Hi over my last few crops I've had some of my plants just dying on me around week 5. Its only been certain strains ( strawberry cough,cheese & northern lights x big bud) that have died. Ive had pineapple express and sensi star growing in the same trays and they've been fine. I have a few black flys about my grow room not many and have been using pyletherum 5ec which seems to kill them but they return in about a week. Up until week 5 the plants look great, then some yellow leaves start to appear and within a week most of the leaves are yellow,the plants start to wilt and fall over. Its pissing me off now I've lost 6 plants since xmas. Any ideas?