Lower Leaves Browning and Curling

Here are pics. The strain is White Widow/Northern Lights in modified Moonshineman soil mix (FFOF, FF Red bag planting mix, Light Warrior, and 40% chunky perlite) and are beginning their 5th week of flower.

The nutes have been: Tiger Bloom (1tsp. per gallon one feeding so far), Big Bloom (2TBS/gal, fed once) Liquid Karma (1tsp/gal fed every other watering all through veg and flower), and Cl Mag+ (every other watering 1tsp/gal). Nute water is always PHed to 6.4-6.5. Temps are in the mid 70s and humidity is now 30 %, but has been higher to help battle spider mites. The last two weeks have been been bad with the mites and I have had to spray neem (which burned all of the pistils).

Is this some king of fungus ? It is showing itself on the new undergrowth in addition to the older fan leaves.

That looks like a simple fert burn.
It would be a good time to flush your soil, and when you do, check the pH of the runoff and compare it to the pH of the water you are using to flush, to get a sense as to whether poor pH may be contributing. 6.4 is a hair on the low side; I shoot for 6.8 as a soil pH.

Thanks Stinky. The runoff Ph is usually about 6.5. I thought Nute burn would show on the leaf tips first, and none of the upper leaves show even a hint of burning. Thats why I am concerned.

yeah the top of the plant looks pretty decent. That's why I'm suggesting a very good flush. You quite likely have that burn symptom from toxic salts building up in your soil rather than a simple overdose of your normal fertilizer. One of those leaves shown also has hints of a Magnesium deficiency, and since you say you are using CalMag as directed, that also points to a lockout from too much salt.
BTW I just bought some of those nice white pots for the first time... what a great product!

The white pots are great. They let you pack alot into a small space since they are square. I guess I'll go flush them out. Thanks again Stinky.

Yeah I have always used square pots- starting with 3", then 1L/4.5", then 1 Gallon (all I could find were these rectangular waste baskets), then for the monster beeyatches, 3 gallon, 12"x18" pails scrounged from restaurant dumpsters, yuck. But when I had WAAAY too many plants it WAS nice that those pails have wire handles on them.