Ecological Implications from a Molecular Analysis of Phytoplasmas Involved in an Aster Yellows Epidemic in Various Crops in Texas

Abstract

In the spring of 2000, an aster yellows (AY) epidemic occurred in carrot crops in the Winter Garden region of southwestern Texas. A survey revealed that vegetable crops, including cabbage, onion, parsley, and dill, and some weeds also were infected by AY phytoplasmas. Nested polymerase chain reaction (PCR) and restriction fragment length polymorphism analysis of PCR-amplified phytoplasma 16S rDNA were employed for the detection and identification of phytoplasmas associated with these crops and weeds. Phytoplasmas belonging to two subgroups, 16SrI-A and 16SrI-B, in the AY group (16SrI), were predominantly detected in infected plants. Carrot, parsley, and dill were infected with both subgroups. Onion and three species of weeds (prickly lettuce, lazy daisy, and false ragweed) were predominantly or exclusively infected by subgroup 16SrI-A phytoplasma strains, while cabbage was infected by subgroup 16SrI-B phytoplasmas. Both types of phytoplasmas were detected in three leafhopper species, Macrosteles fascifrons, Scaphytopius irroratus, and Ceratagallia abrupta, commonly present in this region during the period of the epidemic. Mixed infections were very common in individual carrot, parsley, and dill plants and in individual leafhoppers. Sequence and phylogenetic analyses of 16S rDNA and ribosomal protein (rp) gene sequences indicated that phytoplasma strains within subgroup 16SrI-A or subgroup 16SrI-B, detected in various plant species and putative insect vectors, were highly homogeneous. However, based on rp sequences, two rpI subgroups were identified within the subgroup 16SrI-A strain cluster. The majority of subgroup 16SrI-A phytoplasma strains were classified as rp subgroup rpI-A, but phytoplasma strains detected in one onion sample and two leafhoppers (M. fascifrons and C. abrupta) were different and classified as a new rp subgroup, rpI-N. The degree of genetic homogeneity of the phytoplasmas involved in the epidemic suggested that the phytoplasmas came from the same pool and that all three leafhopper species may have been involved in the epidemic. The different phytoplasma population profiles present in various crops may be attributed to the ecological constraints as a result of the vector-phytoplasma-plant three-way interaction.

Opportunities for Napier Grass (Pennisetum purpureum) Improvement Using Molecular Genetics

“…However, so far no vector has been identified in Uganda [126]. The vector-phytoplasma-host plant three-way interaction plays an important role in determining the spread of the disease [128]. There is the possible involvement of other phytoplasma susceptible food crops and grasses which could act as a reservoir, providing a source of inoculum for the spread of the disease [127] which would present a challenge to the development and implementation of management strategies for the disease.…”

Failure of Flower Bud Formation in Brassica Plants Associated with Phytoplasma Infection

“…These results are in disagreement with our previous findings concerning phytoplasma infection in Brussels sprout (Kamiński et al, 2010) as well as with the results of Wang & Hiruki (2001) and Olivier et al, (2006), who were not able to detect the aster yellows phytoplasma in tested canola plants by direct PCR in Canada. On the contrary to those results, in Italy and Greece (Bertaccini et al, 1998;Marzachi et al, 1999;Maliogka et al, 2009), the USA (Lee at al., 2003) and Iran (Salehi et al, 2007) the titre of phytoplasmas in Brassica spp. plants was high and they were detectable by direct PCR.…”

“…Since the 1980s, in Alberta, Canada, symptoms of canola yellows similar to those of green petal of oilseed rape, have been observed in canola plants (Brassica napus and Brassica rapa) (Wang & Hiruki, 2001;Olivier et al, 2006). In 2000, in southwestern Texas, about 5% of cabbage plants displayed symptoms of purple leaf discoloration and sprouts proliferation characteristic of phytoplasma (Lee et al, 2001;2003). Very recently, in Iran, cabbage disease yellows damaged cabbage up to 50% in certain fields (Salehi at al., 2007).…”

“…Phytoplasma asteris', subgroup 16SrI-B (Bertaccini et al, 1998;Marzachi et al, 1999;Lee et al, 2001Lee et al, , 2003Maliogka et al, 2009). Most phytoplasmas associated with yellows-type symptoms of canola plants in Canada belong to subgroup 16SrI-A or 16SrI-B (Wang & Hiruki, 2001;Olivier et al, 2006), while phytoplasma associated with Iranian cabbage disease is related to 'Ca.…”

Rickettsia-like-organisms and phytoplasmas associated with diseases in Australian strawberries

“…Its absence from other farms in the Caboolture area may indicate that the vector was not present throughout the region to transmit the phytoplasma more widely or an itinerant vector carrying Ca. P. australiense PYL variant strain from another region and/or another host made a brief stop to feed on a limited number of plants in the area (Lee et al 2003(Lee et al , 1998. Disease surveys of alternative host plants in the strawberry growing areas may provide insight into the movement of the vector for this phytoplasma and the distribution of the phytoplasma which would indicate whether this farm represents an isolated ecological niche for the Ca.…”