CHICKWEED, STELLARIA MEDIA - A SOURCE OF VIRUSES AFFECTING CROP PLANTS G . D . HEATHCOTE
Chickweed (Stellaria media (L.) Vill.) (Caryophyllaceae)isonly one ofmany weed species which have been identified as a source of virus infection for crop plants, but its abundance and distribution over all temperate regions of the globe makes it of particular danger to farmers. Many viruses cause only slight or no symptoms in chickweed and it is therefore not an obvious source of disease. It occurs in the semi-natural flora of the shingle banks of Suffolk, but mainly in areas of cultivated ground or wasteland. It has been spread by man to become one of the most completely cosmopolitan weed species, although in the tropics it is usually found only at higher altitudes. The biology of the Stellaria media species group (S. media, S. neglecta Weihe and S. pallida (Dumort.) Pire) has been well reviewed by Sobey (1981). In the following account 'chickweed' refers to S. media ssp. media as in Flora Europeaea (Tutin, et al., 1980). Chickweed is successful as a weed for many reasons. Because of its prostrate habit it is not killed by mowing and, if uprooted by hoeing, will re-root if partially covered by soil. It is resistant to many herbicides, such as MCPA, and can flower and fruit throughout the year, even under adverse weather conditions. It may become brittle and glassy with a severe frost but appears to be undamaged when it thaws. It is, however, sensitive to drought. A single plant may produce 2,200-2,700 seeds, which may remain viable for at least 40 years (Salisbury, 1961), but cultivation will decrease the number of viable seeds in soil by about 60% each year, providing that there is no further seeding (Roberts & Dawkins, 1967). The top 15 cm of soil in an arable field may contain more than 1 million chickweed seeds per hectare ( = 440,000 per acre), and represent 30% of the weed seeds in fields with mineral soils and 80% of the weed seeds in some fenland fields (Bray, 1970). As chickweed seeds are small, 0.8-1.3 mm across, and brown, they are not readily seen on most Suffolk farm soils, but they may form an important part of the diet of some birds, such as sparrows. In mid-winter omnivorous farmland birds such as skylarks may take chickweed seeds from land ploughed after cereals, where few other weeds have survived treatment with herbicides (Green, 1978). Shoots of the plant may also be eaten by many mammals, including deer, and, as the common name suggests, by poultry and other birds such as pheasants. Many insects also feed on chickweed, but few are specific to it. The Sharp-angled carpet moth (Euphyia unangulata Haw.) is one of them, and caterpillars of this local but widely-distributed moth can be found on chickweed in July and early August. In spite of cultivation and foraging by birds many seeds of chickweed remain in farm fields and they may affect the health of crops. Not only will plants which grow from these seeds compete for nutrients with crop plants but they may be infectious with virus diseases carried in the seed. Many soil-borne plant viruses are seed-borne. Murrant (1970) lists several viruses
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carried by nematodes (='eelworms'), many species of which are minute, but they can occur in enormous numbers in some soils where they feed on the roots of plants as ecto- or endoparasites. The list includes four which can be carried by the seed of infected chickweed plants (arabis mosaic, raspberry ringspot, strawberry ringspot and tomato blackring viruses). Tomato blackring virus (TBRV) is carried by needle nematodes (Longidorus spp., Dorylaimina) and attacks not only tomatoes but also sugar beet, causing 'Docking disorder'. Heathcote (1965) records the virus in sugar-beet crops on sandy soil in 5 parishes in Suffolk and 8 in Norfolk, and it has since been found elsewhere in East Anglia. The disease has little effect on the leaves of infected plants, but the roots are 'fangy' and stunted and sugar yield can be greatly decreased. T B R V is a ' N E P O ' virus (i.e. nematode-borne virus with polygonal particles). There is also a 'NETU' virus (i.e. ÂŤematode-borne virus with rubular particles) carried by chickweed seed. It is tobacco rattle virus (TRV), which is carried by stubby-root nematodes (Trichodorus spp. Diphtherophorina), and which is another cause of 'Docking disorder' of sugar beet. Infected sugar-beet leaves may show bright yellow blotches and, as with TBRV, roots are stunted and sugar yield poor. Heathcote (1965) records it from 5 Suffolk and 7 Norfolk parishes. These viruses can only be spread from infected to healthy plants by feeding nematodes, but it is not possible to eradicate the disease by crop rotation when the viruses can survive in infected weed seeds. Few air-borne plant viruses are also seed-borne, but lettuce mosaic virus is carried by chickweed seed (Ainsworth & Ogilvie, 1939), as is cucumber mosaic virus which can be transmitted to 5 - 8 % of the seeds of an infected plant (Tomlinson & Carter, 1970). Both of these viruses are spread from infected to healthy plants by many different species of aphids. They are 'non-persistent' viruses, quickly acquired by an aphid feeding on an infected plant and able to be transmitted immediately should the aphid walk or fly to a susceptible plant. However, the aphid loses infectivity within a few hours even if it does not feed, and loses its infectivity very quickly if it feeds. Although seed-borne viruses are of theoretical interest and local importance, chickweed is of far greater importance in East Anglia as a source of viruses which are not seed-borne. It can carry beet yellows and beet mild yellowing viruses, the cause of 'virus yellows' of sugar beet, a disease which can be widespread and cause serious loss of yield. For example, in 1983 at the end of August 19% of all sugar-beet plants in the Ipswich and Bury St. Edmunds sugar factory areas (which include most of Suffolk) showed symptoms of the disease, and in 1974, when the attack was unusually serious, 97% of sugar-beet plants did so. It must be emphasized that chickweed is only one of many overwintering sources of the disease (such as clamped mangolds, spinach beet and wild beet on the shingle banks of the sea shore) but it may be an important one. Unfortunately, only recently has a serological technique been developed (Smith, 1982) which allows large numbers of symptomless plants to be tested for these viruses both rapidly and conclusively, and a survey of chickweed has not yet been made on an adequate scale.
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The yellowing viruses affecting sugar beet are spread by aphids, and are of the 'persistent' or 'semi-persistent' type. An aphid takes longer to acquire them than 'non-persistent' viruses, and there may be a latent period before the aphid becomes infective, but they are retained by the aphid for days, or even for life. Unlike nematodes, which move only a few centimetres through the soil, winged aphids may fly over great distances, and even wingless aphids can walk several metres between plants spreading virus disease. An aphid can leave a yellows-infected sugar-beet plant in autumn and infect a chickweed plant on the edge of the field, or some distance away. The weed may then survive the winter after the sugar-beet crop has been harvested. Aphids may also overwinter on the chickweed and, when winged individuals are produced in spring, these can fly to young sugar-beet plants and infect them in turn, thus completing the virus cycle. The following aphids have been found on chickweed in Suffolk: the peach-potato aphid ( M y z u s persicae (Sulz.)), the potato aphid (Macrosiphum euphorbiae (Thomas)), the glasshouse-potato aphid (Ă„ulacorthum solani (Kalt.)), the shallot aphid ( M y z u s ascalonicus Donc.) and Myzus certus (Walker). All can act as the vectors (carriers) of plant virus diseases, but they vary considerably in their ability to do so. By far the commonest aphid overwintering on chickweed is M. ascalonicus. It is sand-coloured and shiny, with dark legs, and flies early in the season; it is seldom found in sufficiently large numbers on sugar beet to cause damage by its feeding. When 97 samples of chickweed from sheltered sites were examined in April, 32% were found to be infested with M. ascalonicus, 8% with M. persicae and 5% with M. euphorbiae. M. persicae is the most dangerous of the aphids as it has an extremely wide host rĂ¤nge (more than 290 spp. in at least 63 different families) and is known to transmit more than 100 plant viruses (Kennedy, Day & Eastop, 1962). M. certus is uncommon, but all the other aphids mentioned can be pests of sugar beet and other crops, even if free from virus (e.g. M. euphorbiae causes 'top roll' of potato crops). All of these aphids can transmit yellowing viruses of sugar beet, but M.persicae is by far the most important vector. To summarize this complicated story, chickweed is an extremely successful weed species which is a reservoir for many plant viruses which attack farmers' crops, and some of these viruses are seed-borne in chickweed. It is also a host plant for several species of aphids which can carry plant viruses. As chickweed is only one of several sources of these viruses it is not surprising that in spite of the widespread use of herbicides to control the weed, and pesticides to control the virus vectors, plant viruses can still cause serious crop loss to farmers in Suffolk and elsewhere. References Ainsworth,G. C. &Ogilvie, L. (1939). Lettuce mosaic. Ann. appl. Biol., 26, 279. Bray, W. E. (1970). Report of A.D.A.S. Conference, Lindsey, Lines. 1970. Green, R. (1978). Factors affecting the diet of farmland skylarks, Alauda arvensis. J. anim. Ecol., 47,913. Trans. Suffolk Nat. Soc. 20
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Heathcote, G. D. (1965). Nematode-transmitted viruses of sugar beet in East Anglia, 1963 and 1964. PL Path., 14,154. Kennedy, J. S., Day, M. F. & Eastop, V. F. (1962). A conspectus ofaphidsas vectors of plant viruses. Commonwealth Institute of Entomology, London. Murrant, F. (1970). The importance of wild plants in the ecology of nematode-transmitted plant viruses. Outlook on Agriculture, 6,114. Roberts, H. A. & Dawkins, P. A. (1967). Effect of cultivation on the numbers of viable weed seeds in soil. Weed Res., 7, 290. Salisbury, E. (1961). Weeds and aliens. Collins, London. Smith, H. G. (1982). Use of enzyme-linked immunosorbent assay (ELISA) in the study of beet yellowing viruses. Rep. Rothamstedexp. Stn.forl981, Part 2, 76. Sobey, D. G. (1981). Biological flora of the British Isles. No. 150 Stellaria media (L.) Vill. J. Ecol., 69, 311. Tomlinson, J. A. & Carter, A. L. (1970). Studies on the seed transmission of cucumber mosaic virus in chickweed (Stellaria media) in relation to the ecology of the virus. Ann. appl. Biol., 66, 381. Tutin, T. G. etal. (1980). Flora Europaea, 5, 153. Cambridge. Dr. G. D. Heathcote, 2 St Mary's Square, Bury St. Edmunds.
Bird versus bat Walking in Felsham woods on 27th September, 1983, at about 6 pm, on the path along the edge of Fishpond Fell, we heard a loud squeaking and fluttering in the field next to Bottom Fell. When we reached an opening to the field we saw a kestrel chasing a bat. The bat was weaving and circling in a figure of eight, keeping below treeptop height. The kestrel was about a metre behind and they were Aying fairly fast. We watched for about a minute. The kestrel then veered off into the reserve (it may have seen us) and the bat flew towards us. Leslie Acker & Peter Woodhouse Dr. R. E. Stebbings of the Institute of Terrestrial Ecology, an authority on bats, comments: I suspect many more people see bats being chased by birds than we hear about, so perhaps one could make the plea for people to write in with any such records which normally occur in March/April/May, rather than the end of the year.
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Heathcote, G. D.