Virus diseases of sweet peas generally show up as flecking in the leaves, stunting, and distortion of the flowers and the growing tip. Such plants are readily recognised and should be destroyed immediately. The virus is easily transmitted from plant to plant both by insect vectors, principally aphids, and physically on ones hands or the blade of a knife.
There is, however, another closely related problem that is less well known, and frequently overlooked. This, for convenience, may be referred to as a colour breaking virus. It is not, in fact, a true virus, but is caused by a mycoplasma-like organism or MLO. The symptoms are very limited, being restricted to colour breaking which affects 2-3 flower spikes on an infected plant. Typically the top flowers of one spike, all the flowers on the next spike, and only the lower flowers on the third spike would be affected. Plants are not stunted, leaves are perfectly normal, subsequent flower spikes are unaffected. Due to the transient nature of the symptoms this condition is often overlooked even by experienced growers.
The breaking does not affect all colours equally; in whites, creams and blues the effects can easily be overlooked, while deep pinks and oranges are severely disfigured. The most spectacular symptoms are seen in picotees such as the Spencer variety "Anniversary", illustrated below. Even in these cases, however, the malady can be dismissed as weather damage due to the absence of other symptoms associated with a virus attack.
|Normal flower||Flower with MLO|
Careful examination will often reveal a slight crumpling of the petals of affected flowers, and vase life is dramatically reduced. The infection remains in the plant but whether the symptoms can be reasserted after the initial attack is not clear.
The host plant of the MLO is clover, Trifolium repens, also a noted carrier of several true virus diseases, and growers suffering from this problem should take steps to eradicate this from their lawns and grassy areas. Other species of clover may also be implicated. MLO is transmitted by leaf hoppers, which offers some scope for controlling the spread of the disease where the eradication of clover is not feasible. Personal experience indicates that the MLO can also be transferred physically on the blade of a knife but that this is a very inefficient means of spread. Official wisdom states that "mechanical inoculation did not give positive results". There is some evidence that it can be passed from plant to plant by physical contact, but this is highly speculative. Fortunately, it is not seed borne.
© 2006 P E M Rowland