Virus distribution and seasonal changes of grapevine leafroll-associated viruses

A time-course study was performed over two years to investigate seasonal changes in copy numbers of different Grapevine leafroll associated virus (GLRaV) in infected grapevine (Vitis vinifera) in North America. We examined GLRaV-1, -2, -3, -4, and GLRaV-2RG in 104 grapevines that previously tested positive for one or more of these GLRaVs. Vines were selected from the Davis grapevine virus collection and the USDA National Clonal Germplasm Repository in northern California. Samples were collected from the beginning of the growing season (late April) until February; from April to November, mature leaf petioles were collected, and from December to February, cambial scrapings were taken from dormant cuttings. All samples were tested by RT-PCR and RT-qPCR, RT-qPCR was found to be the more sensitive technique. GLRaVs were more reliably detected in samples collected between August and February. Grapevines infected with GLRaV-2 were evenly detected throughout the year, while infection with GLRaV-1, -3 -4, and -2RG was best observed from November to February for both experimental years. Virus titer was lowest early in the growing season (April/May), and viral load differed among the GLRaVs. The viral load of GLRaV-3 remained unchanged, regardless of the season tested. Viruses associated with leafroll disease in grapevines were not uniformly distributed in both leaf petioles and cambium of grapevines; GLRaV-2 was the most evenly distributed virus in the grapevine, followed by GLRaV-1, GLRaV-3, and finally by GLRaV-4, which had the most uneven distribution.