A count was made of the total number of A. rubicolq on all live, infected plants SPREAD OF RASPBERRY LEAF CURL VIRUS

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1 SPREAD OF RASPBERRY LEAF CURL VIRUS A, T. BOLTON Research Station, Canada Department of Agricultttre, Ottawa. Contribution no.251, received Februata 12, 1970, accepted September 8, ABSTRACT No definite pattern of spread of leaf curl virus from infected to healthy plants did not virus in red raspberry iould be observed follow along the rows but was scattered within an experimental plantation over a throughout the plantation. Despite a definite 7-year period. When a source of the virus increase in the population of the vector, was located at the ends of six 45-meter rows Aphis rubicohz, over the 7-year period, the and allowed to spread over the period from number of newly infected plants each year 1962 to 1968, approximately 50% of the remained quite constant. plants became infected. Transmission of the INTRODUCTION Raspberry leaf curl virus has been an important and extremely destructive disease on red raspberries in eastern Canada for many years. It is generally believed to be spread by the aphid, Aphis rubicola Oestl., (1,2) although it has also been reported that A. idaei v.d.g. is capable of transmitting the virus (6). Frequently, in plantations in Ontario and Quebec where virus-tested raspberry plants have been grown in connection with plant certification programs, leaf curl symptoms have appeared on a limited number of plants scattered through a particular field in spite of very strict spray programs. The aphid vectors were observed on the infected plants in only two out of eight plantations in which this limited leaf curl infection occurred. This erratic spread of the disease was observed in earlier experimental plots at Ottawa and a raspberry plantation was set out to study the pattern of spread of the virus. MATERIALS AND METHODS The experimental plot consisted of two randomized rows of 50 plants of each of the three cultivars Carnival, Rideau, and Madawaska planted in the fall of The plants were 90 cm apart in the rows and the rows were 3.6 m apart. The plants were allowed to form a hedge 60 cm in width, and the space between the rows was kept completely free from weeds. The plot was located over 900 meters from any other raspberries. No insecticide was used at any time. A leaf curl-infected plant of the cultivar Carnival was planted at the south end of each of the six rows at the beginning of the experiment. Aphids had been observed on these plants in September and, although there were no live aphids present when they were transplanted in October, several were found on them the following June; and it was assumed that eggs had been deposited before their removal from the nursery. The number of aphids in the plot was estimated by recording those present on the top 30 cm of each live cane in two 1.5-m sections of each of the six rows. The average number of canes per 1.5-m section was 45. Aphids were counted in early July and mid-september and the two counts were averaged. A count was made of the total number of A. rubicolq on all live, infected plants in 1963 and on 21 7ive, infected plants selected at random in each of the following 5 years. In each case, 20 canes of each plant were examined. The aphids resembled published descriptions of A. rubicola and were shown to transmit the virus from infected to healthy plants under controlled conditions in Can, J. Plant Sci.50: (Nov, 1970) tro /

2 668 CANADIAN JOURNAL OF PLANT SCIENCE the greenhouse. To determine transmission, 200 aphids were collected in 1963 from leaf curl-infected plants in the field and placed on the leaves of red raspberry seedlings (from open-pollinated Carnival seed) in individual cages at the rate of 25 aphids per plant. The same procedure was used to determine the inability of the aphids to transmit the virus to Dundee black raspberry plants. Since the aphids were unable to transmit the disease to Dundee plants in three separate attempts, the virus was assumed to be the alpha strain as described by Bennett ( 1). RESULTS In 7962, 2 yeats after the plot was set out, nine of the healthy plants had become infected with leaf curl. No pattern of spread was established, as the infected clones were scattered throughout the plantation. In 1963, 13 additional plants exhibited leaf curl symptoms, making a total of 22. The spread of the disease had been limited to a small area of the plantation, although vectors were present throughout. In 1964 the spread of the virus was again quite erratic, although there was some indication of a pattern. The prevailing wind in the area (across the rows from west to east) appeared to affect the spread of the disease so that the infected plants were arranged across the plantation rather than along the rows (Fig. 1). During the season 27 additional plants were infected. The following year, and during the next 3 years, the disease continued to spread in a more or less erratic manner. There was still more infection across the plantation from west to east than along the rows. Counts of infected plants were always the same in early July as they were the previous September, indicating that no new infection occurred during late fall or early spring. Progressive infection of red rqspberry rto20 Row I X OOO b,nnaaa a tr al aa 2 X OON' tronaaoo r ll r 3 X N NNNN.Otr 'tr III I 4 X X ONOOtr XXIO s x oon.n, aooo 6 XNN axn.ldror OO Plonls infected during olonts with leof curl Virus 1962 : X 1963 : lr I 1964 z 'l' 1965 : a Y 1966 : o 1967 : N 1968 = O s'on trtr x\1, ooll on, lb,o ol,t! \oaa rl x\oort\nrtr ooo ' \xo\!n oaoa LI trnn\n,i OOOOO xr\l otrtrn,oo a o0 0 Fig. 1. Diagram of six rows of raspberry plants showing the total number infected over the 7-year period and the year in which each plant became infected. Each geometric figure represents the location of the original mother plant. Arrow shows direction of prevailing wind.

3 BOLTON-RASPBERRY LEAF CURL VIRUS 669 Table 1. Relationship betrveen the populatiort of A. rubicola and the progress of infection of red rasoberrv bv leaf curl virus frorn 1962 to No. of aphids* No. aphids per 30 cm No. new infections Total infected plantsf Total dead plantst No. live infected plants Ratio live infected:herlth r 6i L:32 1:13 1:6 *Calculated from counts made in two 1.5-m sections in each row, foriginal infected plants set out in 1960 are not included : rl : ll :3 7:1 The population of A. rubicola increased gradually from 1962 to 1967, and decreased ionsiderably in 1968 (Table 1 ). Aphid counts wele higher in September than in July on the 1.5-m sections containing both healthy and infected plants, in spite of the fact that the aphids occurred in greater numbers on infected plants in futy lfaute Z;. The aphid population on infected plants was consistently approximately double that on the combination of healthy and infected plants in the 1.5-m sections. Winged aphids were not observed in the plantation during the course of the experiments. There did not appear to be a direct relationship between rate of spread of the disease and number of vectors present in the plantation. The actual number of plants infected each year remained relatively constant from 1964 to 1968 while, during this time, the vector population varied considerably. From 1964 to 1968 the ratio of virus source (represented by live, infected plants) to healthy plants remained quite constant (Table 1). Death of a plant usually occurred within 3 years of initial infection. There was some variation between the three cultivars, but this appeared to be closely related to the susceptibility of the particular cultivar to winter injury. The general condition of healthy plants did not vary greatly during the first 6 years of the experiment, but in 1968 all plants suffered considerably from winter injury, necessitating the conclusion of the investigation. 'fabre 2. Comparison t,n",lr""iffii,on""?i,ijft# i;r"!!r!,!..i"'r.jul-v and septernber between On infected plants* Average rrrrtrber of A. r'ubicolc per cane On all plar-rtst Year J ulr Sept. Jul-v Sept ear average Twenty canes of each of 21 plants were examined. teightein meters of row rvittr an average of nine canes Der 30 centimeters rvere examined.

4 670 CANADIAN JOURNAL OF PLANT SCIENCE DISCUSSION In an isolated location where no outside source of infection is within 900 m, a single plant infected with raspberry leaf curl virus can bring about the infection of 5OVo of the plants in a plantation over a7-year period. Since this was possible in the ottawa area where the vector population was approximately five per 30 cm at its highest point, it is conceivable that spread of the disease would be much faster in areas where the vector population was higher. However, under conditions at ottawa, the spread of leaf curl was as rapid when the vector population was two per 30 cm of row as when it was five per 30 cm. The fact that the number of aphids present in the plantation increased from an estimated 2085 to 4290 without an increase in the number of new infections would suggest that the actual population is not a significant factor in the spread of leaf curl virus. A more important factor seems to be the source of inoculum. From 1964 to 1968, although the aphid population varied from 2.3 per 30 cm of row to 4.8 per 30 cm and then decreased to r.4, the number of new infections remained constant as did the ratio of live, infected plants to healthy ones. stace-smith (6) reported transmission tests with A. rubicolq in which he transferred 10 aphids from a leaf curl-infected plant to each of ro Rubus henryi indicators. Six of the 10 plants developed symptoms, indicating that at least 40 and possibly 94 individual aphids did not transmit the virus. These results are in agreement with observations made during the determination of aphid transmission described under Material and Methods, where 25 aphids per raspberry plant resulted in only five out of eight seedlings becoming infected. In this case at least 75, and possibly a much greater number, did not transmit the virus. It seems logical to conclude that, even under optimum conditions, A. rubicola is not an efficient vector of the disease. Rankin and Hockey (5) found that leaf curl spread very slowly arong the rows, aftecting, at most, the two adjacent plants annually. They also observed that newly infected plants appeared annually at some distance from old infections. These observations are in agreement with those made during the present investigation, except that Rankin and Hockey reported the ratio of infection in new areas to infection of adjacent plants was about 1:4 whereas in the present experiments the ratio was about 1:1 from 1963 to After 1966 it was impossible to identify a new area of infection. The absence of virus-infected plants in the central area of the plantation suggests that the prevailing west wind had a definite effect on aphid movement. During the 7-year period no attempt was made to trap flying aphids and none were observed in the plantation. rf, as Swenson (7) reports, winged aphids are continuously present in the air and alight by chance, it would explain the lack of pattern in the occurrence of new areas of infection. It would not explain the absence of the disease in the central area. According to Lees (3) the production of alatae to facilitate aphid movement from one host to another is the result of increased contact or crowding brought about by deterioration of the plant. A. rubicola seldom occurs in great enough quantity on the host to cause crowding. ln 1924, Marcovitch (4) demonstrated photoperiodism in some aphid species. If lower temperatures and shorter days initiate alatae of A. rubicola, these forms would probably occur in eastern ontario in late September; this could explain the absence of winged aphids in the plantation during the summer. No new infections were observed in the

5 BOLTON-RASPBERRY LEAF CURL VIRUS 67I plantation between September 15 and July 1 and, therefore, no transmission occurred. It is possible, however, that the aphids move to new plants in the fall and overwinter there in the egg form. The presence of the uninfected area in the plantation leads one to conclude that most;f the infection came from within the plot. Infection of adjacent plants probably was due largely to active movement of apterous forms as well as passive movement of these forms during harvest, pruning, or cultivation. Movement of the aphids to new areas in the plantation was achieved probably by alatae capable of actively flying short distances but, nevertheless, greatly afiected by wind direction. The conclusion of Rankin and Hockey (5) that leaf curl-infected plants were favored by A. rubicola was partly substantiated by the fact that, in the present investigation, aphids were more numerous on infected canes. However, it is not certain whetheithis was a case of aphid preference or if the plants became infected as a result of heavier infestation by the vectors. If alate forms of A. rubicolo are responsible for the spread of leaf curl infection, and if they can be carried in air currents for a considerable distance, eradication of the disease by roguing infected plants is impossible. The fact that prevailing winds have a definite effect on the local movement of the vectors within a plantation or from one plantation to another indicates that greater care must be iaken to avoid locating new raspberry fields on the windward side of infected ones. Since 50Vo of the plants in an unsprayed plantation can be infected in 7 years,. in spite of the slowneis of spread and inefficiency of the vectors, a strict insecticide spray program is necessary. R.EFERENCES 1. BENNETT, C. W Further observations and experiments on the curl disease of raspberries. Phytopathology 20: CONVERSE, R. H Diseases of raspberries and erect and trailing blackberries. U.S. Dep. Agr. Handb. No Washington, D.C. 3. LEES, A. D The control of po ymorphism in aphids. Advan. Insect Physiol. 3: MARCOVITCH, S The migrarion of the Aphididae and the appearance of the sexual forms as afiected by the relative length of daily light exposure. J. Agr. Res. 27: RANKIN, W. H. and HOCKEY, J. F Mosaic and leaf curl (yellows) of the cultivated red raspberry. Phytopathology 12: STACE-SMITH, R Studies on Rubus virus diseases in British Columbia. VIII. Raspberry leaf curl. Can. J. Bot. 40: SWENSON, K. G Role of aphids in the ecology of plant viruses. Annu. Rev' Phytopathol. 6:

6 This article has been cited by: 1. Robert R. Martin, Stuart MacFarlane, Sead Sabanadzovic, Diego Quito, Bindu Poudel, Ioannis E. Tzanetakis Viruses and Virus Diseases of Rubus. Plant Disease 97:2, [Crossref]