THE RESISTANCE OF SOME POTATO VARIETIES TO SPRAING CAUSED BY TOBACCO RATTLE VIRUS

TRY was detected and identified for the first time in Finland. According to the information available rattle is an uncommon potato disease in our country. Spraing symptoms were detected in tubers grown in a trial field at Tikkurila in 1968, and during the following years it was possible to make observations on the resistance of 10 varieties to the disease. Sieglinde and Pito were most susceptible followed by Jaakko, Eigenheimer, Olympia and Valtti. Amyla, Record and Realta were fairly resistant and Bintje most resistant. According to Calvert and Harrison (1966) there are 2 viruses which cause spraing in potato tubers: tobacco rattle virus (TRY) and potato mop top virus (PMTV). The former is transmitted by nematodes of the genus Trichodorus and the latter by a fungus Spongospora subterranea (Wallr.) Lagerh., the causal agent of powdery scab. To the present, PMTV has occurred chielfly in Great Britain where, according to Calvert (1970), it is widespread. The occurrence of TRY either under the name »rattle» or its synonyms is more common. In addition to its occurrence in Great Britain (e.g. Cadman and Harrison 1959, Todd 1965) it has been detected in the Netherlands (Roozendaal 1947, Roozendaal and van der Want 1948), in Denmark (Hansen 1962, Kristensen 1962, Kristensen and Engsbro 1966), in Norway (Bjornstad 1962, cf. Bjornstad 1969), in Sweden (Lihnell 1958, Persson 1968) and in the USA (Walkinshaw and Larson 1959). This paper is the first report on the occurrence ofTRY in Finland and also summarizes observations of varietal resistance to spraing. Materials and methods Identification of TRY. Tobacco rattle virus was identified by means of test plants Nicoliana tabacum L. (Samsun) and Chenopodium amaranticolor Coste and Reyn, with which TRY can be distinguished from PMTV (Calvert and Harrison 1966, Todd 1967). It was transmitted directly to leaves by rubbing with cut surfaces of tubers having spraing symptoms and by means of juice from infected tobacco roots. The symptoms in test plants were similar to these described by Calvert and Harrison (1966),


Materials and methods
Identification of TRY.Tobacco rattle virus was identified by means of test plants Nicoliana tabacum L. (Samsun) and Chenopodium amaranticolor Coste and Reyn, with which TRY can be distinguished from PMTV (Calvert and Harrison 1966,   Todd 1967).It was transmitted directly to leaves by rubbing with cut surfaces of tubers having spraing symptoms and by means of juice from infected tobacco roots.The symptoms in test plants were similar to these described by Calvert and Harrison (1966), Kristensen and Engsbro (1966), Todd (1967), i.e. necrotic spots in tobacco and chlorotic spots (1 -2 mm in diameter) in C. amaranticolor (Fig. 1).Soil samples from the trial field were examined by Mr. O.Roivainen of the Department of Pest Investigation.The nema- tode Trichodoruspachydermus Seinhorst, a vector of TRV (SOL et al 1960), was detected.
Resistance to spraing.During 1968-71 ten table potato varieties commonly grown in Finland: Amyla, Bintje, Eigenheimer, Jaakko, Olympia, Pito, Realta, Record, Sieglinde and Valtti were grown in sandy soil at Tikkurila to test the resistance of their tubers to mechanical damage.From 1968 through 1970 all the seed potatoes originated from the South Savo Exp.Station, where seed potatoes for trials are propagated under disease-controlled conditions, and they were free from viruses except for virus S in some varieties.In 1971 part of the seed potatoes used were saved from the previous crop and were infected with TRV.
The first spraing symptoms were detected in 1968 in the varieties Eigenheimer, Jaakko and Pito.Valid comparisons of varietal susceptibility were not possible until 1970-71.Only secondary symptoms in the haulm of a few plants were detected in 1971.The leaves and partly also the stems were deformed and dwarfed (Fig. 2).Yellowish mottled leaves were present, but the crop suffered from drought and therefore it was not possible to get any detailed records regarding the varietal responses of the haulms to TRY.Strikingly deformed leaves were present in Amyla, Eigenheimer, Jaakko, Olympia, Pito, Sieglinde and Valtti.Kuva 1, Rattle-viruksen aiheuttamat primäärioireet Nicotiana tabacum'in (Samsun) ja Chenopodium amaranticolor'in lehdissä 9 vrk inokuloinnin jälkeen.
Tubers were harvested normally each year starting in 1968 and examined the follow- ing spring (in February in 1972).Tubers were halved through their largest diameter and the extent of necrosis was rated as follows: Percentages of tubers affected and mean values of the extent ofnecrosis were calculated.This assesment method is not as exact as those used by Harrison (1968) and Alonso and Preece (1970) but is proper for comparing varieties.Furthermore, if tuber necrosis is common and the samples large enough, differences in assessment methods have little or no effect on the results.

TRY in Finland
TRY has probably existed tens of years here without being detected until spraing symptoms were seen in potato tubers grown in a trial field in which potatoes had been grown for a number of years successively.In addition to this, spraing symptoms were recognized in ten samples of tubers sent by growers to the Department of Plant Pathology during the years 1968-71 (Fig. 3).Probably the occurrence of TRY (and PMTV) is more widespread than is indicated in the figure but, in any case, TRY is of no importance in commercial potato production in our country.First, most of the tubers sent in were grown in gardens where potatoes had been grown year after year in the same plot.This has not been done in commercial cropping.Second, in the course of studies concerning the quality of table potatoes and ring rot, attention was paid to the occurrence of spraing, but in more than one thousand samples examined no tubers with spraing were detected even though the susceptible varieties Eigenheimer and Pito made up the majority of the lots (Fig. 4).

Resistance of varieties
Varieties have been examined for the presence of spraing since 1968 but because the field was not uniformly infected the first reliable results were not obtained until 1970.The results in Table 1 show the great variation between varieties.Most susceptible were Sieglinde and Pito; rather susceptible were Jaakko, Eigenheimer, Olympia and Valtti.
Amyla, Realta and Record were fairly resistant and Bintje very resistant.About a fourth of the tubers of Sieglinde and Pito showed necrosis and most of the necrosis was severe (2 -5 rating).*) The index was obtained by using the mid-point values of % necrosis in each of the rating classes (i.e. 6, 25, 50, 75 and 94 % for the ratings 1,2, 3, 4 and 5, respectively), multiplying the number of tubers in each rating class by its respective % necrosis, and dividing by 100.

Discussion
The results obtained concerning varietal resistance to spraing caused by TRY are fairly similar to those reported elsewhere.According to Kostein (1962) Amyla, Olympia and Sieglinde are susceptible.The susceptibility of Sieglinde has also been reported by Kristensen and Engsbro (1966) and that of Eigenheimer by van Hoof (1964).The resistance ofBintje was established by van Hoof (1964), Kristensen and Engsbro (1966),   and Harrison (1968).
It was stated earlier that rattle is very uncommon in Finland.This is fairly surprising because according to Lihnell (1958) spraing is rather common in Sweden (cf.Persson   1968).It is rather common also in Denmark (Hansen 1962, Kristensen 1962, Kristensen   and Engsbro 1966).Although rattle is of no present importance in our country attention has to be paid to its occurrence.Because the number of farmers specializing in potato growing is increasing the probability that all soil borne diseases will increase becomes greater.

Table 1 .
The incidence of spraing symptoms in ten varieties.Means of results obtained in1970 and 1971.