Viruses

The viral diseases of plants

The viruses are obligatory parasites, proliferating to the detriment of their host. They disturb the physiological processes of the plant's organs and tissues. The symptoms are signalled by changes to colour, tissue deformation and necroses. These disturbances can lead to substantial yield losses (including fall in the number of seeds and their weight) and decline in quality (modification of appearance of fruit, texture and so on).

Aphid vectors of viruses

The alate aphids create indirect contacts between plants that are fixed and distant from each other. When they move around, they feed on plants by sucking up the sap, and viruses are ingested with it if the plant is contaminated. In this way they transmit and propagate the highly numerous viral species in the environment. There are several thousand different associations involving a species of aphid, a virus and a plant. A single species of aphid can transmit numerous different viruses: Myzus persicae, for example, transmits over 100 viral species. Just one virus can infect several plant species: many wild and cultivated Poaceae, for instance, can harbour barley yellow dwarf virus (BYDV). Finally a single virus can be transmitted by several species of aphid: the situation for example for BYDV with Rhopalosiphum padi, Sitobion avenae and Metopolophium dirhodum.

Transmission effectiveness

The effectiveness of transmission is linked to the insect’s host-seeking behaviour. When the aphid lands on a plant, it makes a brief trial attack, piercing superficially to taste if the potential host is suitable. If it is, the aphid stays still, thrusts its stylets in as far as the phloem and has a lengthy feed. Otherwise it flies off again in search of another plant. But it can extract a virus as easily in a trial run as during the real feeding action. The viruses and the mode of transmission are specific to one type of piercing and sucking attack.

The principal phases of transmission are:

• Acquisition: the vector takes up a virus from a diseased plant.
• Latent period: lapse of time between virus acquisition and the exact moment the vector will be able to infect a healthy plant. It then becomes infectious. The latent period concerns the vector.
• Retention: lapse of time during which the vector remains infectious after a contaminating meal.
• Inoculation: the infectious vector’s act of injecting the virus into the plant.
• Incubation: lapse of time between inoculation and external manifestation of symptoms on the plant.

Several modes of transmission have ben defined: non-persistent, persistent and semi-persistent, according to whether the virus is circulative or non-circulative in the aphid.