Black spots on my marijuana leaves

A question from a fellow grower:

Can you tell me what in the hell is happening here and how can I fix it. Everything was fine, green and beautiful. The plants started their flowering stage, so I gave everybody Fox Farms “Tiger Bloom” and then all hell broke loose it’s been over a month since I did it. I gave them straight water for a couple weeks, then I fed them with “Big Bloom” and nothing seems to help them. The flowering doesn’t seem to be affected at this point, however I’m concerned because last year I lost 6 big plants to bud rot within a couple of weeks of harvest. Total bummer! Any help here?

What is the PH of your water or your soil?

It is hard to say but it really looks like it could be a manganese or potassium deficiency.

I had one to do that and i dint try to give it any kind of nutrients or feed of anyking becouse i dint want to make it anyworser than it was. So i just gave it a good flushing. And then i just gave plain water the last 10 day. Then i gave them some 20-20-20 one time just to balance it out little. then just plain water again when it needs it for the next 10 to 15 days and then i went back to the regular feeding time with my bloom bust 5-52-28. And mine was two weeks into flower when it happend. And mine went rite back growing and all the leafs are back green and looking good. And now mine are 3,4,and 5. weeks i.nto flower. But anyway it worked for me and I’m growing white widows. Thanks from Tennessee

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I’m having the same problems with my WW babies. The tops look great and are starting to flower, but there are brown dots all over the lower fan leaves. I just watered today with a mix of 18-18-21. I sure hope that helps them. I checked my “grow log” and realized I hadn’t watered them in a while. Wondering when it’s time to switch to a high phosphorous/potassium mix. I don’t know what the ph of the soil is.

How are your plants doing now?

Knowing the PH of the water you are mixing with your nutrients and the PH of the soil is very important, it could be a direct cause of the nutrient imbalance, toxicity or lock out. Get a [soil probe]
? at the very least, you can get one pretty cheap at a local nursery, homedepot, lowes, or what have you for probably under $20, and you can find those or a digital ph pen online, ebay, amazon for around $10, you gotta be careful with those, they have a delicate glass bulb around and really fine wire inside the plastic enclosure side of the pen that you put in the water, and these bulbs need to stay moist and washed with distilled water after every use, keep maybe barely a drop of water in the cap when you put it on the pen and store it standing cap side down, say in a coffee mug. The long metal soil probe you can find at any gardening supply store or also online is very durable and you simply drive it fairly deep down on the soil, be sure it is not too close to the center where the tap root would be, and the aluminum rod will have a chemical reaction with the PH of the soil, enough to generate a flow of current through the meter, giving you an fairly accurate reading, and as long as you are about between 6 and 7 PH you will be ok.

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MacGyver … thanks for the info. I’ll look for the PH thingy at Lowes. When do I switch to a higher phosphorous/potassium blend of fertilizer? My WW’s having been flowering for about a week now.

Yeah, go ahead and start.

I would probaly give that one a good flushing then give it more ph and less nitra. The other ones look ok. But i just try that. Can’t hurt.

williamrayKUSH, the PH we’re talking about is potential hydrogen, not phosphorous. PH is a rating of how acidic or how alkali things are, this can cause a lot of problems if it is far outside the ranges of about 5.5 PH to about 6.5 PH.

Thanks everyone … will change up the fertilizer this week and get the PH checked.
They’re blooming like crazy right now and look pretty good. I only topped them once about a month ago.
They aren’t very big (one is about 3 ft the other two are about 2 1/2 ft). I thought they would get taller outside. I think the pots they’re in may be too small.
Is it okay to transplant to a larger pot at this stage of the bloom?

Some leaves will self prune and die off when a plant gets root bound or if the leaf isn’t getting enough light but they almost never look quite like these, but I don’t grow outdoors and the leaves might look different or die differently.

There is no harm at transplanting, a root bound plant already has some damage to the outer most roots, as long as you take all other transplanting precautions, wait till it’s mostly dried out as too avoid damaging many roots, etc, and don’t bother going too much larger as the plant won’t do a whole lot more growing after starting flowering. The transplant alone will help average out the PH, adding all that well balanced fresh soil around the root ball. There is a chance that the PH is either too acidic, as a flowering plant can cause this change in some soils at the flowering stage, or it may have become too alkali if you have really hard and alkali water, as tap water is often this way, and again, the PH leading to a nutrient imbalance.

Yea, I was talking about what to do after a good flushing. He was wanting to know when to start feeding. It looks to me it woulnt hurt it a bit if it was flushed, then start feeding after a few days. Or like u said said transplant it in a one size biger pot. I think all. the lower leafs start self prune there self at about the six week of flower. I guess i been lucky when it comes to PH probalams. It may be the Tennessee. soil l.o.l.

What is your bloom mix NPK ratio going to be?

What do u think about the (water soluble) Bloom Booster Flower Food and its 15-30-15 with of course all its other trace min. But this is all i use when its in flower. And mine are 4, 5 and 6 weeks into flower. And I’m growing 2 white widows and 1 girl scout cookie. And they don’t look bad at all. So far so good. Is there anything u would do or give it. But i was told if it ant broke don’t fix it. Thanks u guys for all the help u give the newbys.

Sounds fine. I agree, if it ain’t broke don’t fix it.

So is it safe to repot ??? Im having this same problem with my Purp

Meaning is it ok to change out the soil ??

It could be a pH issue, or a nutrient deficiency. Maybe something else. We need more info.


Take a look at this:


I’m a fairly new grower this is my 3rd attempt but only my second success with some difficulties learning these diseases and different things and pH Etc. However last week I hit it hard between the molasses and Big Bloom it was doing fine. And then the last couple of days we’ve been having a tropical storm they were in some rain the day before yesterday but I brought them in the house and put them under the Skylight to have the most light exposure as possible I put a small heater pointed in the opposite direction just to give them a little bit of heat to dry them out cuz I’m scared about Bud rot because I had that problem last year and now yesterday I started getting these black spots