DISEASE
 
   NAME
     Virus (Generally)
     Various Virus Species

Viruses can infect any plant and there are hundreds of types, but unlike fungi and bacteria viruses can only reproduce inside a living cell. Viruses are small requiring a magnification of x 10,000 to be seen and infection can be transmitted from one plant to another causing a variety of symptoms. Infected cells normal function, in the plant is diverted to aid the virus to reproduce by supplying nutrients and energy.  The virus can spread from cell to cell or travel through the vascular system in the plant and may only infect a specific part of the plant, normally the new growth or the whole plant. 

Lethal Yellowing
This fatal disease is not a virus, but of uncertain origin that is associated with mycroplasmas or mycroplasma-like-organism and infects palms, particularly Cocos nucifera (Coconut Palm).
The symptoms include premature dropping of the fruit at all stages (shelling) and these have brown-black water soaked areas under the calyx.  The second symptom is the blacking of the new inflorescence as it is exposed from the spathe. Male flowers are also blackish and fruit will not set. 
The third stage of this infection causes the lower fronds to turn yellow, eventually engulfing the entire crown. The fronds turn brown and easily fall from the plant and the infection kills the entire plant within 3 to 6 months with the crown falling away leaving a bare trunk.
Some palm species only exhibit the third stage making it difficult to diagnose, but generally a solitary frond will turn yellow, known as ‘Flagging’ and indicates Lethal Yellowing. 
It is known to be transmitted by a plant-hopper (Myndus crudus) and currently there is no practical control.

Mosaic Virus generally is a viral problem that involves several viruses altering the production of chlorophyll in the leaf. It may only be seen on certain parts but the whole plant will be infected.

Symptoms

Mosaic Virus on Roses form variable yellow pattens on some of the leaves that are fine lined, imitating the veins with blotches or larger marks forming streaks.

Mosaic Virus on Cabbage, cauliflower and broccoli form yellow rings on immature leaves that turn shades of green as the leaf matures and eventually turning purplish-black in cooler weather.

The virus (Marmor iridis) infects Iris and Watsonia species leaves develop pale green streaks on the leaf that become larger then turn brown eventually killing the leaf. The virus prevails in the rhizome from season to season and is a serious problem reducing and damaging flower production.

Phloem Necrosis (Morsus ulmi) is a serious disease of Ulmus species. The initial symptoms occur at the apex and on branch tips where the leaves droop as the petiole is affected. The leaf margins curve upwards and the leaf turns greyish then yellow and then falls, overall giving the tree a sparse appearance. As the virus matures it infects the phloem tubes turning them yellowish, and then brown eventually killing the cells. The roots die and the entire tree may gradually decline over eighteen months, or in acute cases a tree may die within three weeks. 

Pumpkin and zucchini are attacked from a virus commonly known as ‘Watermelon virus’ which affects the leaves by making them mottled in light or dark green and may become distorted. This inhibits the size of the plant and the fruit may have sunken areas with concentric rings.

Tobacco is infected by a Tobacco Mosaic virus (Marmor tabaci) and a Ring virus (Annulus tabaci).

Tulip Breaking (Marmor mite) causes yellowish-white, light coloured striping or spotting of the flower and reduces the size and vigour of the plant in Tulipa species. It may also affect the growth of the bulb and offsets and is more prevalent in double-flowering varieties.

 Yellows (Chlorogenus callistephi) is a viral disease that causes leaves to turn yellow and produces shoots that are simular to Witches Broom or flowers that have poor colouring. The infection will not kill the plant as it overwinters and spreads infection from the living host via the Leaf Hopper (Macrosteles fascifrons). It is not found in the soil or in seeds but is a major problem for Aster species.

Source and Dispersal
The source and the dispersal vary depending on the particular Virus that is involved. In roses the virus overwinters on fallen leaves or pruned pieces of plant material. It is also found on infected plants and is spread by infected propagation material. Aphids transmit this Mosaic Virus.

Infected cabbage, cauliflower and broccoli are an excellent source for aphids, such as the green peach aphid to disperse.

Iris rhizomes are the source and the virus (Marmor iridis) is spread by aphids and infected rhizomes during propagation.

Phloem Necrosis is thought to be transmitted by the Elm Leafhopper (Scaphoideus luteolus) or by infected rootstocks.

Pumpkin and zucchini are infected from surrounding weeds or other infected plants by several aphids including the cotton aphid and the green peach aphid.

Generally plants are infected by viruses by, rubbing against each other or through grafting, budding and other vegetative means such as wounds. Insects also transmit viruses by sucking sap or transferring infected pollen. 
 
Favoured Conditions
Viruses prefer warm humid climate.
	
Affected Plants
A wide range of plants are affected from a range of Mosaic viruses causing simular symptoms. These include roses, cabbage, cauliflower, broccoli, iris, pumpkin, zucchini, turnips, capsicum, swedes, stocks, alyssum and wallflowers. Shrubs and trees such as Ulmus species are also infected.

Abutilon species may be infected by the Mosaic Virus ((Marmor abutilon) which causes variegation of the leaves.

Aquilegi, Berberis Passiflora and Tulipa species are infected by the Cucumber Mosaic Virus (Marmor cucumeris). This causes yellow streaking or mottling in the leaves and dark coloured patches to appear on the flowers. It also stunts the plant and prevents flowering.

Lilium and Tulipa species are infected by the Mosaic virus (Marmor mite) causing yellow mottling on the leaves.
 
Palms are infected by Lethal Yellowing include, Pritchardia martii and Phoenix canariensis.

Non-chemical Control
With roses if the problem is minor it may be ignored as the flower production will be normal. Heavily affected plants should be disposed of.
Other plants that are infected should be removed and destroyed. The spread may be arrested by the eradication of aphids by applying soapy water. 

Chemical Control
Once the virus is seen it is too late to spray. Preventative spraying to control aphids should be carried out in spring using Dimethoate. There is no satisfactory chemical control for viruses.