Crop Management and Diagnosing Virus Damage in Field Peas

Infected pea plants develop mosaic and chlorotic vein flecking (appears as translucent windows) and veinal enations (blisterlike outgrowths), which are very characteristic for PEMV . Plants are stunted, and proliferation of basal branches is common. Pods are distorted, split open, and may show prominent enations. Many American cultivars are resistant to PEMV. The method of virus transmission and the behavior of the pea aphid are two factors aiding in the control of PEMV. Spraying plants with insecticides reduces the incidence of the virus because of the persistent nature of spread and because the pea aphid readily colonizes peas and broadbean. The aphids can be controlled before much secondary spread occurs. Removal of perennial legumes bordering the planting area also reduces the primary virus reservoir. Pea streak diseases are a general grouping of virus diseases in which the symptoms of infected plants appear as conspicuous purple brown streaks extending the entire length of the stem and petioles . The plant growing points are curved and brittle; and if pods are present at the time of infection, they remain flat and turn dark purple brown. Pods already formed before infection develop purple pitting. Pea streak may be caused by a specific virus, pea streak virus (PSV), also called Wisconsin pea streak, by cucumber mosaic virus or alfalfa mosaic virus' or by the combination of bean yellow mosaic virus and red clover vein mosaic virus. The latter two viruses are discussed separately in this fact sheet. PSV is very widespread and prevalent in other pea-growing states such as Wisconsin, Idaho, and Washington. The virus is not seedborne in legumes, but can be overwintered in perennial legumes such as red clover. Because so many viruses or combinations of viruses can cause streak, symptomatology alone cannot be relied upon to identify true pea streak virus. The pea aphid is the principal vector of PSV, and transmission is in a nonpersistent manner. Control measures discussed under pea enation mosaic apply. Red clover vein mosaic virus (RCVMV) belongs to the same virus group as pea stunt virus, but can be distinguished from the latter virus by symptomatology and laboratory tests. RCVMV was formerly called Wisconsin pea stunt virus. Plants appear stunted, with leaves showing vein chlorosis and curling, resulting in apical rosetting

RCVMV has a slightly larger host range than PSV and is reported to be seedborne in red clover. The virus naturally infects many legumes besides pea; many are common forage crops such as red, white, Alsike, and Ladino clover. A number of aphid species, including the pea aphid, can transmit the virus in a nonpersistent manner. There are no known resistant commercial cultivars. Controls rely on isolation of pea plantings from virus sources. Bean yellow mosaic virus (BYMV), also called bean virus 2 in older literature, includes pea mosaic virus strains, which some authors treat separately. BYMV is widely distributed in the state and is transmitted by at least 20 aphid species in a nonpersistent manner. This virus is also discussed under the virus diseases affecting bean. The symptoms of BYMV depend upon the strain of virus involved. The pea strain, for example, causes a very bright yellow mosaic , whereas the more typical isolates produce a dull light and dark green mosaic. Symptoms are masked or delayed at low temperatures (below 60° F), but develop normally at 65° -75° F. Resistance for BYMV has been identified and extends to the Perfection types used for both canning and freezing. Resistance is conditioned by a recessive factor, which, when it occurs in a heterozygous condition, is strongly influenced by temperature. Seed transmission is absent or very rare in pea and several other legumes. Aphid control may help to reduce the rate of spread in susceptible cultivars. Clover yellow vein virus (CYVV) causes a serious disease of pea. A severe strain can cause intense veinal chlorosis or severe yellowing, followed by apical necrosis and premature death. Plants are usually stunted, and pods show some deformation. The virus is transmitted in nature by at least four aphid species (including the pea aphid) in a nonpersistent manner. This virus also naturally infects bean and cucurbits, as well as many cultivated and wild legumes, and ornamentals such as wild violet. Symptoms of CYVV infection in pea consist of veinal chlorosis, mosaic, curling, and apical rosetting, with suppression of pod size and number . Cultivars resistant to BYMV are also resistant to this virus. Isolate plantings from potential virus reservoirs or take steps to remove these sources before the crop is planted. Pea seedborne mosaic virus (PSBMV) is not a problem for commercial fields in New York at this time. The virus is readily seedborne in pea, with up to 90% infected seed reported in some commercial seed lots. varieties. This characteristic is not unique to virus infection. The virus has been found in the "plant introduction" collection, its distribution being increased by exchange of infected seed lots.

Diagnosing virus damage in field peas It can be difficult to distinguish plant disease symptoms caused by viruses in pea plants, as viral foliage symptoms are often similar to those caused by nutritional deficiencies, herbicide damage or waterlogging. Pea seed-borne mosaic virus (PSbMV) is non-persistently aphid-borne and reaches high seed transmission rates. Symptoms are: • Usually mild, and plants may be infected without showing symptoms. • Margins of young leaves roll downwards, there is mild chlorosis and mosaics, mild vein clearing may develop. • Terminal leaves are often reduced in size and tendrils excessively curled. • Infection at later growth stages may result in top leaves turning pale. • Infected plants may also produce distorted flowers, which give rise to small distorted pods and fewer seeds. The seed coats may split as the seeds mature, and dark brown rings and tan spots develop. Alfalfa mosaic virus (AMV) is non-persistently aphid-borne, and develops low rates of seed transmission in peas • Chlorosis and necrosis of new shoots. Necrotic spots or streaking of older leaves. • Pods may be malformed and fail to develop seed. Bean yellow mosaic virus (BYMV) is non-persistently aphid borne, and develops a low rate of seed transmission • Variable symptoms, but plants sometimes become infected symptomlessly. • When foliage symptoms develop, these consist of vein clearing and mottles and mosaics especially on young leaves.

veins. Cucumber mosaic virus (CMV is non-persistently aphid borne and develops a low rate ofseed transmission.Foliage symptoms are mild and difficult to observe. Possible leaf chlorosis and slight stunting. Beet western yellows virus (BWYV) persistently aphid-borne; no seed transmission.BWYV causes symptomless infection in peas in WA PSbMV is the most important viral disease in pea in Western Australia because most seed stocks are infected and it both reduces seed yields and damages seed quality. BYMV is the second most important virus as it causes severe symptoms in pea despite occurring at lower levels than PSbMV. BWYV is the most widespread virus but is of less economic importance because its infection is symptomless in pea in WA. Some viruses causing severe symptoms in peas in the eastern states are not found in peas in WA.

• Seed-borne virus infection causes stunted plants that are scattered thoughout the crop and act as within crop infection foci for virus spread by aphids. • When the virus source is external to the crop, plants often become virus-infected first and in greatest numbers close the the paddock edges or around thin or bare areas, particularly on the paddock,s windward side. • Patches of virus infection are common with infection spreading out from initial infection foci in their centres. Plant • The intensity and types of virus symptoms depend on the strain of the virus, climatic conditions (especially temperature), and stage of plant growth at infection. • Depending on the virus, field pea plants can develop various degrees of stunting and mature later than healthy plants • In plants infected early by aphids and in plants infected via seed, all the foliage may show symptoms, which vary from pale and mild mosaic to necrosis and plant death depending on the virus present. leaves. When a virus causes mild symptoms in foliage, typical symptoms are often most clearly visible in young leaves. • Peas are rarely colonized by aphids under WA conditions

The field pea agronomic package available to growers is one offering simplicity and reliability. Producing high yielding field pea crops is now limited more by seasonality and rainfall rather than technology and genetics. With new varieties and agronomy package field pea is a profitable break crop option.

When selecting a paddock in which to grow field pea you need to consider the following points:

Field pea can be successfully grown on a wide range of soil types, but prefer sandy loams or heavier, and pH (measured in CaCl2) above 5.0. Duplex soils with neutral to alkaline subsoils are suitable if they do not become waterlogged, although the sandy surface may pose an unacceptable erosion risk after harvest. Field pea does not tolerate salinity or waterlogging well, so avoid soils where these are likely.

This is largely determined by the rotational history of the paddock, and of surrounding paddocks. Choose paddocks that have not grown field peas for at least three years, and go no closer than 500 metres (m) to paddocks that grew field peas in the previous year. It is preferable to be at least 50m downwind of two and three year old stubbles, but no separation is necessary upwind of these older stubbles.

This has important implications for ease of harvest and soil erosion after harvest. Avoid paddocks with a rough or uneven surface that will be difficult to follow closely with the header front and avoid paddocks with stones or stumps that could be picked up by the header. These can cause considerable damage if they pass through the threshing drum. Some obstacles are acceptable if they are buried by rolling post-sowing. Avoid paddocks with loose surface soil if it will be exposed to strong winds after harvest.

Field pea is sensitive to residues of sulfonylurea herbicides that may have been applied to a preceding cereal crop, so avoid paddocks where this will be a problem. Residues are likely to be greater on alkaline soils, and after a dry growing season and a dry summer. Sulfonylurea herbicides also differ in their breakdown rates. Information on persistence and safe withholding periods is given on the herbicide label and should be consulted.

Field pea is not a strong competitor with weeds, so the best paddocks will have low weed burdens, especially of broadleaf weeds. However, remember that the ability to plant late means that some cultural weed control is usually possible and that there is a greater range of post-emergent herbicides available for use in field pea than in other pulse crops.

Frost during early pod filling can cause devastating yield losses in field pea. Avoid paddocks with a high probability of frost from 1-3 weeks after flowering.

Choosing when to sow field pea requires a compromise between being early enough to avoid end of season drought and late enough to avoid bad blackspot infection. Since other crops, such as wheat, lupins and canola, benefit much more from early sowing, field pea should be sown after these sowing crops is complete. It is unlikely then that field pea will be sown too early and the important question will be how late is it safe to sow field pea?

Field pea, being a legume, has the capacity to derive its nitrogen requirements from the atmosphere but, in order to do this it must be inoculated with Rhizobium bacteria that will form nodules on the crop's roots. Field pea should be inoculated with Group E inoculum before sowing, irrespective of whether the paddock has grown field pea before. This is because the field pea Rhizobium does not survive well in Western Australian soils. Field pea can also be inoculated with Group F inoculum, although remember that the Rhizobium strains in the two groups may not always be equally effective on peas. Achieving good nodulation is crucial to growing a good crop, so take care to inoculate properly. Slurry inoculation is the best way to ensure a well nodulated crop. Fungicide seed dressings will further reduce the life span of the inoculum so, as seed dressings have little value in preventing disease in WA; their use is not recommended. Granular inoculum is now available and can be applied to the soil separately in much the same way as fertiliser. This provides increased flexibility at sowing as it removes the need to inoculate within three days of sowing and will make achieving good nodulation compatible with fungicide seed dressings, should these ever become worthwhile in WA.

Sow sufficient seed to establish 45 plants/m for conventional-leaved varieties and 55 plants/m for semi-leafless varieties. The necessary sowing rate will depend on germination percentage and seed size. Field peas grow well in conventionally spaced rows (18-25 centimetres (cm)). Limited research in the 1990s showed no adverse effects on yield of planting in rows as wide as 36cm, but poorer standing ability (with a conventional-leaved prostrate cultivar) was observed, and consequently more difficulty harvesting. For this reason, wider rows are not recommended for field peas. Standing ability in semi-leafless cultivars is likely to be impaired in wide rows too, since plants will experience less support from their neighbours than in narrow rows.

Trouble-free harvesting requires an even soil surface and this is best achieved by rolling. This is most important for prostrate cultivars, but applies also to semi-erect cultivars because these will lodge under some circumstances. Use either steel or rubber-tyred rollers. Heavy steel rollers do a better job leveling heavier soil types, especially the larger ridges left by no-till seeding, and pushing small stones and sticks into the ground. However, they have a greater tendency than rubber-tyred rollers to smear and seal the soil surface if it is moist. It may be necessary to wait for the soil surface to dry after seeding before rolling with a steel roller where a rubber-tyred roller could follow directly behind the seeder. Less pressure is required to knock down ridges on sandy soils and on them rubber-tyred rollers are just as effective as steel rollers. Roll is best done straight after sowing, as long as the soil surface is not too moist. However, field peas can

three and 10 node stages. Rolling earlier risks damaging emerging seedlings and older plants are usually too tall to recover completely from being knocked down in the rolling operation. Don't roll for two weeks before or after applying post-emergent herbicides, as the stress of rolling will predispose the crop to damage from these herbicides. Rolling emerged crops is best done in the afternoon, as then the plants are more flexible and less likely to be damaged.

The virus has previously been known as pea leafroll mosaic virus or pea fizzletop virus. In pea, the symptoms consist of leaf narrowing and downward leaf rolling, accompanied by a mild mosaic. Plants are somewhat reduced in size. The virus is predominantly spread by seed, although it is also spread in nature by aphids in a nonpersistent manner. Most Perfection-type pea varieties are very susceptible, and no resistant varieties are presently available. Resistance is conditioned by a recessive factor.

Alconero, R., Provvidenti, R. and Gonsalves, D. (1986). Three pea seedborne mosaic virus pathotypes from pea and lentil germ plasm. Plant Disease, 70: pp. 783–786. doi:10.1094/PD-70-783. Ashby J.W., Teh, P.B. and Close R.C. (1979). Symptomatology of Subterranean clover red leaf virusand its incidence in some legume crops, weed hosts, and certain alate aphids in Canterbury, New Zealand. New Zealand Journal of Agricultural Research, 22: pp. 361–365. doi:10.1080/00288233.1979.10430760 Baggett J.R. and Hampton R.O. (1991). Inheritance of viral bean leaf roll tolerance in peas. Journal of the American Society for Horticultural Science, 116: pp. 728–731. Bruun Rasmussen, M., Moller, I.S., Tulinius, G., Hansen, J.K.R., Lund, O.S. and Johansen, I.E. (2007). The same allele of translation initiation factor 4E mediates resistance against two Potyvirus spp. in Pisum sativum. Molecular Plant–Microbe Interactions, 20: pp. 1075–1182. doi:10.1094/MPMI-20-9-1075 (2009). Quantifying effects of seedborne inoculum on virus spread, yield losses, and seed infection in the Pea seed-borne mosaic virus–field pea pathosystems. Phytopathology, 99: pp. 1156–1167. doi:10.1094/PHYTO-99-10-1156 Coutts B.A., Prince, R.T. and Jones, R.A.C. (2008). Further studies on Pea seed-borne mosaic virus in cool-season crop legumes: responses to infection and seed quality defects. Australian Journal of Agricultural Research, 59: pp. 1130–1145. doi:10.1071/AR08113 Crampton M.J. and Watts L.E. (1968). Genetic studies of pea leaf-roll (top-yellows) virus resistance in Pisum sativum. New Zealand Journal of Agricultural Drijfhout, E. 1968. Testing for pea leafroll virus and inheritance of resistance in peas. Euphytica, 17: pp. 224–235. Fletcher, J.D. (1993). Surveys of virus diseases in pea, lentil, dwarf and broad bean crops in South Island, New Zealand. New Zealand Journal of Crop and Horticultural Science, 21: pp. 45–52. doi:10.1080/01140671.1993.9513745 Freeman A., Spackman M., Aftab M., McQueen V., King S., van Leur J.A.G., Loh M.H., Rodoni B. (2013). Comparison of tissue blot immunoassay and high throughput PCR for virus-testing samples from a south eastern Australian pulse virus survey . Australasian Plant Pathology 42 (in press) Research, 11: pp. 771–783. doi:10.1080/00288233.1968.10422409 Kumari S.G., Makkouk, K.M., Katul, L. and Vetten H.J. (2001). Polyclonal antibodies to the bacterially expressed coat protein of Faba bean necrotic yellows virus. Journal of Phytopathology, 149: pp. 543–550. doi:10.1046/j.1439-0434.2001.00674.x Kyriakou A. (1984). Studies on the luteoviruses Beet western yellows virus and Subterranean clover red leaf virus. PhD thesis, University of Canterbury.

Latham L.J. and Jones R.A.C. (2001b). Alfalfa mosaic and pea seed-borne mosaic viruses in cool season crop, annual pasture, and forage legumes: susceptibility, sensitivity, and seed transmission. Australian Journal of Agricultural Research, 52: pp. 771–790. doi:10.1071/AR00165

Research Scholar E-Mail – lmsgroupglobal@gmail.com