Waltheria indica is a New Host of Phytoplasma Belongs to 16SrI-B Subgroup Associated with Virescence Symptoms in China

Waltheria indica L. is a kind of medicinal plants belonging to the family of Sterculiaceae distributed in China, which extracts with many active compounds used for treatment of rheumatism and sore pains (Hua et al., 2019). During September to November 2020, the plants showing abnormal symptoms including floral virescence, leaf chlorosis and leaflet, as shown in Fig.1, were found in Dingan county of Hainan province, China, with about 70% incidence. The disease symptoms which were suspected to be infected by the phytoplasma, a phloem-limited cell-wall-less prokaryotic pathogen could not be cultured in vitro, severely impacted Waltheria indica growth resulting in financial loss and ecological damage in the location. For identification of the causal pathogen, the total DNA of symptom or symptomless Waltheria indica samples were extracted using 0.10 g fresh plant tissues using CTAB method. PCR reactions were performed using primers R16mF2/R16mR1 (Lee et al., 1993) and AYgroelF/AYgroelR (Mitrović et al., 2011) specific for phytoplasma 16S rRNA and groEL gene fragments. The target productions of the two gene fragments of phytoplasma were detected in the DNA from four symptomatic plant samples whereas not in the DNA from the symptomless plant samples. The PCR productions were sequenced and the data were deposited in GenBank. The two gene fragments of the DNA extracted from the symptom plant samples were all identical, with the length of 1340 bp 16S rRNA (GenBank accession: MW353909) and 1312 bp groEL (MW353709) gene sequence fragments, putatively encoding 437 (groEL) amino acids sequence. The phytoplasma strain was named as Waltheria indica virescence (WiV) phytoplasma, WiV-hnda strain. A Blast search based on the 16S rRNA gene fragment of WiV-hnda phytoplasma strain revealed the highest level of sequence identities (99.85%) with that of 16SrI aster yellows group members (16SrI-B subgroup), such as Onion yellows phytoplasma strain OY-M (AP006628) from Japan (Oshima et al., 2004); Periwinkle virescence phytoplasma strain PeV-hnhk (KP662136), Chinaberry witches’-broom phytoplasma strain CWB-hnsy1 (KP662119) and CWB-hnsy2 (KP662120), all the strains from Hainan island of China (Yu et al., 2017). A Blast search based on the groEL gene sequence fragment of WiV-hnda indicated 99.92% sequence identity with that of 16SrI aster yellows group members (16SrI-B subgroup) such as Onion yellows phytoplasma strain OY-M (AP006628). Homology and phylogenetic analysis by DNAMAN 5.0 and MEGA 7.0 software indicated that the phytoplasma strains of WiV-hnda, OY-M, PeV-hnhk, CWB-hnsy1 and CWB-hnsy2 were clustered into one clade based on the 16S rRNA gene fragments. WiV-hnda, OY-M and Aster yellow witches’-broom (AYWB) (CP000061) phytoplasma strains were clustered into one clade based on the groEL gene fragments. To our knowledge, this was the first time that Waltheria indica virescence disease induced by 16SrI-B subgroup phytoplasma strain was reported in China. Genetic analysis showed that WiV-hnda was closely related to the phytoplasma strains causing Onion yellows in Japan, Periwinkle virescence and Chinaberry witches’-broom disease in China.

‘Candidatus Phytoplasma asteris’ subgroups display distinct disease progression dynamics during the carrot growing season

Aster Yellows phytoplasma (AYp; ‘Candidatus Phytoplasma asteris’) is an obligate bacterial pathogen that is the causative agent of multiple diseases in herbaceous plants. While this phytoplasma has been examined in depth for its disease characteristics, knowledge about the spatial and temporal dynamics of pathogen spread is lacking. The phytoplasma is found in plant’s phloem and is vectored by leafhoppers (Cicadellidae: Hemiptera), including the aster leafhopper, Macrosteles quadrilineatus Forbes. The aster leafhopper is a migratory insect pest that overwinters in the southern United States, and historical data suggest these insects migrate from southern overwintering locations to northern latitudes annually, transmitting and driving phytoplasma infection rates as they migrate. A more in-depth understanding of the spatial, temporal and genetic determinants of Aster Yellows disease progress will lead to better integrated pest management strategies for Aster Yellows disease control. Carrot, Daucus carota L., plots were established at two planting densities in central Wisconsin and monitored during the 2018 growing season for Aster Yellows disease progression. Symptomatic carrots were sampled and assayed for the presence of the Aster Yellows phytoplasma. Aster Yellows disease progression was determined to be significantly associated with calendar date, crop density, location within the field, and phytoplasma subgroup.

Oviposition Behavior and Development of Aster Leafhoppers (Hemiptera: Cicadellidae) on Selected Host Plants From the Canadian Prairies

Some plant pathogens are capable of manipulating their insect vectors and plant hosts in a way that disease transmission is enhanced. Aster leafhopper (Macrosteles quadrilineatus Forbes) (Hemiptera: Cicadellidae) is the main vector of Aster Yellows Phytoplasma (Candidatus Phytoplasma asteris) in the Canadian Prairies, which causes Aster Yellows (AY) disease in over 300 plant species including cereals and oilseeds. However, little is known about the host range of Aster leafhoppers or their host-choice selection behavior in this geographical region. Several crop and noncrop species commonly found in the Canadian Prairies were evaluated as food and reproductive hosts for Aster leafhoppers through no-choice bioassays. To study possible effects of pathogen infection, AY-uninfected and AY-infected insects were used. Cereals and some noncrops like fleabane were suitable reproductive hosts for Aster leafhoppers, with numbers of offspring observed in treatments using both AY-uninfected and AY-infected insects, suggesting an egg-laying preference on these plant species. Development was similar across the different plant species, except for canola and sowthistle, where growth indexes were lower. Sex-ratios of Aster leafhopper adults did not differ among the plant species or with respect to AY infection. Potential fecundity differed across plant species and was affected by the infection status of the insect. These findings have implications for AY epidemiology and suggest that while cereals can be suitable host plants for Aster leafhopper oviposition and development, some noncrop species could act as alternate hosts for leafhoppers that migrate into the Canadian Prairies before emergence of cereal and canola crops.

Candidatus (Ca.) phytoplasma asteris subgroups display distinct disease progression dynamics during the carrot growing season

Aster Yellows phytoplasma (AYp; Candidatus (Ca.) Phytoplasma asteris) is an obligate bacterial pathogen that is the causative agent of multiple diseases in herbaceous plants. While this phytoplasma has been examined in depth for its disease characteristics, knowledge about the spatial and temporal dynamics of pathogen spread is lacking. The phytoplasma is found in plant’s phloem and is vectored by leafhoppers (Cicadellidae: Hemiptera), including the aster leafhopper, Macrosteles quadrilineatus Forbes. The aster leafhopper is a migratory insect pest that overwinters in the southern United States, and historical data suggest these insects migrate from southern overwintering locations to northern latitudes annually, transmitting and driving phytoplasma infection rates as they migrate. A more in-depth understanding of the spatial, temporal and genetic determinants of Aster Yellows disease progress will lead to better integrated pest management strategies for Aster Yellows disease control. Carrot, Daucus carota L., plots were established at two planting densities in central Wisconsin and monitored during the 2018 growing season for Aster Yellows disease progression. Symptomatic carrots were sampled and assayed for the presence of the Aster Yellows phytoplasma. Aster Yellows disease progression was determined to be significantly associated with calendar date, crop density, location within the field, and phytoplasma subgroup.

Complete Genome Sequence of “Candidatus Phytoplasma asteris” RP166, a Plant Pathogen Associated with Rapeseed Phyllody Disease in Poland

ABSTRACT The complete genome sequence of “Candidatus Phytoplasma asteris” RP166, which consists of one 829,546-bp circular chromosome, is presented in this work. This bacterium is associated with rapeseed phyllody disease in Poland and belongs to the 16SrI-B (i.e., aster yellows) group.

Identifying Leafhopper Targets for Controlling Aster Yellows in Carrots and Celery

Aster yellows phytoplasma (Candidatus Phytoplasma asteris) is a multi-host plant pathogen and is transmitted by at least 24 leafhopper species. Pathogen management is complex and requires a thorough understanding of vector dynamics. In the American Midwest, aster yellows is of great concern for vegetable farmers who focus on controlling one vector, Macrosteles quadrilineatus—the aster leafhopper. However, vegetable-associated leafhopper communities can be diverse. To investigate whether additional species are important aster yellows vectors, we surveyed leafhopper communities at commercial celery and carrot farms in Michigan from 2018 to 2019 and conducted real-time PCR to determine infection status. Leafhoppers were collected within crop fields and field edges and identified with DNA barcoding. Overall, we collected 5049 leafhoppers, with the most abundant species being M. quadrilineatus (57%) and Empoasca fabae—the potato leafhopper (23%). Our results revealed the most abundant aster yellows vector in Michigan in both crops is M. quadrilineatus, but we also found that E. fabae may be a potential vector for this pathogen. While several taxa reside in and near these crops, we did not find strong evidence that they contribute to phytoplasma infection. These findings indicate that M. quadrilineatus should be the primary target for controlling this pathogen.

Genetic Variation Among Geographically Disparate Isolates of Aster Yellows Phytoplasma in the Contiguous United States

Aster Yellows phytoplasma (AYp; Candidatus Phytoplasma asteris) is associated with diseases of herbaceous plants, including ornamentals and important commercial vegetable and grain crops. The aster leafhopper (ALH; Macrosteles quadrilineatus Forbes) is the predominant vector of these bacteria, though other leafhopper species can acquire and transmit AYp. Potentially inoculative leafhoppers are reported to overwinter in the southern United States and migrate to northern latitudes in the spring. Examining the genetic similarities and differences in AYp associated with southern and northern populations of ALH may provide insight into the role that migrating ALH play in AYp disease development. To investigate similarities among geographically distinct populations of ALH and characterize the variation in AYp associated within these populations, we identified genetic variations in subgroup designation and the relative proportions of secreted AY-WB proteins from field-collected populations of AYp isolated from ALH from select locations in the southern (Arkansas, Kansas, Oklahoma, and Texas) and the northern United States (Wisconsin) in 2016, 2017, and 2018. Isolated phytoplasma were tested for variation of AYp genotypes, numbers of potentially inoculative (AYp-positive) ALH, and presence of specific AYp virulence (effector) genes. Geographically distinct populations of ALH collected in northern and southern regions were similar in CO1 genotype but carried different proportions of AYp genotypes. While similar AYp strains were detected in geographically distinct locations, the proportion of each genotype varied over time.