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Symptoms of GY diseases are very similar in Europe, Israel, Australia and North America (Caudwell, 1983, 1990; Pearson et al., 1985; Magarey and Wachtel, 1986; Caudwell et al., 1987; Prince et al., 1993; Wolf et al., 1994; Marcone et al., 1996; Davis et al., 1998).
The leaves of affected plants are thicker than normal, brittle, rolled downward and show veinal yellowing followed by necrosis. Shoots of affected branches exhibit rows of black pustules that develop along the internodes. Due to incomplete lignification, these shoots are more flexible than normal and confer a drooping aspect to the plants. Flowers and bunches are whiter and desiccated.
Use of healthy plant material, removal of infected plants and effective control of insect vectors are important measures for reducing the incidence of GY phytoplasmas. Healthy plants should be used for establishing new vineyards. Therefore, rootstocks and scionwood should be indexed by an appropriate method to ensure that they are free from infection. Also, GY phytoplasmas seem to be readily eliminated from plant material by hot-water treatment at 50°C for 45 minutes (Caudwell et al., 1997; Rivenez and Bonjotin, 1997; Bianco et al., 2000; Murari et al., 2000). Diseased grapevine plants as well as alternative plant hosts and insect vectors of the pathogens should be eliminated as they are identified.
Weber et al. (1998) reported a high disease incidence (30-34%) of VK disease in vineyards with a high number of field bindweed plants and an abundance of Hyalesthes obsoletus, which are dependent on field bindweed plants. Moreover, they found that some agricultural practices such as ploughing negatively affected populations of this planthopper vector.
In France, control of FD disease is subject to several legislative directives. One reason is the high adverse impact of this disease on the viticulture industry. Furthermore, the successful control of the disease is only feasible by well coordinated activity by producers within a particular area, due to the high mobility of the vector. All mother plots used as a source of propagation material must be treated with insecticides three times a year to protect against infestation by Scaphoideus titanus while all nurseries must be treated throughout the time when larval stages or adults of S. titanus occur.
Directives from the local authorities regulate the mandatory control of FD disease for particular areas where the disease is present. They also regulate the mandatory control of S. titanus by insecticides and the measures to be taken in order to reduce inoculum.
Prophylactic activities include the destruction of FD-infected vines as well as uprooting of both cultivated and wild Vitis plants in abandoned vineyards, both of which are potential sources of inoculum and provide a breeding reservoir for the vector. These measures, as well as the mandatory uprooting of complete vineyards when the disease incidence exceeds a certain level, are subject to prefectorial directives in France.
Control of the vector depends on insecticide treatments, which are applied either against the eggs during winter or against larvae and adult leafhoppers during the growing season. Pruned plant parts that carry the eggs of S. titanus should be burned. The number of viable eggs can be reduced by a treatment of vines in March before bud-burst. A wide range of insecticides, mainly organophosphates and pyrethroids, are available for summer treatments, which are usually applied three times. The first treatment is usually applied in June and should not be done later than 1 month after the beginning of hatching. The time of the second application depends on the stability of the compound used. The second treatment is usually applied in combination with insecticidal control of the second generation of grape berry moth. Adult leafhoppers immigrating into vineyards from surrounding areas are the target of a third treatment in August.
For further information see Caudwell (1965), Caudwell et al. (1971b, 1974) and Boudon-Padieu and Maixner (1998).
GY diseases are of considerable economic importance, especially in areas where outbreaks occur. In affected plants, flowers and bunches whiten and desiccate so that the yield is drastically reduced. In addition, the quality of wine is decreased by high acid and low sugar contents of infected clusters.
According to studies by Maixner and Reinert (1997), the proportion of VK-infected vines in some vineyards in Germany, increased from 18% in 1990 to 26% in 1995. The disease incidence within individual vineyards varied from 5% to 65%. It was increasing in seven of the 12 vineyards investigated and decreasing in three. The increase in the number of symptomatic vines was highest in young plantings, which often provide favourable conditions for the insect vector Hyalesthes obsoletus and the alternative host of the VK phytoplasma, field bindweed. These vineyards are severely damaged, since young grapes systemically develop symptoms and die.
In the Emilia-Romagna region (northern Italy), Credi and Callegari (1988) found that the percentage of GY-infected vines varied in the period 1983-1987 from 0.1 to 42.8% while crop losses of 47% were recorded in affected vineyards of cv. Sangiovese (Credi, 1989). Disease incidences of more than 70% with very heavy crop losses have been recorded in GY-affected vineyards of cv. Chardonnay in the Veneto region of northern Italy (Belli et al., 1994) while a disease incidence of 76% was recorded in the Latium region (central Italy) (Del Serrone et al., 1995).
In Slovenia, during a survey carried out during 1991-1995, the percentage of GY-infected grapevine plants varied from 2 to 38% and the crop losses in infected vineyard ranged from 20 to 40% (Koruza, 1996).
For details about economic impact of FD-phytoplasma and Candidatus Phytoplasma australiense see the corresponding data sheets.