-0)
Indonesian Bat Guano (0-13-0)
Rainbow Mix Bloom (1-9-2)
Earth Juice Bloom (0-3-1)
BIO BLOOM (2-6-3.5)
AGE OLD BLOOM (5-10-5)
ALASKA MORBLOOM (0-10-10)
METANATURALS ORGANIC BLOOM (1-5-5)
Any of these will cure your phosphorus deficiency. Affected leaves will not show recovery but new growth will appear normal.
Now if you added to much chemical ferts and or organics,( which is hard to burn your plants when using organics) You need to Flush the soil with plain water. You need to use 2 times as much water as the size of the pot, for example: If you have a 5 gallon pot and need to flush it, you need to use 10 gallons of water to rinse out the soil good enough to get rid of excessive nutrients.
Note: Blood Meal, Dried Blood, Guanos, Kelp Meal, Cotton Seed Meal, Peat Moss,
Sulfur and fish meal are all acidic and can bring your ph down, so if you add these please monitor your ph when using those.
Note: Bone Meal, Rock Phosphate, Wood Ashes pretty much all ashes, Shellfish Compost and Crab Meal are all alkaline and can make your ph go up, so if you add any of these please monitor your ph.
Picture 1 is a Phosphorus deficiency during vegetative growth.
Picture 2 is what a phosphorus deficiency looks like in flowering.
(Picture 1 is Mine)
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These pictures is what heat stress looks like.
Only way to fix this is to Move the lights away from the plant!
And get better ventalation, and or add more fans! Heat - If the lights are too close to the plant, the tops may be curled,
dry, and look burnt, mimicking a
nutrient problem. Your hand should not feel hot after a minute when you hold it at the top of the plants.
(Picture 1 and 2 Provided by Mosfed)
