First report of Flavescence Dorée-Related Phytoplasma in a Productive Vineyard in Germany

Flavescence dorée (FD) and Bois noir (BN) are the principal grapevine yellows in Europe caused by distinct phytoplasmas: BN by Candidatus Phytoplasma solani, FD by 16SrV-C and -D phytoplasmas (FDp) transmitted by the introduced Nearctic Deltocephalinae Scaphoideus titanus. FDp is listed as a quarantine pest in the European Union (Regulation (EU) 2019/2072). Black Alder (Alnus glutinosa) is a common asymptomatic host of 16SrV phytoplasmas in Europe and considered the original host of FDp (Malembic-Maher et al. 2020). Palatinate grapevine yellows (PGY) transmitted from alder to grapevine by the Macropsinae Oncopsis alni is not transmissible by S. titanus (Malembic-Maher et al. 2020). Germany is considered free from FD in grapevine and from its vector. A single case in a nursery in 2014 was eradicated (EPPO 2017), and FD was never before detected in a vineyard. Since S. titanus appeared in 2016 in the neighboring French Region of Alsace, monitoring of FD was carried out in Germany following a risk based strategy. It was focused on vineyard plots within a distance of 100 m from stands of alder. A geodata-based risk map (Jalke 2020) was used to localize those plots. All symptomatic vines sampled until September 2020 proved to be infected by BN or, occasionally, by PGY. Eight vines with typical symptoms were sampled in vineyards adjacent to alder stands in the winegrowing region of Rheinhessen in September 2020. Symptoms comprised leaf rolling and discoloration, incomplete lignification, and black pustules arranged in lines along the shoots. Diseased shoots were black and necrotic in December. Leaf midribs were sampled for total nucleic acids extraction. The phytoplasma 16S rRNA gene was amplified by generic primers R16F2/R2-mod followed by a nested PCR using 16Sr(V) group-specific primers R16(V)F1/R1, and primers R16(I)F1/R1 (Lee et al. 1995) to detect ‘Candidatus Phytoplasma solani’, associated with BN. While BN was detected in seven vines, one sample tested positive for 16SrV phytoplasma. This result was confirmed by triplex real-time Taq-Man assay based on rpl14 gene sequences (IPADLAB), by multiplex real-time PCR of map locus as well as by Loop-mediated isothermal amplification (LAMP) according to the EPPO diagnostic standard PM 7/079(2) (EPPO 2016). PCR-products of the map and the vmpA genes (Malembic-Maher et al. 2020) were sequenced and compared to reference sequences to distinguish between FD- and non-FD genotypes. The isolate from the diseased vine (GenBank MW 727272) exhibited 100% identity with map-M38 (GenBank LT221933), a genotype of the map-FD2 cluster. The same genotype was detected in A. glutinosa and Allygus spp. sampled at the infested site. A 234 bp sequence of the first repeat of the vmpA gene (GenBank MW727273) showed 100% identity with the homologous part of isolate FD-92 (GenBank LN680870) of the vmpA-II cluster. It can be concluded, that the symptomatic grapevine was infected by FD and not PGY This is the first report of FD in a productive vineyard in Germany. The infected vine of cv. Silvaner was 25 years old. While infected planting material is an unlikely source of the infection, a transmission of FDp from alder is highly probable. Finding a single FD-infection after several years of testing implies a low risk originating from the wild compartment, but the approach and possible establishment of S. titanus expected to be able to colonize the area (Jeger et al. 2016) justifies further monitoring activities. The infected vine was eradicated.

Detection and Distribution of Viruses Infecting Field-grown Sweetpotato in East Africa

Sweetpotato is an important staple food crop in Sub-Saharan Africa, with production being concentrated in East Africa, particularly around Lake Victoria. Productivity of the crop is greatly constrained by viral diseases. Four main viruses have consistently been detected from various surveys done in the region viz., sweetpotato feathery mottle virus (SPFMV), sweetpotato chlorotic stunt virus (SPCSV), sweetpotato mild mottle virus (Sp.m.MV), and sweetpotato chlorotic fleck virus (SPCFV). The most severe symptoms have been caused by co-infection with SPCSV and SPFMV, resulting in the synergistic sweetpotato virus disease (SPVD). Some local sweetpotato genotypes have been reported to recover from, or have localized distribution of SPVD, suggesting that the disease is not fully systemic. This has led to the suggestion that uninfected cuttings may be obtained from previously infected plants. Experiments were set to determine the possibility of obtaining cuttings long enough for propagation that are free from virus infection. This would form a basis for recommending to the local small-holder farmers of a way to reduce losses due to the disease. Field-grown sweetpotato vines were cut into three pieces (15, 15–30, and >30 cm from the apex) and tested for SPCSV and SPFMV. Nine genotypes were selected from a group of 21 local clones and used for this study. The two viruses were equally present in all the three sections of infected vines, indicating that it is not easy to obtain a virus-free cutting for field propagation from an infected vine. Virus assays in the past has mainly been limited to the use of serological methods. Use of PCR resulted in detection of begomoviruses infecting sweetpotatoes for the first time in the region.

VIRUS DISTRIBUTION IN FIELD GROWN SWEETPOTATO VINES IN AFRICA

Sweet potato virus disease (SPVD) is a major constraint to sweetpotato production in East Africa. The disease is a result of co-infection with sweet potato feathery mottle virus (SPFMV) and sweet potato chlorotic stunt virus (SPCSV). Some local sweetpotato genotypes have been reported to recover from, or have localized distribution of SPVD, suggesting that the disease is not fully systemic. This has led to the suggestion that uninfected cuttings may be obtained from previously infected plants. Experiments were set to determine the possibility of obtaining cuttings long enough for propagation that are free from virus infection. This would form a basis for recommending to the local small-holder farmers of a way to reduce losses due to the disease. Field grown sweetpotato vines were cut into three pieces (15, 15 to 30, and >30 cm from the apex) and tested for SPCSV and SPFMV. Nine genotypes were selected from a group of 21 local clones and used for this study. The two viruses were equally present in all the three sections of infected vines, indicating that it is not easy to obtain a virus free cutting for field propagation from an infected vine.

Research Note virus transmission by grape phylloxera (Daktulosphaira vittfoliae Fitch)

Grape phylloxera (Daktulosphaira vitifoliae FITCH) infests immature roots of both Vitis vinifera L. and phylloxera resistant rootstocks. A capability to transmit viticultural viruses would make grape phylloxera a phytosanitary threat even under conditions where direct damage by the insect is not likely. We tested the hypothesis that phylloxera could transmit grapevine fanleaf virus (GFLV) by planting infected and non-infected vines in common 10 liter pots and infecting roots of the infected vine with grape phylloxera. In this test infection of a previously non-infected plant in the absence of nematode population suggests that grape phylloxera is a vector of GFLV.