Identification of an emaravirus in a Common oak (Quercus robur L.) conservation seed orchard in Germany: implications for oak health. novel virus in ringspot diseased oak

We observed the health status of oak trees in a conservation seed orchard for more than twenty years focusing on characteristic virus-suspected symptoms. The orchard was established in 1992 in Kreuztal, North Rhine-Westphalia (Germany) with altogether 1302 seedlings in 186 cluster.The number of seedlings showing chlorotic ringspots and mottle on leaves fluctuated annually, but increased from 3.3% to 12.1% during the last 20 years; the number of affected clusters rose from 8% to 25.9%. A scientific breakthrough was the identification of four genome segments of a novel emaravirus in diseased oak by high-throughput sequencing (HTS). Screening of the oak seedlings in three consecutive years using a newly established virus-specific diagnostic RT-PCR confirmed the virus infection and revealed a close to 100% association between the observed leaf symptoms and the novel virus. As no other plant virus could be identified in the HTS-datasets, we assume that the novel virus is primarily causing the symptoms. To reliable detect the novel virus in oaks, RT-PCR targeting the viral RNA3 and RNA4 should be applied in routine testing of symptomatic plants. It was obvious that most groups with virus-infected plants cluster. Only five out of the 42 affected groups are offside and do not border on an affected group of plants. So far, there is no clear correlation between the detection of the virus and the overall vitality of the seedlings. Thus, the clearly enforcing tree may be virus-infected, or may remain in growth in favour of another seedling of the group. Forecasts on the future growth behavior of emaravirus-infected oak trees are therefore not possible.