As Dutch elm disease spread across Britain in the 1970s, the country fell into mourning. When the sentinel trees that framed our horizons were felled, their loss was a constant topic of sad and angry conversation. Today, just a few years into the equally devastating ash dieback epidemic, and as the first great trees are toppled, most of us appear to have forgotten all about it. I’ve travelled around much of Britain this summer, and seen the disease almost everywhere. A survey published this spring found infected trees across roughly three-quarters of England and Wales: the spread has been as rapid and devastating as ecologists predicted. But in this age of hypernormalisation, only a few people still seem to care. Ash to ashes: our memories wither as quickly as the trees.

And almost nothing has been learned. Our disease prevention rules, whose scope is restricted by the European Union and the World Trade Organization, and whose enforcement is restricted by the British government’s austerity, do little to prevent similar plagues afflicting our remaining trees. Several deadly pathogens are marching across Europe. While it is hard to prevent some of these plagues from spreading across land, there is a simple measure that would stop most of them from spreading across water: a ban on the import of all live plants except those grown from tissue cultures, in sterile conditions.

But bans are more or less banned. Nothing must be allowed to obstruct free trade. Instead, the world’s governments rely on hand wringing. Take, for example, a lethal plague called Xylella fastidiosa, which is ravaging olive groves in Italy and threatens a remarkable variety of trees and shrubs, including oak, sycamore, plane and cherry. The system for preventing its spread depends on inspections of random consignments of known host plants, and a passport scheme to ensure they aren’t imported from infected areas. This system is likely to be useless. The EU keeps a list of plants that can carry Xylella. It has been updated 12 times in four years, as new carriers emerge. No one knows how many more host species there might be. Visual inspections won’t reveal plants that carry the disease without symptoms. Random sampling won’t protect us from a plague that can be introduced by a single plant.

Nor do we know whether Xylella is the most urgent risk to our remaining trees, or whether an entirely new contagion will hit them instead. Many plant pathogens evolve at extraordinary speed, jump unexpectedly from one host to another, suddenly hybridise with each other, and behave in radically different ways in different environments. A system that regulates only known risks is bound to fail.

Even in economic terms, the live plant trade is senseless. Ash dieback alone, according to a paper in Current Biology, will cost this country around £15bn. But the UK’s import and export of all live plants amounts to £300m a year – 2% of the costs of this disease. The paper estimates that another 47 major tree pests and diseases now threaten to arrive in Britain, and these are just the known plagues. In ecological terms, this legislative failure is a total disaster. For the sake of deregulatory machismo, we face the prospect of tree species everywhere eventually meeting their deadly pathogens. Where logging and climate breakdown have so far failed to eliminate the world’s forests, imported diseases threaten to complete the job.

What will come next? Will our beech trees succumb to Phytophthora kernoviae, a disease that appears to have been imported to Cornwall on infected shrubs from New Zealand? Will Sitka spruce, on which commercial forestry in this country relies to an extraordinary extent, be hammered by the larger eight-toothed spruce bark beetle, found for the first time this year in a Kent woodland? Will it be hit by another marvellously named plague, Neonectria fuckeliana? Or by something else entirely? As the trade in live plants reaches almost every corner of the Earth, nothing and nowhere is safe.

Just as we need a precautionary approach, every lid is being ripped off, every barrier smashed, facilitating trade in everything, including pathogens. In response to a parliamentary question about Xylella, the environment minister, Thérèse Coffey, claimed that Brexit creates an opportunity to introduce “stricter biosecurity measures”. It does, but will it be used? Given that, for the monomaniacs who now run this country, the main purpose of leaving the EU is to escape its public protections, the chances of Brexit leading to stricter regulation of plant imports seem remote. Never mind that this trade makes neither ecological nor economic sense. Our government, like many others, favours a global trade regime that places the free movement of goods above all other values (while imposing ever tighter restrictions on the free movement of people).

There’s nothing good about ash dieback, but there is one useful thing that could be done: wherever possible, leave the dead trees to stand. There is more life in a dead tree than in a living tree: around 2,000 animal species in the UK rely on dead or dying wood for their survival. But (except in politics) there’s a dearth of dead wood in this country. Many species, such as the lesser spotted woodpecker, the pied flycatcher and the stag beetle, are severely restricted by the shortage of decay, caused by our tidy-minded forestry.

And there’s another reason to let the dead giants stand: as memorials to the repeated failures of government. Let us remember our losses, and learn from them.

• George Monbiot is a Guardian columnist