The verticillum wilt is soil borne, and may be troublesome in houses in which tomatoes have been grown in a previous season. In both cases the tubers can sometimes be saved by cutting away the diseased parts until clean flesh is reached, but this is not always possible. All debris and the rotted tubers should be destroyed by burning, and the soil in which they have been grown removed and replaced by fresh soil.

The most common, dahlia mosaic, is transmissible by greenfly and occurs amongst certain other of the sub-order Helianthea (to which the dahlia belongs), being thus transmissible from these also; however, this is unlikely to be a serious risk as such plants are somewhat rare. The main symptoms are yellow-green banding along the line of the vein, which in extreme cases causes the resemblance to a mosaic which prompted the name. It is often associated with a bumpiness in the texture of the leaf.

If it does get into the cutting beds, then remove the damaged plants to prevent it spreading throughout the bed and increase the ventilation. Out of doors other bacterial and fungoid diseases may affect the dahlia. Two of them, fortunately extremely rarely seen, cause the comparatively quick collapse of the dahlia, usually at ground level.

Both are fatal, for nothing can be done to save the dahlia if attacked by either the fungoid Schlerotinia rot, or by the bacterial blight. If any plants collapse or show strong signs of collapsing, then these should be removed and burnt straight away, and the ground sterilised. It is best to use commercial formaldehyde for this purpose, made up to the maker’s instructions, and to water the affected ground thoroughly with the solution.

It will be apparent that it is not easy to identify correctly the exact virus that has attacked a plant as symptoms vary greatly, particularly if more than one virus is present.

As well as the virus infections, the dahlia is liable to attack by some diseases of bacterial or fungoid origin. The average dahlia grower is familiar enough with some of them, for example the brown stem rot which affects the tubers after these have been set up, causing the developing shoots to collapse and the flesh of the tuber to become brown at first, and later to collapse completely. This, if it is wished to identify it, is verticillum wilt, a fungoid disease which occurs rather more frequently in tomatoes than in dahlias. The black rot which affects tubers in store is another unfortunately too common visitor to the dahlia.