Different viruses of the family Luteoviridae infecting Poaceae (barley yellow dwarf-MAV, -PAS, -PAV; cereal yellow dwarf-GPV, -RPS, -RPV) and two other viruses not assigned to a genus are collectively called YDs. The distribution ofthese viruses is dependent on environmental conditions. BYDV-PAV is probably the most widespread. In fields mainly infected by BYDV-PAV, yield reduction from 5% to 20% and nearly 40% have been reported in susceptible wheat and barley cultivars, respectively. Symptoms in infected fields occur in patches. Yellowing (mostly in barley) or reddening (mostly in wheat) are characteristic of YDV infection. These symptoms appear at the two- to three-leaf stage in barley and often after heading in wheat. YDVs are transmitted in a circula-tive-persistent manner by one or several aphid species. The capsid of YDVs includes a CP and a read-through protein, both needed for aphid transmission.
In temperate areas, YDV vectors have two main flight periods. At the beginning of autumn, winged aphids leave summer hosts (ripening maize, perennial grasses, regrowths, and volunteers). These aphids are attracted to low plant density areas in young winter cereal fields. Depending on the temperature, one to several cycles of virus infection occur before aphids are killed by low temperatures. In oceanic and in other warm areas, low concentration of apterous aphids is maintained during winter. In spring, aphids leave their hosts and infect spring crops and then maize crops. Perennial grasses maintain a low but permanent reservoir of YDVs and aphids.
Risk assessment systems have been developed. In Australia, forecasts of vector incidence are based on the temperature and rainfall in late summer/early autumn. In France, a decision-support system based on temperature-driven simulations of aphid populations was proposed. In UK, a computer-based decision-support system associating the number of aphids found in suction traps and the numbers of foci of YDV infection per unit area of crop was developed. Using these systems, a risk index allows rationalization of foliar pesticide usage (usually synthetic pyre-throids). Treating seeds with an aphicide is necessary in some areas when early sowing of susceptible cultivars is decided or when the pressure of YDV inocula is known to be regularly high. Imidacloprid (nicotinic agonist) or fipro-nil (chloride channel agonist) gives a good protection during several weeks.
Only minor sources of resistance are known in wheat. Several sources of tolerance have been detected in other Triticinae genera. Several lines derived from crosses between wheat and Thinopyrum sp. are very tolerant to YDVs. The basis for Thinopyrum-derived resistance to CYDV-RPV is associated to resistance via inhibition of viral systemic infection.
In barley, a major gene of resistance mapped to chromosome 3H, Ryd2, has been introduced into present-day cultivars. The level of protection conferred by this gene varies according to strains of BYDV-PAV and has a low efficiency for CYDV-RPV. Recently, a new major gene Ryd3 mapped to chromosome 6 has been characterized. In resistant plants, these genes tend to increase seed yield compared to healthy controls.
Recent data on the mechanism and genetics of transmission by aphids and evidence of the glycosylation of virus CP open new avenues to control these viruses in the future.
See also: Barley Yellow Dwarf Viruses; Brome Mosaic Virus; Cereal Viruses: Maize/Corn; Cereal Viruses: Rice; Furovirus; Hordeivirus; Maize Streak Virus; Nepovirus; Pecluvirus; Plant Reoviruses; Plant Resistance to Viruses: Engineered Resistance; Plant Resistance to Viruses: Natural Resistance Associated with Dominant Genes; Plant Resistance to Viruses: Natural Resistance Associated with Recessive Genes; Plant Rhabdoviruses; Tenuivirus.
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