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The flower breaking of tulip has been a fascinating phenomenon for centuries. The Persian poet Mushrrifu'd-din described multi-coloured tulips in his poem Galistan in 1258 (Botschantzeva, 1982). The variegation of the flower colour in striped and flamed patterns of extreme fineness were highly valued in the 16th and 17th century. The desire to produce broken tulips came to an end at the beginning of the 19th century (McKay and Warner, 1933). Viruses which infect tulips are considered undesirable.
In the early literature three types of flower breaking were recognized: 'full break' in which colour is removed in areas of the petals of coloured varieties except in yellow and white ones; 'self break' in which the colour is intensified in similar areas; and 'average break' in which full and self break appear in the same petals (Cayley, 1928; McKenny Hughes, 1930, 1931, 1934; McWhorter, 1938; Slogteren and Bruyn Ouboter, 1941; Slogteren and Asjes, 1970; Slogteren, 1971). Very little mention of the leaf symptoms has been made.
McWhorter (1938) stated that two viruses were involved as causal agents, whereas Slogteren and Bruyn Ouboter (1941) proposed that the effects were due to strains of the same virus, i.e., a severe and mild strain of tulip breaking potyvirus which are associated with the full and self break, respectively. However, a number of cultivars appeared incapable of showing full and/or average break (McWhorter, 1938; Asjes and Elbertsen, 1982).
Information on the symptomatology of tulip breaking potyviruses has increased considerably in the past few decades (Asjes and Elbertsen, 1982). The symptoms can vary according to the cultivar concerned, the stage of development of the plants, the effect of different growing conditions. Symptoms appear consecutively throughout the growing season in different infected plants, i.e., before flowering, or during flowering, but after flowering newly formed symptoms may still appear in some cultivars. Symptoms in the form of a stringing appearance of the tulips and reddish-brown-green mosaic may be visible in plants emerging from soil. In the leaves, patterns of light/dark green or purplish-red to green mosaic may develop depending on the cultivar. The flower stem may show a deviating colour slightly before and after flowering. The flower-bud colour may be abnormal. The flower may show a different tint in the initial flowering stage of yellow- and white-coloured cultivars, or breaks of the flower colour in its various forms irrespective of its possible association with virus strains still to be found. Self-break patterns in flowers may additionally be due to a current-season infection, or a late-season infection in the previous year. The flower shape may be changed substantially by virus infection. The stigma colour may change from green to yellow, green to white, or green to reddish-brown, as observed differentially in several cultivars. The leaves may show different patterns of mosaic, or conspicuous ringspots appear during and after flowering in some cultivars. As a rule, symptoms appear in plants as secondary symptoms. In their severe forms, symptoms characteristic to cultivars will be easily recognizable, but it is generally difficult to attribute the symptoms to different viruses, for example, tulip band-breaking and tulip top-breaking virus.
Other viruses also induce different forms of breaking of the flower colour apart from the leaf symptoms, for example, tobacco rattle tobravirus (Slogteren, 1958), tobacco necrosis necrovirus (Kassanis, 1949), lily symptomless carlavirus (Derks and Asjes, 1975), tulip X potexvirus (Mowat, 1982; Asjes and Blom-Barnhoorn, 1998) and tulip mild mottle mosaic virus (Morikawa et al., 1995).
Tulip cultivation is largely concentrated in the Netherlands. In Great Britain, Japan and Australia minor parts of the total world acreage of tulips are grown. Therefore, measures of control dealt with in this script mainly describe the control situation in the Netherlands, in which over 10,000 hectares of tulips are grown each year.
The control of the viruses is determined by several factors which will be concisely described. The overall incidence of tulip breaking potyviruses (mainly tulip breaking potyvirus) is very low. The symptomatology of tulip breaking potyviruses is fairly clear and growers have an extensive knowledge (Asjes and Elbertsen, 1982). The reduction in quality of virus-diseased tulips is felt to be generally unacceptable. The detection of tulip breaking potyvirus infection is routinely checked by DAS-ELISA in dry bulbs during storage (Schadewijk and Eggink, 1984; Asjes, 1997). The supervision and inspection of the tulip crops in the field and the sampling of the bulbs during storage is compulsorily performed by the Flowerbulb Inspection Service in all tulip bulb growing regions (Asjes, 1997). The necessity of strict inspection to keep the virus health situation at an appropriate level is obvious as tulip cultivation is concentrated in some regions, compared, for example, with the tulip culture situation in Australia (Sutton and Garrett, 1978). Virus spread in May and June in the Netherlands generally tends to be fairly slow as a result of the low incidence of tulip breaking potyviruses, the differential susceptibility of cultivars to infection (Asjes, 1978; Romanov et al., 1990; Straathof et al., 1996), the occurrence of mature plant resistance in June (Asjes, 1997) and the curtailment of virus spread by weekly sprays of synthetic pyrethroids (Asjes, 1981, 1985, 1997).
Controlling potyvirus spread with mineral oil sprays is more efficient than with pyrethroid sprays, but this is not applied because of the low incidence of the breaking viruses and the enhanced susceptibility to Botrytis fungal infection after the application of mineral oil (Asjes, 1975; 1997). The control of aphids capable of spreading breaking viruses during the storage period from July until October/November is necessary in order to prevent the neutralization of measures such as the rogueing of virus-diseased plants and the curtailment of virus spread by pyrethroids in the field (Asjes, 1997).
The overall loss caused by tulip breaking potyvirus is difficult to estimate as the level of infection is often very low. Nevertheless, if a low tolerance rate is not met by growers they are not allowed to sell planting material intended for propagation.