Contribution to the Knowledge of the Distribution of Crucifer Viruses in Southern Europe

The most important viruses which in nature predominantly attack Cruciferae are: turnip mosaic virus (TuMV), cauliflower mosaic virus (CIMV), radish mosaic virus (RaMV), turnip yellow mosaic virus (TYMV), turnip crinkle virus (TCV), and turnip rosette virus (TRosV). Some of them are spread in many countries while the others were found in limited areas. TuMV has been detected in 7 or 8 European countries (Klinkowski 1968, Vukovits 1956, Blaszczak 1968), and also in the USA, Japan and China as well as in some other countries (Yoshii 1963); recently, it has been found in British Columbia (Stace-Smith and Jacoli 1967). CIMV is also widespread in Europe where it was observed in 13 countries (Klinkowski 1968; Kovacevski 1968, Giunchedi 1968); it is also known to exist in the USA (Tompkins 1937), New Zealand and Australia (Randles and Crowley 1967). Finding places of TYMV are apparently limited to Europe (comp. Matthews and Ralph 1966); this virus was first found in Scotland (Markham and Smith 1966); and later in 5 other countries in continental Europe. TCV was reported from Great Britain and India and TRosV only from Scotland (Hollings and Stone 1969; comp. Broadbent and Heathcote 1958). RaMV has been so far detected in the USA and Japan (Campbell and Tochihara 1969). Recently, Stefanac and Mamula (1971) found RaMV in Europe. From the six above mentioned crucifer viruses the following four have been repeatedly found in Yugoslavia up to now: TuMV (Stefanac-Uđbinac et al. 1963), CIMV (Mamula and Milicic 1968; comp. Panj an and Prpic 1954), TYMV (Mamula et al. 1966) and RaMV (Stefanac and Mamula 1971). In order to obtain some evidence of the distribution of these viruses we have investigated them in many places of Yugoslavia, especially in the western part of the country (Fig. 1, Table 1 a. 2). On the basis of distribution analysis we have established that the mentioned viruses are rather spread in Yugoslavia. The finding places of the viruses are denser in those parts of the country where they were investigated more intensively (Fig. 1). We have particularly closely investigated these viruses on turnip plants (Brassica rapa var. rapifera). Observations on the frequency of the viruses inside turnip fields were made during three years in Tuhelj and Ivancc near Zagreb and they are presented in Table 2. The most frequent virus was RaMV, the second in order of frequency being TYMV. CIMV and TuMV were not found so often. Mixed infections with all of these viruses were detected in very low percentage. In several fields near Ivanec turnip was infected only with RaMV (Table 2). In some localities (Tuhelj) the viruses were found repeatedly for several years. During the autumn 1969 we visited southern Austria and established that TYMV and RaMV were rather spread in turnip fields (Fig. 1). Identification of virus isolates was carried out by means of the following methods: 1. Observation of symptoms in field, 2. Analysis of intracellular changes, 3. Testing of isolates by serological method, 4. Investigation by means of test plants. At least two of these methods were used for the identification of each isolate (Table 1). On the basis of characteristic symptoms on field plants, especially on turnip, it is sometimes possible to identify crucifer viruses. In this plant TYMV produces outstanding variegation and mosaic with yellow, sometimes nearly white areas on leaves which are rarely deformed (Markham and Smith 1949). Field plants infected with RaMV mostly show necrosis on the whole leaf, specially on the midrib and the pedicie (Fig. 2a). Besides, they often have mosaic and chlorotic as well as necrotic rings and lines on leaf blades (Campbell 1964, Stefanac and Mamula 1971). Turnip plants affected by TuMV often exhibit mosaic and variegation, but the difference between the brighter and the darker areas of the leaf is not so remarkable as in the case of TYMV. Turnips attacked by C1MV show vein-banding, assymetrical leaves with laterally curved midribs and often get appearance of stunted rosette (Tompkins 1937, Mamula and Milicic 1968; comp. Broadbent 1957). C1MV infected plants often have well expressed vein-clearing on leaves as the first sign of infection; this symptom also appears in TuMV and TYMV infections and is particularly conspicuous in the latter. It is, however, absent from plants infected with RaMV. Only in TYMV infections stunting of plants is a symptom which is not often very expressive. Identification on the basis of symptoms in turnip plants can be done with a fairly high degree of certainty in the case of TYMV and RaMV, and somewhat less sure concerning TuMV and C1MV infections. In the cells of their hosts crucifer viruses induce characteristic intracellular inclusion bodies and also some other changes, especially in plastids (Fig. 3a), which can be very helpful in their identification. Inclusions of TuMV, C1MV and TYMV have already been minutely studied and described (Rubio 1956, Milicic and Stefanac 1967, Milicic et al. 1969). Plants infected with RaMV also contain very specific inclusions (Stefanac and Ljubesic 1971, Stefanac and Mamula 1971). They are usually fairly big, specially in relation to inclusions of TuMV, TYMV and CIMV. In inclusions of RaMV, which sometimes contain distinct granula, one, or often more, fairly large vacuoles are mostly present (Fig. 3b). Conspicuous intracellular changes have been noticed in turnip, white mustard, charlock, Chinese tabbage, Sisymbrium officinale and some other plants, whereas they are not so distinct in some varieties of Brassica oleracea. It would be useful to investigate inclusion bodies in B. oleracea varieties in more detail. Nevertheless, despite the difficulty in working with this species, the method of cell inclusions can be very useful for identification of crucifer viruses. The knowledge of intracellular changes has helped us very much in rapid identification of numerous isolates of all investigated viruses. This was especially the case in TYMV and RaMV infections, because intracellular alterations which arise under the influence of these viruses are very conspicuous (Fig. 3a, b)