Disease Cycle and Epidemiology

Disease Cycle

The virus particle, or virion, is deposited in a phloem cell by an aphid vector. The virus replication cycle (Figure 11) begins when the ssRNA is released from the virion. This ssRNA is a positive sense or “+ sense” RNA that serves as a messenger RNA

Figure 11

Early gene products (proteins) are translated from the ssRNA. These early gene products are believed to be part of the complex required to make copies of the viral RNA.

Complementary (negative sense or “- sense”) RNA strands are produced from the + sense ssRNA. The - sense RNA strands are then used as templates for the production of many copies of full-length + sense RNA. It is believed that these new + sense RNA strands are transported from cell to cell in the host, initiating more replication cycles and thus spreading the infection within the plant.

Subgenomic + sense RNAs also are made from the - sense RNA strands, and late gene products are expressed from these subgenomic RNAs. Among the late gene products are the structural proteins of the virions.

Full-length + sense RNA and structural proteins are assembled into virions, which can be ingested by an aphid vector that transmits the virus to an new cell, in the same or another plant, where the replication cycle can begin again. Newly formed virions also spread within the host plant through the phloem (Figure 12), but are not believed to initiate new infections within the plant.

Figure 12

Epidemiology

The viruses that cause BYD are transmitted from plant to plant by at least 25 different species of aphids (Figure 13). The viruses cannot be transmitted mechanically (by rubbing), and there is no evidence that they multiply in their aphid vectors or are present in new-borne aphids. Thus, all aphids must acquire the viruses by feeding on infected plants. The viruses travel up the aphid’s stylet, through the food canal and into the gut, where they are transported into the aphid’s body cavity, or hemocoel (Figure 12). The viruses then circulate through the hemocoel to the aphid’s accessory salivary gland where they pass into the saliva and can be expelled into the phloem of another plant. The viruses cannot be transmitted by an aphid until they follow this path through the body of the insect, so there are usually several hours after an aphid acquires the viruses during which it cannot yet transmit them. This period of time is called the latent period.

Figure 13

This type of aphid transmission has been called “circulative” because the virus circulates in the body of the insect, or “persistent” because the aphid can retain the virus in its body for days or weeks. A single virus-carrying, or viruliferous, aphid can spread the virus to many plants as it moves and feeds. As nonwinged (apterous) aphids crawl to and feed on new plants in a field, small patches of infected plants will develop (e.g. Figure 7). Winged (alate) aphids (Figure 14) often develop as host plants begin to deteriorate or when the aphid population is overcrowded. Circulative transmission allows alate aphids to carry the viruses that cause BYD over long distances as they migrate, seeking new hosts. Thus, the sources of infection in a crop may be either local or at a great distance from the affected cereal field or grass pasture.

Figure 14

The different viruses that cause BYD are transmitted more efficiently by different species of aphids, a fact that was originally used to distinguish the viruses. For example, BYDV-MAV is efficiently transmitted by the aphid Sitobion (formerly Macrosiphum) avenae. The acronym MAV came from Macrosiphum avenae virus. Similarly, CYDV-RPV is most efficiently transmitted by the aphid Rhopalosiphum padi. These differences in vector transmission are due to the fact that the viruses are selectively transported across the gut and salivary gland membranes of the aphids. For example, some of the viruses that cause BYD will be transported across these membranes in R. padi, while others will not.

Environmental factors play several roles in the BYD disease cycle. High light intensity and relatively cool temperatures 15-18ºC (59-65ºF) generally favor expression of symptoms, such as leaf discoloration which may attract aphid vectors to virus-infected plants. The reproduction rate of subsequent aphid populations is affected by environmental conditions, as is the efficiency with which the aphids transmit the viruses. For example, transmission of the BYDV-RMV by the inefficient vectors R. padi and S. avenae is dramatically increased at high temperatures (30ºC/86ºF).

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