August 2010
Tales from the Green Dragon - a Virtual Pub
MORE SICK TOMATOES
By Dr Hugh Glen

The other day I came across my friend John-the-Nursery sighing into his beer, which is unusual for him. It turned out that he’d had a rash of customers bringing in tomatoes (and other plants) that were clearly not in the best of health – sometimes not in any state of health at all. He had, at least, seen part of the problem written up in the Royal Horticultural Society’s magazine The Garden (almost the same name as ours, but not quite). I read the story and realised I’d heard the same story in the BBC Radio 4 programme Gardeners’ Question Time, which you can get access to on the internet if you have a reasonably fast connection (ADSL rather than dialup) – go to www.bbc.co.uk/radio4 and navigate from there.

The Gardeners’ Question Time version started with the deformed leaves and dead plants that John was seeing here in KwaZulu-Natal, but these were in various parts of Britain. They, too, found the source of the problem. That trail started with the plant-detective noting that all the plants showing this particular disease came from organic gardens, or at least trying-to-be-organic gardens. It turned out that they were all using bought-in compost or manure. A little more detecting revealed that the animals that produced the manure all grazed on fields that were not so perfectly organic. In fact, the farmers were using plenty of herbicides to deal with the weeds and encourage the grass. So the animals were getting a good dose of herbicide along with their grass. And the herbicide (or some of it) was passing straight through the animal and landing in the compost, where it didn’t degrade. So the innocent gardeners were trying to grow broadleaf plants in soil with organic fertiliser heavily laced with herbicide aimed at? Broadleaf plants! No wonder the tomatoes did so badly. So about a year or a bit more ago there was a major hoo-ha and loads of red-tape type enquiries and the offending organic material disappeared from British garden centres. I gather they did eventually find a way to neutralise the herbicide, and the organic gardeners are back in business and happy again.
John’s problem was a little different; he lives in a farming area just outside a town. That, of course, means that he has to contend with spray drift from all around. I think many of his customers have the same problem. He showed me the news item I mentioned in the first paragraph. That led me to believe that the problem has been known for at least a couple of years, though I didn’t know that it had been seen here before last weekend.
The culprit is an active ingredient called 2,4-D, which is a well-known plant hormone. It controls the growth of stems and leaves, and spraying more than the plant makes by itself causes the plant to become deformed and then die. As it happens, broadleaf (dicotyledonous) plants are much more sensitive than grass-like plants (monocotyledons), and so the hormone is sold as a broadleaf herbicide for lawns, mealie fields and the like. If it degrades in the soil at all, it does so slowly. So those tomatoes remain at risk for quite some time.
So what can we do to make sure that our (and our neighbours’) tomatoes survive our efforts at “tender loving care”? Two things, basically. First, never spray if there’s any wind at all. Preferably, don’t use herbicides at all, but if you must, then drip the herbicide on the plant you want to kill, and don’t spray the stuff over half the country. For one thing, chemicals are expensive, and you’ll save money by only putting it where it’s really needed.
Second, make your own compost; then you know what’s in it. Try to get manure of known origin: mushroom compost is relatively safe, because mushrooms don’t enjoy foreign compounds, either. The stuff sold on the side of the road is usually lethal; beware of it! If you are in the habit of throwing doggy-do in your compost heap then be aware that if your dog is being treated by the vet with antibiotics, most of its medication will end up in the compost, and stop the microbes working there too.
Maybe we need to be a little more careful than we have in the past, and start thinking more of the consequences of what we do.

Article by Dr Hugh Glen, Specialist Scientist at the KwaZulu-Natal Herbarium (a division of SANBI) and Vice President (Africa) of the International Association for Cultivated Plant Taxonomy. Visit www.sanbi.org for more information on the work done at the South African National Biodiversity Institute.