THE PEA MOSAIC VIRUS AT THE EXPERIMENTAL FARMS. VIIK AND MUDDUSNIEMI Kyösti Kaihinko Department of Plant Pathology, University of Helsinki Received May 13, 1964 The occurrence of virus diseases on red clover and their causal agents in Fin- land have only in recent years been investigated. Recently Tapio (15) has published the first reports on the appearance of pea mosaic and bean yellow mosaic virus in red clover. There are several aphid species in Finland known to be able to transmit virus diseases of legumes (7). Tapio (15) found that at least the Acyrthosiphon pisum Harris, Aphis fabae Scop, and probably the Megoura viciae Buckt. are vectors of pea mosaic and bean yellow mosaic virus in Finland. In the summer of 1960at the Viik Experimental Farm, symptoms were observed in red clover which suggested the presence of virus diseases. The first observations were made on experimental plots, but since the disease appeared to occur elsewhere, too, observations were made also on the ordinary fields of the farm. The observa- tions were continued in 1961 and 1962; in these years virus symptoms occurred also in peas. The occurrence of the virus was also investigated at the Muddusniemi Ex- perimental Farm in Lapland (69°5' N). The symptoms of this disease varied greatly in red clover (Fig. 1). The most common symptom was mottling of the leaves, in which the chlorophyll appeared as dark-coloured blotches distinct from the lighter surrounding colour. In some places the chlorotic areas formed striations parallel to the leaf veins. These striations were light green, yellowish green or sometimesyellow. A yellow colour also appeared often in the form of small spots which were elongated and parallel to the leaf veins. The leaves in general were of normal shape and size, although occasionally small and https://www.c-info.fi/en/info/?token=6YDjMUD61ixYBL8c.chyGcqQiX2cQg6nc7_J4mA.G-uRihocpRtLpjlYKlZiO0LS_IXwBMIvfNecaH5x8zY47SfQOzpw5hrFTZop-dAp7F87lUX3PI_UdfTduAKUwn8DUDJ0OaKUfoCXJFH_JgChTFE63kQ4r3WeUUt266msB3SlPcm6Yr_v5LT5Le54x6ntRaI5WA2BuapSy-bSbydM3w wrinkled. The infected plants were often smaller than the healthy plants, but some times as large as the latter. In the leaves of the pea plants, localized chlorosis appeared which was clearly distinct from the dark green surrounding colour (Fig. 2). The spots were almost completely yellow. Similar chlorosis occurred in the stipules and sometimes also in the youngest parts of the shoots. The growth of the plant did not appear to be hinder- ed by the disease. Virus testing The causal agent of the disease was studied by transmission tests. In order to prevent the virus-transmitting insects from coming in to contact with the test plants, these were grown in isolation cages with dimensions of 60 X 60 X 60 cm. The top and two sides of the cages were made of glass, while the remaining two sides were made of gauze for ventilation. As a safety precaution, the plants were sprayed with in- secticide (metasystoxin) at intervals of two weeks. Fig. 1. Symptoms of pea mosaic virus on red clover 115 Inoculations were performed by mechanical applications of sap from infected plant to the leaf of the healthy plant, upon whose surface carborundum powder had been sprinkled; application was made by means of a glass rod, the finger, or the leaf of an infected plant. In most cases potassium phosphate or sodium sulphate solution was used as buffer. The sap used as inoculum was obtained from infected pea or clover plants grow- ing in the field. The sap from both pea and clover caused similar symptoms when inoculated into the same species of test plant (Table 1). Although the virus symptoms of the clover plants from which the sap inoculum was obtained were very variable, the symptoms appearing in the test plants were identical. When mechanically inoculated, the virus caused a systemic infection in red clover, pea, broad bean, white sweetclover, yellow sweetclover and localized necrosis in Chenopodium quinoa. The common bean and soybean, on the contrary, was not infected by the virus. Fig. 2. Symptoms of pea mosaic on pea 116 Table 1. Inoculations to test plants Reaction of test plant Test plant Inoculum Inoculum Symptoms from clover from pea Trifolium pratense, Tammisto ** * 10—15 days after inoculation chloro- l Jokioinen ** sis appeared; the colour of the spots varied from light green to yellow. Pisum sativum, Kalle *** 9 —l4 days after inoculation the leaf » Alaska ** *** veins began to turn yellow. Later, » Laxton progres intense chlororsis appeared. Phaseolus vulgaris, Bountiful » Erstling Viciafaba, Hangdown ** 12 days after inoculation chlorosis appeared; the chlorotic areas were light green. Melilotus alba. Common ** ** 9—12 days after inoculation chlo- rosis appeared. Melilotus officinalis, Madrid * About 20 days after inoculation mild chlorosis appeared. Soija max, Fiskeby 111 Chenopodium quinoa * ** Only localized necrosis 4 5 days after inoculation *** more than 50 % of inoculated plants became **» »20%* * » » infected » * less * 20 % » * * » » no symptoms The effect of the virus on the growth and overwintering of red clover According to observations made in the field, at least half of the clover plants showing virus symptoms were more or less stunted. In the greenhouse the virotic clover plants grew very poorly. The average weight of healthy plants, two months old, was approximately twice that of infected plants which had been inoculated at the age of three weeks: No. of plants Aver, fresh weight, g Healthy 30 10.5 Infected 9 5.6 Observations on the overwintering of virotic clover plants were made in a trial in which both healthy plants (120 in number) and virotic plants (30) were transplant- ed in an outdoor trial plot in the spring of 1960. During the following winter 21 of the virotic plants (70 %) died, while only 10 % of the healthy plants succumbed. In a large red clover breeding material consisting of about 2000 plants located on the same trial field, 12 % died during the winter 1960—1961. During the same period 25 out of 52 virotic appearing plants in the same material died (nearly 50 %). 117 Frequency of the virus In the summer of 1960 the frequency of red clover virus in different parts of the Viik Experimental Farm was visually estimated on the basis of the symptoms in the plants. The method used was to count the number of virotic plants on each of the 30 test plots, generally 20 m 2 in size. In June there were on an average 1.2 infected plants per 10 m 2 in first-years leys and 2.2 in second-year leys, or 0.06 and 0.12 % of the clover plants, respectively. In September a second count showed 3.0 infected plants per 10 m 2 in first-year and 4.0 in second-year leys, or 0.20 and 0.25 %. The virus was considerably more abundant in the trial fields than in the ordinary fields of the farm (cf. 15). For example, in June 1960 0.1 % of the clover plants in the trial plots showed virus symptoms and in September the figure was 0.5 %. The virus spread particularly heavily in the red clover spaced plant trials, in which as many as half of the plants in the most infected areas were virotic. In one greenhouse where an abundance of pea aphids (Acyrthosiphon pisum Harris) appeared, nearly all the clover plants were infected by the pea mosaic virus. Observations made at the Muddusniemi Experimental Farm at Inari showed a certain extent of virus occurrence with an average of 0.25 % of the clover plants being virotic. The occurrence of the virus in different varieties of red clover was studied at Viik in the summer of 1960 by counting the numbers of virotic plants in the red clover variety trials (Table 2). The highest numbers of virotic plants were found in Tepa (Jo TPA 1) and in Jokioinen red clover (Jo 37). Practically no virus appeared in the three clovers imported from the Soviet Union: Permian, Siberian and Russian. Discussion The mosaic virus appearing in red clover and pea at Viik is caused by the pea Table 2. The numbers of virotic clover plants in first-year red clover stands, summer 1960. The figures are averages from 5 replicates. ■?3 O c c ~ • 3 c Ä '