Verticillium wilt is caused by two parasitic fungi - which commonly occur in garden soils. The fungus enters the plant via the roots, disrupting both the xylem and phloem systems within the plant and directly affecting osmosis and transpiration. This causes the plant to become limp and yellow as it grows upwards inside the stem. When growth is vigorous, symptoms may not be noticed until growth slows down for bud initiation. If an infected stem is split longitudinally a brown discolouration is usually seen penetrating the wood from the base upwards. A consequence is flower quality deterioration and in severe cases stunting will also occur. It is possible for the fungus to be active in some laterals while others remain unaffected. 

Cuttings must only be taken from plants that have remained vigorous for the whole season. Sterilised components should be used for propagation and growing on in pots. The commercial use of heat therapy can do much to control this disease since tip cuttings produced by this method are likely to be clean of the disease even if the parent stools are infected. If plants have to be grown in an open bed that has previously produced diseased stock it would be sensible to treat the ground with a soil sterilant prior to planting.

Controls
There is no chemical control for this disease.

Alternative control actions include the following:
- only take cuttings from healthy, vigorous plants

- Remove infected plants or parts of plants and dispose of, but do not compost.

- Clean all implements thoroughly as the fungus can be spread in soil on spades and muddy boots, and perhaps other common tools.  

- The fungus can remain in infected soil for years, so once the disease has been discovered, resistant plants should be planted.

- Lightly affected plants may recover. This can be aided by application of nitrogenous fertilisers (but not nitrates) to boost the growth of new wood, which may remain disease free.