A short word about pests! If you have problems with aphids we recommend you use a systemic insecticide. We have also found that earwigs in particular enjoy eating the petals, whereas slugs and caterpillars will destroy the leaves.
A soil-borne disease called verticillium wilt (kills the roots) may cause the loss of some plants, especially if your soil is poorly drained. However, in the few years we have been growing these plants we have lost only 2 or 3 plants from this disease, despite our heavy clay soil.
In early 1999 we discovered a problem with some of our Osteospermum stock plants that we'd never previously seen. The leaves of many of our Osteospermums had a strange mottling or mosaic pattern. At first we suspected that it was caused by aphids. However, on closer examination of the leaves, we could see no major infestation that might account for the severity of the problem. It was also suggested to us that it might be the red spider mite. Our instinct told us that it was something completely different! But what? The answer came from a chance discussion with the head of propagation at a nearby RHS garden. We were told of a problem they had experienced that had wiped out almost all their stock of Osteospermum. The symptoms sounded exactly the same as what we had. It was the Lettuce Mosaic Virus (LMV). This was later confirmed by virus testing. We'd never previously read any articles that suggested that Osteospermums were prone to plant viruses. Unfortunately, there is no cure for this problem. We had no option but to destroy all the affected plants and find out more about this problem.
How do you test for plant viruses?
You must send samples to a specialist laboratory. A test called sap inoculation can be performed. A small amount of the affected material is taken and ground in a buffer. The sap is then rubbed on to indicator plants. The reactions these produce if they have a virus
infection can sometimes tell what the virus is. However, to test for specific virus, a more specialist virus test is required called ELISA.
Prices start at about £15 for ELISA (per virus tested) or £35 for sap inoculation. For more information see website from Central Science Laboratory or E-mail: [email protected]
What are the main symptoms ?
Mosaic like patterns on the leaves, yellow mottled patches or blotches, ringspots and irregular line patterns. Please note that these symptoms may also be caused by other factors such as nutritional deficiencies.
What other viruses can be transmitted to Osteospermums?
Apart from the Lettuce Mosaic Virus Osteospermums are also prone to the following viruses:
- Chrysanthemum virus B
- Tomato spotted wilt virus
- Impatiens necrotic spot virus
How do plant viruses spread ?
Mainly by insects such as aphids and western flower thrips. Some insects may only be able to transmit a virus for a very short time, while others may remain capable of this lifelong.
How can plant viruses be controlled?
All affected plants must be destroyed immediately. Do not re-plant in the same area as the virus can remain in the soil. It may be necessary to change the soil to prevent re-infection by soil-borne pests such as nematodes. Wash hands and any tools used thoroughly with hot soapy water . Four Oaks Nursery in the UK sterilize their tools with Hydrogen Peroxide (highly alkaline), although it's not guaranteed that even this can kill a virus completely. Insecticides may offer some control against aphids, but won't solve an existing virus problem. Do not propagate any infected plants as the virus will be transmitted via the sap. This is because plant viruses are thought to be systemic, meaning that they exist throughout the plant. Any tools you use will only infect the next plant you come in contact with. Another tip given by Four Oaks is that when taking cuttings, (even of plants that seem virus free) dip your cutting tools in skimmed milk after each use. It would appear that the virus remains in the milk rather than infecting the plant material. It is unknown how this may work! It is also advisable to keep your weeds under control as it's thought that these may also harbour viruses.
Generally it's recommended to buy plants that are grown from virus free elite stock where possible. The commercial growers often have specialist companies who ensure that their mother plants remain free from all known pests and diseases. (ELITE stock)
At present a couple of institutes in France and Germany are using genetic modification in an attempt to produce Osteospermums that are resistant to LMV, the virus that is considered to cause most problems. It is already possible to remove the virus from a plant with heat treatment or meristem cultures. However, both are tedious techniques that require specialist facilities.