Adam, have a look at this...
https://www.rhs.org.uk/advice/profile?PID=391
Also, from the picture you have your plants too close together. You gotta let the air circulate and the closer they are together, the easier it is for disease to spread. You can trim off the lower leaves as well, they'll get bigger as the plant grows so get shot of them. Don't forget to feed the plants as well after the first truss sets