Plants have own bugs to battle
PLANTS can get hit with viruses but so far not those that affect humans. With the spotlight firmly on viruses that make us ill, especially coronavirus, it is an opportunity to look at viruses that make plants ill and why they are so hard to control.
While there are many plant viruses, they don’t affect human health. However, like the one we are dealing with at the moment, plant viruses don’t have cures. Some virus problems encountered in plants can be devastating and lead to plant decline and death. Others are less drastic and may just lurk in the plant unnoticed until the plant falls under some type of stress.
A virus many gardeners may have seen is apple mosaic virus, which causes yellow patterning in apple leaves. It’s a very old virus and is mostly likely to appear in susceptible old varieties including Golden
Delicious and Jonathan along with crabapples. This virus is spread during propagation and doesn’t seem to have adverse affects on the apple tree so it isn’t necessary to remove affected trees.
Many plant viruses encountered in the home garden will be found in the vegetable garden, especially in tomatoes, beans, potatoes and strawberries. One of the worst is tobacco mosaic virus, which can kill tomato plants.
As the name suggests it also infects tobacco plants and was one of the reasons the growing of some ornamental forms of tobacco such as Nicotiana alata was restricted in gardens for many years. Tobacco mosaic virus was the first plant virus identified.
Other viruses found in tomatoes are tomato spotted wilt virus and tobacco yellow dwarf virus, which causes a disease in beans known as bean summer death, which affects beans and tomatoes.
Strawberries too have been plagued by virus problems, which is why when you grow strawberry plants you are encouraged to buy and plant certified virus-free strawberry plants. These are the ones sold at your garden centre. Replanting every couple of years keeps strawberry plants healthy and productive.
Certified seed potatoes (tubers) also give rise to healthy potatoes as they won’t be carrying viruses such as potato leaf roll that can devastate the crop.
While plant viruses have no cure, many can be controlled. Improvements in virus control in plants have gone hand-inhand with the development of molecular screening and tissue culture.
Many virus-free plants are propagated by tissue culture from plants that have been cleared of virus. In the case of potatoes, which are grown tubers (sold as seed potatoes), the tissue-cultured potato plants produce virus-free tubers. Viruses that are spread from seed can often be controlled by using virus-free seed or by vegetative propagation from virus-free plants.
Once the plant is growing it needs to be protected from infection by good hygiene and pest control. Good hygiene practices including using clean tools – especially secateurs – and even washing hands.
Insects spread plant viruses so keeping plants pest free, particularly free from sapsucking pests such as aphids, leaf hoppers, thrips and whiteflies, can restrict the spread of viral diseases.
Aphids are very good disease vectors as they feed on plant sap and can fly between plants. If an aphid feeds on an infected plant it can then spread the infection around.
When a plant develops a virus the usual control advice is to remove it promptly. This is particularly important with vegetables.
While virus problems are often well recognised in productive plants they may be less well known in ornamentals. Garden plants that often carry viruses include roses, orchids and daphne. If you’ve ever noticed curious wriggly yellow lines in rose leaves then your rose has rose mosaic disease. While this is a virus it is one that rarely does much damage so there’s no need to remove affected plants unless they are failing to thrive.
Some virus diseases in orchids also appear as streaks or discolouration in the leaves although often there is a general loss of vigour and poor flowering when plants are affected.
Cymbidium mosaic virus is one viral disease that may be seen in orchids.
Daphnes, which are very popular and fragrant winterflowering shrubs, carry a number of viruses. Often these lie unnoticed in the plant until it experiences other stress factors such as poor drainage. Sudden death of a daphne may be due to a virus rather than poor gardening.