Plant viruses are economically important pathogens, reducing yield and quality of many agricultural crops.
There are over 700 known plant viruses that can cause serious damage to the plants they infect. Globally, plant viruses can cause a complete crop failure even with updated pest and disease management measures.
In this chapter the main virus diseases of pulses, cereals and canola, their symptoms, disease cycles and management are discussed.
Viruses cause a variety of symptoms in plants they infect and can be present in plants without showing any symptoms.
Common virus symptoms are local lesions, mosaic patterns, yellowing, leaf rolling or curling, ringspots, necrosis and total plant death.
Some viruses are seed-borne, others are mechanically transmitted however, most viruses are transmitted by insect vectors.
In western Victoria, the most common non-persistent (and/or semi-persistent) viruses are Alfalfa mosaic virus (AMV), Bean yellow mosaic virus (BYMV), Cucumber mosaic virus (CMV) and Pea seedborne mosaic virus (PSbMV) in pulses, Wheat streak mosaic virus (WSMV) in cereals and Turnip mosaic virus (TuMV) and Cauliflower mosaic virus (CaMV) in canola. The main persistent (phloem limited) viruses are Turnip yellows virus (TuYV), Bean leafroll virus (BLRV) and Subterranean clover stunt virus (SCSV) in pulses, yellow dwarf viruses (YDV) in cereals and Turnip yellows virus in canola.
Non-persistent Viruses
The primary source of infection of non-persistent viruses is infected seed. If the seed infection level is one per cent, one plant in every 100 will be virus-infected, therefore it will be randomly distributed across the field. The secondary infection happens by aphids. Aphids acquire the virus from primary infected plants or weeds and spread them to healthy crops within a short time span from a few minutes to a few hours. Aphids lose infectivity after a few probes because these types of viruses are not kept long on aphid’s mouth parts and aphids do not remain infective throughout their life.
These viruses do not go through a lifecycle like many other pathogens. They cannot survive for very long outside the plant and they rely solely on aphids to spread them between live plants. These viruses can survive for long periods of time in seed, but do not have stages which allow them to survive in the soil or stubble as sources of infection for future crops.
Persistent Viruses
Persistent viruses are phloem restricted and only transmitted by aphids. Aphids need to feed on plants for hours to days to transmit the virus. The aphids remain infected for their whole life. These viruses are not seed-borne. The source of these viruses lie in over-summering perennial weeds.
