I expect that, by now, all those of you who are going to grow petunias have already bought them or raised them from seed, and are now impatiently waiting for the chance of frost to be over before planting them out. But do you remember, way back in 1995, when petunias made the headlines in the newspapers. It is not very often that people read of plants at breakfast time. But they did that year, when the news broke that the trailing Surfinia strain of petunia had been found to be virus infected. Supplies were withdrawn from sale - first the retailers and then the wholesalers and the plant raisers. Why, you may ask, was all this fuss made over a few bedding plants. I don't suppose the state of the nation would have suffered all that much if all the petunias went into decline and did not flower very well. No, that wasn't the trouble. The reason was mainly that the particular virus or viruses concerned could be spread by aphids to other plants of the same family as petunias; like potatoes and tomatoes. And a drop of one tenth in the year's crop of those two items would have been very serious.
What is a virus. First and foremost, viruses are exceedingly small. You will never see one, and this tends to make them mysterious. They exist within the cells of living plants, (which means that, by themselves, they cannot move from plant to plant), and, what is important, they cannot live on tissue that is itself dead. So a virus will not kill a plant. That would be suicide. If the plant dies, so does the virus.
So, if a virus doesn't kill a plant and can't move from plant to plant, why are they so important? The answer is that they are highly specialised parasites and they weaken the plant so that its leaves, flowers and fruits grow less well than they should. But the real concern about the Surfinia petunias was one of the most important of features of virus biology. While they are unable to spread under their own steam, they can be transferred very readily from one plant to another by a variety of means. Aphids are the main culprits in this transfer process. An aphid, having sucked up sap containing a virus from one plant will move on to another plant and will introduce the virus into its sap. In this way the virus begins to multiply and so the disease is spread. They can also be spread by other creatures like eelworms. Viruses that are spread by aphids tend to travel very quickly. Viruses transmitted by eelworms or even microscopic soil fungi travel much more slowly so infection may take several years.
But many viruses can also be spread by you and me. If you have handled a virus-infected plant, it can very often be spread on your fingers or on your pruning tools to contaminate the next plant that you touch. Some viruses can be spread from parent to offspring in seed, though this is not always the case. But anything that exists in plant material can be passed on from one generation to the next during vegetative propagation, that is, propagation from cuttings (and that includes grafting), plant division, and multiplication of bulbs and tubers. Any plant that is routinely propagated in this way can have serious virus problems. So potatoes, dahlias, carnations and bulbous plants are particularly badly affected.
But is it safe to eat plants which are infected by a virus? The answer is definitely yes. Plant viruses can not affect people (at least, I‘ve never caught one), just as a plant can't catch a cold. In fact, viruses tend to be fairly specific, and only attack plants of the same family. This was the trouble with the Surfinia petunias. They are the same family as tomatoes and potatoes. Unfortunately, there is one notable exception - the cucumber mosaic virus, so called because it was first identified on a cucumber - which has the ability to spread across family boundaries and can affect an enormous number of plants in a lot of different families.
But can viruses be controlled? In a direct sense, they can't. You can‘t give antibiotics to roses. There are no virus-killing chemicals like there are fungicides and insecticides, and they don't have any predators. But insecticides are valuable in an indirect way, for they can be used to kill the aphids or other insects which transmit the virus from plant to plant. And you can do your bit. Firstly, don't propagate from stock known to be affected by a virus. Secondly, keep your plots weed-free. Viruses such as cucumber mosaic can exist in weed plants and from which they can be carried to your own plants. Thirdly, don't handle healthy plants after touching diseased ones. And if you grow potatoes, don't be tempted to put in a few you have saved from last year. Always get new seed potatoes every year which are certified virus-free tubers which have been grown in areas where aphids are few and far between. And if all else fails and the problem threatens to get out of hand, do as the Surfinia petunia growers did and destroy everything that is infected. In the long run, the benefits will outweigh the losses.
Happy Gardening,
written by Bill Hutchings
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