Effect of temperature on symptom expression and sequence polymorphism of grapevine yellow speckle viroid 1 in grapevine.

Symptom expression of yellow speckle disease was studied in a row of 32 individual Vitis vinifera cv. Chardonnay vines in the warmer summer of 2009/2010 as compared with the cooler summer of 2011/2012 in South Australia. RT-PCR analysis showed that all these vines were positive for hop stunt viroid, grapevine yellow speckle viroid 1 (GYSVd-1) and grapevine rupestris stem pitting-associated virus. Four vines named Vines 1, 8, 11 and 15 were selected for further analyses. Vines 1 and 8 had never shown yellow speckle (YS) symptoms, Vine 11 had always been symptomatic, and Vine 15 showed YS symptoms only in the summer of 2009/2010, but not in 2011/2012. Analysis of partial nucleotide (nt) sequence of GYSVd-1 from these vines showed two major sequence polymorphisms in the pathogenicity domain coinciding with the YS symptoms and the prevailing temperature in each season. One group designated UA group had a uridine (U) at position 309 and an adenine (A) at position 311, while another group designated AU group had the other way around: an "A" at position 309 and a "U" at position 311. The AU group had never been reported before. The AU group was a minor variant in the GYSVd-1 population and not present in symptomatic Vine 11. In contrast, the UA group was dominant and present in all the vine samples. Surprisingly, all the asymptomatic vines, but not symptomatic vines, had the signature of the AU group. Whether the AU group is associated with the YS symptom expression is interesting. Our result provides a new insight into the sequence variability of viroid-inducing symptoms during two significantly different growing seasons.