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Phytoplasma and phytoplasma diseases: vectors, control, and research topics |
Cahiers Agricultures. Volume 11, Number 2, 115-26, Mars - Avril 2002, Synthèses
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 Résumé
Article gratuit
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Author(s) : Marie-Thérèse Cousin, Elisabeth Boudon-Padieu |
Summary : Phytoplasma vector species belong to hemipters (Figure 1) that are phloem-feeding insects. Three families (Jassidae (Figure 2), Cixiidae and Psyllidae) contain the known vector species. However, information is still lacking on vectors of numerous phytoplasma diseases.
A high specificity between vector and disease is frequently observed. It has been shown that phytoplasma circulate, multiply and persist in the body of leafhopper vectors. Though detection of phytoplasma in eggs, nymphs, and adults of Scaphoideus titanus specimen reared on healthy plants has been reported, it is considered as a rule that phytoplasma are not transmitted vertically to the progeny of infected specimen. Among planthoppers, species in the cixiid family are known to transmit phytoplasma belonging to the "stolbur" group and to a closely related group. Among psyllids, several species have been shown to transmit diseases of fruit trees, which are all associated with phytoplasma in the same group.
The transmission cycle of phytoplasma depends on the life cycle and ethology of their vectors. Scaphoideus titanus, the vector species of "flavescence dorée", is a monovoltine species restricted to Vitis sp., thus conferring to this grapevine yellows the very particular traits of an epidemic disease. In the case of Euscelis plebejus, a vector of "clover phyllody" (Figure 3a), the number of cycles per year and hibernation sites differ with climate, hibernation as larvae occurring in oceanic areas and hibernation as eggs in continental areas. In the case of Hyalesthes obsoletus (Figure 3b) and Pentastiridius beieri, most of the life cycle is carried out underground where eggs and larvae can be found on the plant roots. The aerial stage is limited to winged adults that migrate for about six weeks in summer. These cixiid vectors of "stolbur" phytoplasma are polyphagous species, that ensure to "stobur" phytoplasma complex transmission cycles, with various hosts and a few preferential host plants that can act as phytoplasma reservoirs threatening susceptible crops.
The circulation of phytoplasma between insects and plants (Figure 4) and the mechanism of phytoplasma transmission (Figure 5) are described. Phytoplasma are always "intracellular", located inside the cell walls of their hosts, either as "extra-cytoplasmic" in mature sieve tubes of plants, in haemolymph, gut lumen and saliva of insects or as "intra-cytoplasmic" in young sieve tubes of plants and in insect cells. It has been shown that circulation and multiplication of "flavescence dorée" phytoplasma in the body of an experimental leafhopper vector (Figure 6) meet with two important specific events, i.e. trespassing of the gut membrane after ingestion and penetration of specific secretor cells of salivary glands. Molecular factors for the specific transmission of phytoplasma by insects are not known.
Transportation of phytoplasma-infected plants is dangerous or forbidden (quarantine diseases) even if the specific vector species is absent in the importation area, since potential alternative vector species may be present. Similarly, introduction of an exotic insect species in a new area is dangerous and should be avoided by all means. Scaphoideus titanus was introduced from North America to Europe as eggs inserted in the bark of vine planting material. Quarantine of "stolbur" phytoplasma transmitted by cixiids, should be recommended between Europe, Australia and the Near East where related phytoplasma isolates have been detected.
Conventional control methods include sanitary selection, in some cases pruning of branches of woody plants with localized symptoms, destruction of phytoplasma reservoir plants, monitoring of vector species, thermotherapy, cross protection and genetic selection for tolerant or resistant varieties. Phytoplasma are not transmitted through seeds but through vegetative parts. Sanitary selection should include scion and rootstock material. Mother-plants should be carefully screened because of latency of infection or tolerance which may result in healthy-looking, nevertheless infected branches that greatly contribute to propagation and long-distance diffusion of diseases.
Pruning of symptom-showing parts of woody plants may be efficient to limit the economical effects of the disease, for instance in the case of "bois noir" or "vergilbungskrankheit" where vine stock is a "dead-end host" for the associated "stolbur" phytoplasma transmitted by the occasional vine-feeder Hyalesthes obsoletus. Similar pruning would be dangerous in the case of "flavescence dorée", unless Scaphoideus titanus has been completely eliminated by insecticide treatments. If this is not so, pruning could mask the disease but it would not prevent diffusion of phytoplasma by the vector. In the case of the "witches'broom of Populus nigra cv Italica", pruning of lower branches reduces the movement of vector larvae towards the upper parts of poplar.
Destruction of reservoirs is recommended. In the case of "flavescence dorée", a quarantine disease, it includes uprooting of diseased vine stocks and of abandoned vine plots which are also a shelter to vector leafhoppers. Direct vector control is available if the vector is restricted to the crop and if the number of generations per year is limited. Compulsory insecticide application programs against Scaphoideus titanus have been developed according to the life cycle of the species and to the transmission cycle of the phytoplasma. Search for natural enemies to be used in biological control is currently being developed. Control schemes are more difficult to set up in the cases of vectors with several generations per year, such as Euscelis plebejus, vector of "clover phyllody" or with numerous host plants including weeds such as Hyalesthes obsoletus, vector of "bois noir", "vergilbungskrankheit" and "decline of Lavandula sp.". The weeds should be eliminated by cultivation pratice.
Curing of dormant plant material by soaking into hot water (50° C, 45 min) is efficient on grapevine, use of aerated steam therapy (52° C, 1 hr) on sugar cane. Cross protection has been reported in the case of fruit trees. More should be known on the mechanism to avoid latently diseased plants.
Natural resistance and tolerance are being investigated, as well as possible cultivar feeding preferences of vector species. True resistance to phytoplasma disease is rare but it has been reported in the case of "coconut lethal yellowing". Tolerance has been used to control diseases of fruit trees (Prunus, Malus), forest trees (Fraxinus), ornamental (Prunus virginiana), and aromatic (Lavandula) plants. Feeding preference is suspected in the case of Hyalesthes obsoletus on Chardonnay, a grapevine cultivar which is very susceptible to "bois noir".
Monoculture and cultivation of clonal material on large areas may have favoured the development of epidemic diseases or increased the incidence of occasional diseases. Thus, controlling of phytoplasma diseases requires a diversification of the genetic material and an integrated approach of biological, ecological, climatic factors and cultivation practice.
Research should aim at a better knowledge of resistance, tolerance and recovery of cultivars. Molecular technology approaches include the study of phytoplasma genes involved in pathogenicity and transmissibility, of the role of major membrane proteins in recognition mechanisms with insect organs and the comprehensive study of the phytoplasma genome. Prospective research might deal with breeding of plants modified by genetic engineering and screened for resistance either to phytoplasma or to vector-insect. |
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