Hey Macyverup
If you'd ask me it looks like powdery mildew.
Powdery mildew - Wikipedia, the free encyclopedia
From some of my past google research, i concluded the cause of it mostly originates due to quick drops and rises in temperature, along with large differences in night and day time temp and usually accompanied with high humidity. But most important cause is a too quick drop in temperature or fast rise of temperature from day night cycle. The fungus profits from the plant's decline of resistance when these fast drops and increases in temperature differences occur.
Normally mari jane can take large temp differences for night and day, even can stimulate flowering, sativa's ( high tree line like in Nepal, very windy ) and a large number of indica's like an afghanica ( 35C during day, 14C during night ) are used to it, some have more resistance than others. So i guess the quick fluctuation in temperature is the biggest cause of it, and they need a lot of fresh air, also helps preventing.
When i encountered the powdery mildew, i noticed that i used to turn off room circulating fan during the nights. After that, i did not turn off the nighttime grow room circulation anymore. So that seemed very important too, maybe differences in night and day RH count as well.
I encountered late during flowering period, so kinda stressed i was. I used a light garlic extract sometimes to spray the leaves, but what helped mostly was a mixture, if i recall correctly, 10% full fat milk, as long as it's fatty, on 90% water to spray the infected leaves with. That helped a lot, and i even got some reveg clones, who were totally covered with it, taken from the same plants, to become totally fungus free, i think the garlic helped in later phase for general resistance, the leaves really got a nice shine from some garlic spraying. I threw the revegs away later, since i had some nice growing seedlings, i regret that a bit still, but they had too many 1 finger leaves, though large.
Spraying the infected leaves like one day with garlic, then 2 or 3 days with milky solution, the garlic, etc. For the flowering plants, got rid of most of it.
Professionals often use sulfur stuff, i believe H2S, which is a gas to release in the grow room to get rid off it or to prevent. From reading here and there i read the full fat milk option of spraying is a succesfull way of preventing mildew for some plants, not necessarily curing. It helped for me to get small clones fungus free, and the soil also gets saturated with milk and garlic, which didnt harm them.
I think the cause in my case was already there when i planted seedlings in their big containers to go into flowering. At a later time I remembered i didnt disinfect the containers and i bought them from someone who stored them outside, so they were probably already infected from very early on. From now on everything i transport from outside to inside my room gets disinfected with light chlorine solution.
Also i try to prevent large temperature fluctuation during day cycle and especially try to achieve slow temperature drop when going to night and slow temperature increase when coming out of night into day cycle.
A simple method working for me if the timers dont break all the time, is to stop some of the intake/outtake/circulation when going into night for about an hour.
Example: When lights go off, turning off room circulating fan, which kools down your grow room quite fast for 30 minutes, then 15 minutes on, then again 15 to 30 minutes off, and then on during the rest of the night.
For going from night to day cycle, sometimes i just turn room circulation off for the first 15 minutes sometimes.
At the moment, after 2 timers broke, never i will buy them again, occasionally am doing it by hand. Just turning room circulation a bit down also works, as long as temperature doenst drop too fast within short period of time. I dont change intake/outtake at the moment, but when in winter time i might also do that.
Hope that helps you, if you can create environmental circumstances to prevent fast growth of the fungus, then it helps a lot, and it doesnt have to influence your yields too much.
Oh yeah, digital timer may be excellent for this job, since it's only for low wattage circulating fan control.
PH! Sorry I missed this post. I'm usually a lot more prompt with my responses. Great info and thanks for taking the time to share it with me. 
My GR is in the garage which is insulated so my temps don't vary too much, 10 deg overnight and it is slow. But great info and I bet others here will benefit from it. 
and plus reps +
//// imtryn my hardest!!
