What’s causing the yellow streaks on leaves

In both spring and summer you may find the tell-tale signs of virus in the form of yellow streaks on the leaves of plants. A gardening friend recently contacted me with this picture of an underperforming Brugmansia (which used to be called Datura), complaining that the growing point of the plant was looking deformed and not developing as it should. On closer inspection, the leaves are showing wiggly discoloured lines in amongst the normal bright green, more obvious in the youngest leaves where the virus is really active and taking advantage of the sugars and proteins that the plant is making.

Such viruses in plants are easily spread, too. Sap sucking insects, most especially aphids are responsible for transporting the disease from plant to plant by their feeding activity. As they probe into the plant tissues with their pointed mouthparts, virus contaminated sap from an infected plant which they may have previously visited is injected into the new victim, precisely where it can thrive.

Sadly, there is no cure for such plant viruses. The plants will continue to decline and become weaker, all the time providing a source of infection for healthy plants of the same species. The best thing you can do for yourself and your fellow gardeners, is to destroy the infected plants as soon as possible.

How do viruses infest plants

This kind of infection takes place very rapidly in the spring and early summer when winged forms of aphids are flying around to find the best food sources. A virus infected plant may not be growing as strongly as an unaffected plant, meaning that there is less sugary sap available and it has a lower water pressure in the stems. The insects will fly to plants where there is more likely to be a reliable food source so that the live young they produce stand a better chance of survival.

And of course, once these new young complete their growing up and become winged adults they can then fly off to infest another plant – and if t is of the same type, then infect it with the virus.

The popularity of plants and virus infection

Virus disease can easily be spread by other means too. Any form of vegetative propagation that is done to an infected plant will produce new plants that are already ‘pre-dosed’ with the virus. Cannas are a prime example of this and divisions from large, infected clumps will always produce diseased, weakened grow. This is one of the reasons why these exotic-looking plants have fallen from favour in the past.

They were popular in Victorian and Edwardian times, before virus took hold and decimated the propagation stocks; and again in the 1950s their numbers were hit hard. Each time, small amounts of uninfected material has remained to be bulked up to commercial levels and then made widely available to gardens. And each time the amount of canna built up, the virus was spread around by aphids. The popularity of brugmansia also ebbs and flows with levels of virus accumulating and then decreasing as infected plants are discarded.

Secateurs and other pruning equipment can also spread virused sap between plants, so it is worth cleaning the blades in white spirit to limit their contaminating effect. And it is also easy to important the disease into your garden or greenhouse by either buying or being given infected material.

It pays to know the signs and to destroy infected plants before they get chance to contaminate other stock. Daffodils and other members of the lily family are prone to virus infection too, while cucumber and tomato plants will often become infected. There are many other forms of plant virus.

Virus-free stock

Modern methods of micropropagation do offer a way to ‘clean up’ stocks of susceptible plants. This involves tissue culture – small collections of cells from the actively growing tip of a plant. The virus is less likely to have been transferred into this material and in many cases can be treated to destroy the infection without killing the cells.

And breeding work has been done to develop virus-resistant strains of food crops, where there is an economic incentive. There is less commercial development in ornamental plants.

So check your brugmansias and cannas this summer for signs of virus, and do your bit for plant health.

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