Have you a picture of the leaves when they have turned black?
Does this happen soon after the leaves have come on or much later?
Are the trees in the garden or in pots?

Could possibly be a root problem have they had enough water since you planted them its been a dry couple of years, either that or they got caught by the frost?

another thing . dont believe everything your told thow turning black doesnt sound so good . people selling things have a way of putting words cleverly and being colourful with the truth . i mean however many blades will a razer have in 20 years ? you mite need a long vehicle sign to buy one . mars bars bye my calcultatiopns if theyve grown the amount they actualy say shopuld way about 12 metric tons bye now lol . ow and a 100% meat in jelly you here on i think kitty cat adverts i think . well that could be a bath full of jelly with an ice cube of meat in it and it would still be 100% meat in jelly . theres another cat food advert that sais normal cat food is 80% water but the dried one isnt . well this is true i guess but the cat itself is made up of 90% water lol just like there natural food ie mice have in them lol . ow going back to shaving surely if the shave gets any closer youl walk out with a skeletal head with 2 eyes in it lol .