Phytophthora ramorum is now in larch as well as woody shrubs. Oak processionary moth is heading north. Leaf miner and canker have been made worse by drought in horse chestnuts.
But Johnson, whose latest book - Trees: A Lifetime's Journey Through Forests, Woods and Gardens - is published by Octopus this autumn, is not too concerned. "Whether tree disease is more prevalent, or we just know more now, is not clear to me. Knowledge is deeper than before.
"Science has moved on and maybe there are more chances to nip in the bud. Scares go round very rapidly - it's all modern life. Who can judge whether there are more diseases than in the past? We now know DED had been around for many times before it hit really hard in the 1970s and sweet chestnuts were eradicated in America in the 1930s and haven't struggled back."
Nevertheless, Johnson, who is also a world-renowned wine writer, admits: "I get a fright every time I see a sick tree, especially common ones." He is at a loss on what to do about P. ramorum. "I have tested the corpses of several plants and never nailed it. A succession of junipers died and baffled me. Wisley could not identify what it was."
The "fundamental" draw of trees led Johnson to spend years researching them for his masterwork. "The appeal of collecting trees is so fundamental to our surroundings that part of the appeal is hardly a mystery. The mystery is why a great majority of the population don't notice trees and are not curious. I think: 'How pathetic. Did no-one teach you how to look?' I use my time to see how trees behave. It is hugely rewarding."
He adds: "Trees are one of the things that are totally beautiful. You can collect wherever you go and it doesn't matter whether it belongs to you - you can still collect it. If I see a tree I don't recognise when bolting along the motorway, I turn off and find it and dig it off."
They are planted at his Essex estate, Saling Hall, bought in 1973 and developed since then. "I have about 1,000 varieties of woody plants including about 500 trees. They're all too close together these days," he says.
"Forty years ago, Hillier was just about the only source, but I have travelled and taken cuttings and smuggled seedlings (of Calocedrus decurrens - California Incense-cedar) from Oregon 30 years ago that are now beautiful trees. I had a plant import licence every year for 20 years and no-one checked up, so I thought what the hell."
One of the biggest issues for gardeners this year has been drought, but Johnson says: "It all looked sad by the end of July after three months without a drop but we had three inches in August and, bingo, everything is overgrown. There's no permanent damage."
He explains: "Most trees are pretty drought-tolerant. It depends on the soil more than the tree. I have clay/gravel and they all find what they need. Even Japanese maples have done beautifully this year. But the press is the press. People don't read stories that say 'business as usual'."
He does admit, however, that "shade is so valuable in the garden. People think they must have sun and plants are not happy." This leads onto Johnson's career as a garden journalist, now not as active as it was after being editorial director of RHS The Garden magazine for many years.
"It has changed so radically with the growing membership, so it doesn't say a great deal to people like me any more who have been deeply rooted in gardening over many years. People might think it's sour grapes but The Garden doesn't thrill me as it used to," he says.
"When I gave it its title and format in 1975 there was a moment of real thrill. We had wonderful writers because I was a magazine editor from way back and needed to have the best writers writing for me so I went and got them. It was a more stylish publication - not necessarily totally specialist. We had Christopher Lloyd, Arthur Hellyer, Anne Scott-James, Elizabeth David on food - people I had contact with. I did an editor's job rather than gardener's job."
Johnson founded the more high-brow magazine The Plantsman in the 1970s and says: "That interests me more than The Garden, which now, like all press, is about celebrities and personalities. Perhaps we were more modest in our day."
He is no longer involved in The Plantsman - "it got away from me". He says: "I tried to get the RHS to back it from the beginning but the old regime was not interested so I found my own sponsors. Then, finally, the RHS decided to start a new Plantsman. It staggered on and then they changed their minds and turned it back into what it was."
Johnson takes the purist line on another modern trend, grow your own: "The boom is an offshoot of celebrity culture and who is on TV. It is rather sad that people not on TV are hardly listened to, however much they have to say."
1973: The International Book of Trees published in six languages
1975-2006: Tradescant's Diary, a column of garden jottings appeared in RHS magazine The Garden
1979: The Principles of Gardening published
1979: Editorial director, The Plantsman
1994: The International Book of Trees new edition published
1996: Hugh Johnson's Gardening Companion (reissued as The Principles of Gardening) published
2007: Gardens Illustrated published
2010: Trees: A Lifetime's Journey Through Forests, Woods and Gardens published by Mitchell Beazley on 4 October