Viral diseases and their management in potato production

Viral diseases and their management in potato production Ravinder Kumar Division of Plant Protection, CPRI, Shimla-171001 (H.P.) The potato (Solanum tuberosum L.) is an important crop worldwide. Potato production is not an easy task, as potatoes are affected by multiple key pests, including several viruses which contribute to "running out", or degeneration, of seed stocks. Viral diseases are prevalent throughout the India but are most severe in North-eastern plains and plateaux where population of aphid vectors is high throughout the crop season. Tomato leaf curl New Delhi virus (ToLCNDV), potato leaf roll virus (PLRV) and potato virus Y (PVY) are the most important viruses in India. Viruses once introduced may persist and spread in agricultural environments and under favourable conditions they may multiply rapidly and cause significant yield loss and economic damage. Therefore, sustainable potato production is possible only if the diseases are kept under check, especially in sub tropics where the weather is highly conducive for common viral diseases. Major potato viruses have been discussed below. 1. Tomato leaf curl New Delhi virus (ToLCNDV): Potato apical leaf curl disease was first reported in northern India by Garg et al. (2001). The association of a geminivirus with this disease has been found by immune electron microscopy using polyclonal antibodies of the Indian cassava mosaic virus and based on symptoms, the virus was named tentatively as Potato apical leaf curl virus (Garg et al., 2001). Later, Usharani et al. (2003) confirmed that this virus is a strain of Tomato leaf curl New Delhi virus (ToLCNDV) belonging to the genus Begomovirus within the family Geminiviridae. Apical leaf curl disease has emerged as a new disease in potato during the last decade in India due to a change in planting date and an increased whitefly population. Its incidence is on the rise threatening the cultivation of potato across the country. The affected plants show curling/crinkling of apical leaves with a conspicuous mosaic symptom. When the infected tubers are used for planting, the entire plant show symptoms with severe stunting due to high virus titre (Fig. 1). The plants recover from symptoms as the maximum temperature falls below Fig 1. Infected potato plants showing symptoms of apical leaf curl disease. 25 C. Recently, high prevalence of this disease was recorded in some of the popular high yielding commercial potato varieties from different potato growing regions of the country. Transmission: The virus is transmitted by whiteflies and the infection is more common in crops planted during October than in November because of the large whitefly population (Chandel et al., 2010). The incidence correlates positively with the whitefly population and the whitefly infestation period of potato crops. Around 40 75% of infections were recorded in the cultivars grown in Indo-Gangetic plains of India (Venkatasalam et al., 2005). Lakra (2002) reported even upto 100% of infection from

the Hisar (Haryana) area of India causing heavy yield losses in susceptible varieties. The primary infection in the field appears within 40 45 days after planting. 2. Potato leaf roll virus (PLRV): It belongs to genus Polerovirus, family Luteoviridae and causes severe yield loss (up to 90%) as well as in some cultivars a quality reduction due to internal damage to tubers (net necrosis). Single infected plants can have their yield reduced by 50% or more, although neighbouring uninfected plants will compensate. However, where widespread infection is present, severe yield reduction will occur. PLRV is probably the most important potato virus worldwide. One estimate has suggested the virus is responsible for 20 million tonnes yield loss globally. Primary infection, arising from infection in the growing season, appears in the youngest leaves and mostly results in a pale discoloration and in-rolling of leaflets starting at the leaflet base (Fig. 2). Some purple discoloration of affected leaflets may occur. Primary symptoms tend to occur only where infection of the plant occurs early in crop development or in hot climates. Secondary infection, where symptoms develop from infected tubers, is always more severe. Inward rolling of lower leaflets, extending ultimately to the upper leaves, is typical. The leaves become dry and brittle, and if touched the plant makes a characteristic rustling noise. Leaves are chlorotic and often show purple discoloration. Net necrosis may not be apparent at harvest but can develop in store. It can develop in tubers from plants showing both primary and secondary haulm infection (Fig. 3). Fig. 2. In-rolling of lower leaflets as a result of PLRV infection. Fig.3. Net necrosis of vascular tissue of tubers infected by PLRV. Transmission: PLRV can be spread long distances by winged aphids. The virus persists in the aphid throughout its life cycle in a persistent (circulative) way. All instars (stages) of the aphid can transmit the virus, but the nymph stage is more efficient than the adult. The extent of transmission depends on the degree of acquisition of the virus from an infected host and this, in turn, depends on the duration of feeding. Similarly, the transmission to an uninfected host depends on the amount of virus in the aphid and the duration of feeding. Virus accumulates in tubers and, if planted, the virus is transmitted to daughter tubers as well as into the foliage. 3. Potato virus A (PVA): It belongs to genus Potyvirus, family Potyviridae and cause up to 40% yield reduction. The virus is found wherever potatoes are grown. It consists of flexuous filamentous particles 730 x11 nm. PVA causes mild mosaic symptoms not

dissimilar to those caused by PVX (Fig 4). Differences can be difficult to detect visually, but PVA mottles may appear on the veins, and infected leaves look shiny. Infected plants may have a more open habit. Although visually similar to PVX, this virus is related to PVY and is spread by aphids. Transmission: At least seven aphid species are capable of ransmitting PVA (including Aphis frangulae, Macrosiphum euphorbiae, and Myzus persicae). The virus is non-persistent and is lost from aphids as they go through their life cycle. As with PVY, the virus can be acquired rapidly from an infected plant (<1 minute) and transmitted equally rapidly. 4. Potato Virus M (PVM): It belongs to genus Carlavirus and yield reduction in potatoes is usually low, at worst 15-45%. It consists of slightly curved filamentous particles 650 x 12 nm. Causes mottle, mosaic, crinkling and rolling of leaves (paracrinkle), and stunting of shoots. Symptoms mainly occur in plants infected at very young stage. Transmission: For most isolates, natural spread is by aphids in a non-persistent manner. Some isolates, however, may be transmitted mechanically (e.g. machinery) including plant to plant contact. 5. Potato virus S (PVS): It belongs to genus Carlavirus and the most frequently found virus in potato. Yield reduction is usually low, at worst 10 20%, but might be slightly worse in combination with PVX. It consists of slightly flexuous filamentous particles 660 x 12 nm. PVS is symptomless on the majority of cultivars, with occasional mild leaf symptoms of rugosity, vein deepening and leaf bronzing. Transmission: Commonly spread mechanically (e.g. machinery) including plant to plant contact. Some isolates are spread in a non-persistent manner by aphids, particularly Myzus persicaea and Aphis nasturtii. 6. Potato virus X (PVX): It belongs to genus Potexvirus and found wherever potatoes are grown. In nature it is largely confined to members of the family Solanaceae. When occasional plants are infected, yield loss is negligible. With more widespread infection, yield losses of 15 20% can occur. However, through effective control, in many countries the virus has become of limited importance. PVX is one of the potato viruses that cause mosaic symptoms. Visually, symptoms range from absent through a faint or fleeting mottle to a severe necrotic streak (Fig. 5). Only occasionally does leaf distortion, rugosity, necrotic spotting or stunting occur. More severe symptoms can occur when PVX is present with other viruses. In combination with PVA or PVY it causes leaf distortion and crinkle. Transmission: Transmission is by contact, either plant to Fig 4. Mild mosaic symptoms in a plant affected by PVA. Fig 5. Mild mosaic symptoms in a plant affected by PV X plant or by humans, animals or machinery passing through a crop. PVX is highly contagious, and once attached to a surface such as clothing the virus can remain

infective for many hours provided the surface remains wet. Consequently, a virus picked up from an infected plant can be transmitted to many other plants when moving through a crop. The virus accumulates in tubers, and the process of cutting seed tubers can spread the virus from one tuber to another. PVX is not transmitted by true seed or by aphids. 7. Potato virus Y (PVY): It belongs to genus Potyvirus, family potyviridae. It consists of flexuous, long filamentous particles 740 x 11 nm. PVY O is one of the most damaging potato viruses in terms of yield loss. In combination with PVX, it causes an even more destructive disease known as rugose mosaic. PVY contains strain groups which cause different symptoms in potato and other Solanaceous crops. Primary symptoms of common strain (PVY O ) are necrosis, mottling, yellowing of leaflets, leaf drops and premature death of plants (Fig. 6). Necrosis of foliage is a hypersensitive-type of reaction and generally starts at a veinal point on foliage and may result in spots or rings. Affected leaves may drop (leafdrop streaks) or remain clinging to the stem. These symptoms may be restricted to a few leaves or to a single shoot. Secondary symptoms result in a dwarf plant with mottled or crinkled foliage (Fig. 7). Fig 6. Comparison of healthy leaf (right) and leaf with PVY infection (left). Fig 7. Severe mosaic PVY O symptoms. Transmission: The main sources of PVY inoculum are infected seed tubers. Aphids feeding on plants emerging from infected tubers acquire PVY within a few seconds and also inoculate the virus to healthy plants within seconds. Thus, aphids probing on potato plant are potential vectors of PVY. The peach potato aphid, Myzus persicae is the most efficient vector in many areas. 8. Potato spindle tuber viroid (PSTVd): Viroids are the smallest-known infectious agents causing diseases in higher plants. They consists of small (241 399 nucleotides), single-stranded circular RNAs. Aerial symptoms develop in warmer conditions but are masked in cooler ones. Primary haulm symptoms are seldom evident in potato plants. Stem and blossom pedicels are slender, longer than normal, and remain erect. Leaflets are slightly reduced with fluted margins, tend to curve inward and overlap the terminal leaflet. As the season advances, diseased plants are restricted in growth (Fig. 8) and become harder to identify because of intertwining with neighbouring healthy plants. Tubers may be reduced in size and may be misshapen with spindling and conspicuous eyes (Fig. 9).

Fig 8. Stunted plant infected with PSTVd. Fig 9. Surface cracking as a result of PSTVd. Transmission: The viroid is highly contagious and readily transmitted to plants by contaminated cultivating and seed cutting tools. The viroid is transmitted through pollen and true potato seed; therefore, breeding and release of new cultivars can be one of the sources of its introduction to fields. Persistent and Non-persistent Viruses: Potato viruses are transmitted by aphids in two basic ways. The virus is either non-persistent (stylet-borne) or persistent and circulative (they are ingested and persist in the aphid throughout its life). The most common persistent and circulative virus affecting potatoes in India is potato leaf roll (PLRV). Common non-persistent or stylet-borne viruses are PVY, PVA, PVS and PVM. Because the aphid has to feed for more than several minutes to transmit persistentcirculative viruses, control by insecticides is highly effective, whereas with a stylet-borne virus the virus is transmitted as soon as the aphid stylet penetrates the epidermal cells of the leaf- too soon for any insecticide to kill the aphid and prevent virus transmission. Integrated management of viruses: Viral diseases of plants cause enormous economic losses particularly in the tropics and semi tropics which provide ideal conditions for the perpetuation of viruses and their vectors. Intensive agricultural practices necessitated by the ever-increasing demands of the rapidly growing population and the introduction of new genotypes, cropping patterns and crops have further aggravated the problem of viral diseases. Many diverse approaches have been tried to minimize the losses caused by these diseases. The approaches are mainly based on avoidance of sources of infection; avoidance or control of vectors; modification of cultural practices; use of resistant varieties obtained though conventional breeding procedures; cross protection; systemic acquired resistance; and use of transgenic plants containing alien genes that impart resistance to viruses. Although the use of resistant varieties has been found to be the most economical and practical, for effective management of viral diseases an integrated approach is essential in sustainable agriculture. Development of integrated management practices also requires correct identification of the causative viruses, because symptoms can be misleading, and adequate understanding of the ecology of viruses and their vectors. Control of viruses involves a number of general cultural techniques to reduce inoculum. Most seed lots originate in tissue culture to remove all pathogens. The resulting disease-free

plantlets are used to produce minitubers in the greenhouse, which in turn go to the field to produce several generations of seed potato. In the field, general practices of sanitation reduce viruses- destroying cull piles to prevent sprouting, and roguing and removing infected plants from the field. Our certified seed program that starts with virus free planting stock, 100% testing of nuclear stocks, field inspection with virus testing winter testing, requiring that all seed planted for certification have less than 0.5% virus infection and a flush out requirement after G4 has minimized our virus problem and is the basis for a virus management program. Insecticides will reduce numbers of both potato colonizing and non colonizing aphids. Seed growers should select insecticides that alter aphid feeding behavior and that do not agitate aphids causing them to move. No gap insecticide protection from emergence till vines are dead is important. It should be remembered at the time of vine-kill potatoes are often the only green things around and are thus highly attractive to flying aphids. Since most growers use one of the neonicotinoid products (imidacloprid-based products-admire Pro, Gaucho, Nuprid, Mana Alias, etc. To avoid aphid or other insect insecticide resistance, growers should never use the same class of insecticide in successive applications. Suggested Readings Chandel, R.S., Banyal, D.K., Singh, B.P., Kamlesh Malik Lakra, B.S., 2010. Integrated management of whitefly bemisia tabaci (Gennadius) and Potato Apical Leaf CurlVirus in India. Potato Res. 53, 129 139. Garg, I.D., Paul-Khurana, S.M., Kumar, S., Lakra, B.S., 2001. Association of a geminivirus with potato apical leaf curl in India and its immuno-electron microscopic detection. J. Indian Potato Assoc. 28, 227 232. Lakra, B.S., 2002. Leaf curl: a threat to potato crop in Haryana. J. Mycol. Plant Pathol.32, 367. Usharani, K.S., Surendranath, B., Paul-Khurana, S.M., Garg, I.D., Malathi, V.G., 2003.Potato leaf curl a new disease of potato in northern India caused by a strain oftomato leaf curl New Delhi virus. New Dis. Rep. 8, 2. Venkatasalam, E.P., Singh, S., Gawande, S.J., Malathi, V.G., 2005. Detection of white fly transmitted geminivirus associated with potato apical leaf curl by serological and molecular tools. In: Proceedings of the Annual Meeting of Indian Society of Plant Pathologists and Centenary Symposium on Plant Pathology, 7/8 April, Central Potato Research Institute, Shimla, p. 18.