Plant viruses are extremely minute infectious particles consisting of a protein coat and a core of nucleic acid. They have no means of self-dispersal, but rely on various vectors (including humans) to transmit them from one host to another. Once viruses penetrate into the plant cells they take over the cells’ nucleic acid and protein synthesis systems and ‘hijack’ them to produce more virus.
Plant viruses come in all shapes and sizes from small round (isometric) particles to rigid rod-shaped, bullet-shaped and long and flexuous, depending on the viral species.
Virus infections in plants can cause quite dramatic symptoms but, more often than not, they can be mild and sometimes even symptomless. Virus symptoms are seen more often in the spring when the temperature warms and the plant becomes active. With some viruses and plant species, virus symptoms may be transient in that they disappear later in the growing season when the temperature increases further.
Viruses rarely kill plants, and usually plants can tolerate infections if they are grown well and kept in good health and vigour.
Viruses have no means of movement and generally rely on other organisms (vectors) to transmit them from diseased to healthy plants. These are often sap-sucking insects such as aphids, thrips and whiteflies. Other vectors include leaf-feeding beetles, plant-feeding mites, soil-inhabiting nematodes (eelworms), and root-infecting fungal and fungus-like organisms (e.g. Olpidium and Polymyxa species). Some viruses are also very contagious and can be transmitted on pruning knives or gardeners’ hands. Viruses are also transmitted from one plant generation to the next via infected cuttings or grafted material (vegetative propagation) and a relatively small number can pass through infected seed or pollen.
Plant viruses can be transmitted in different ways by insects, depending on the length of time the vector can harbour the infectious particles. Thus transmission is classified as non-persistent (minutes to hours), semi-persistent (days), and persistent (life-time and passed onto progeny).
Many viruses are unable to survive long outside their host plant or vector and are rapidly killed by exposure to the heat and ultra-violet light in sunlight. However, some are robust enough to survive and be transmitted via pruning tools, and a few can survive composting.
The host ranges of some viruses are very wide, others are specific or restricted to a few closely-related plants. Plants may be infected by a single virus or more than one virus at any one time. The symptoms that a plant exhibits are determined by which viruses are present, the relative proportions of each, the order in which they infected the plant, the plant’s natural resistance to the strains of the viruses, and the environmental conditions: some symptoms are temperature dependent, for example. Some virus infections are symptomless, allowing symptomless infected plants to act as reservoirs of infection.
A note on virus names: Viruses are named according to the type of symptom they cause in the first host in which they were discovered, though this is often only the first of many. Thus Bean yellow mosaic virus actually infects a wide range of plants as well as beans; some are in the family Iridaceae, completely unrelated to legumes.