Potato leafroll is caused by the potato leafroll virus (PLRV). This is the most serious potato virus in New England, and is responsible for huge losses worldwide. It can infect potato, tomato and other related plants. It can cause a loss of yield and quality. It is transmitted by aphids in a persistent manner, which means that it takes the aphid a while to acquire the virus from an infected plant. It is able to infect healthy plants with the virus for the rest of its life.
Symptoms. When a healthy plant is infected by aphids, the young leaves stand upright, roll, and are slightly pale. Young leaves may turn pink to purple to reddish, starting at the margins. They may only roll at leaflet bases. These symptoms may spread to lower leaves. Plants infected late in the season may have no visible symptoms.
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Potato leafroll virus. Note the upright growing habit and rolled leaves.
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When an infected tuber produces a plant, the plant is very stunted and rigid. Leaves are stiff, dry, and leathery. When crushed in the hand, the leaves crackle. Lower leaves roll and may be pink or dead at the margins. Leaves higher in the plant are slightly pale.
When a tuber grown on an infected plant is cut open, a light to dark brown net-like pattern may be visible in some cultivars. If the tuber sprouts, the sprout may be long and spindly.
Potato virus Y disease is caused by the virus, potato virus Y (PVY). This virus can cause disease on members of the potato family, pepper, tobacco, datura, dahlia, henbane and goosefoot. The virus may kill plants in severe cases, but may only cause a loss of yield. It is transmitted by aphids in a non-persistent manner. The aphid acquires the virus from infected plants almost immediately, but it is only able to infect healthy plants for a short time, usually a few days to a week.
Symptoms. Symptoms vary widely depending on the cultivar. Leaves may be mottled, yellow or have dead patches. The dead areas usually start as patches or rings on the leaflets and then grow to affect the entire leaflet. The leaflet collapses and may drop from the plant or remain attached. In addition to the mottling, severe rugose wrinkling may occur. Brown streaks may occur on the veins, the petioles and down the stem.
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Plants grown from infected tubers are usually stunted and have mottled, crinkled leaves. Generally, the yellow-green mottling is of a very fine pattern. The leaves and stems occasionally die.
Tubers may show light brown rings on their skin. In cases where the infection occurred late in the season, the tuber symptoms may be the only symptoms.
Potato virus A is a disease caused by potato virus A (PVA). This virus infects only potatoes. It is one of the most widespread potato viruses and is found in most potato growing areas. The virus may lower yield slightly. It is transmitted by aphids in a non-persistent manner.
Symptoms. A mild pattern of yellowish or light green patches alternating with patches of very dark green is present on most potato cultivars. The patches vary in size and can cross veins. The leaf surface is usually rougher than normal. Edges of infected leaflets may be slightly crinkled or wavy. Infected leaves usually look shiny. The stems of the plant bend outward, giving the plants an open look. Tubers are usually unaffected, except for a slight decrease in size.
Potato virus S is a disease caused by potato virus S (PVS). In temperate regions, the only host is potato, although there are other hosts in more tropical climates. This disease was not discovered until the early 1950's because the symptoms are very inconspicuous. The disease may cause a slight decrease in yield. It is transmitted by aphids in a non-persistent manner. Some strains of this virus are transmitted only by planting infected tubers.
Symptoms. The plants must be infected early in the season, because most potatoes become naturally immune as they mature. Most potato cultivars show no symptoms. On others, a slight deepening of the veins occurs. The leaves may be slightly roughened. The growth of the plant may be more open than normal. Some plants may have mild mottling or bronzing of the leaves, and may have tiny dead spots on the upper surfaces of the leaves. When the leaves are older and shaded, greenish spots may remain as the rest of the leaf turns yellow. Tubers may be slightly smaller in size.
Potato virus X is a disease caused by potato virus X (PVX). This virus can affect potato and tomato. It is the most widespread of potato viruses and can cause reduced yields, even if the plants appear healthy. This virus is transmitted by planting infected tubers.
Symptoms. Often there are no symptoms present on plants infected with this virus. Symptoms range from a mild yellowish-greenish mottle to a severe mottling of the plant with roughening of the leaves. Mottling is more evident after a few days of cloudy weather, and may be almost non-existent after a few days of bright sunny weather. Plants may be stunted and have small leaves. In some cases, the tips of the plants may die. This top death is usually associated with corky symptoms in the tuber which lead to tuber death. This disease is usually transmitted by planting diseased tubers.
Prevention of all potato viruses. Since some of these viruses are often symptomless or nearly so, it is important to use certified or foundation disease-free seed tubers. Resistance is the best line of defense against the potato viruses and is available for PVY, PVA, PVS and PVX. Cultivars with some level of resistance are available for PLRV and there are some cultivars that do not produce the net-like browning in the tubers even though the plants are infected. Remove visibly diseased plants from the field as soon as possible. Controlling volunteer plants from the previous season has been shown to help in PLRV prevention.
Aphid transmitted viruses (PVY, PLRV, PVA, PVS). Plant early to avoid heavy aphid times. See current recommendations for chemical aphid control. Chemical control is not always completely effective when viruses are transmitted in a non-persistent manner, as the aphids can infect many plants before the insecticide is able to kill them. An oil spray can be used to prevent aphids from transmitting the virus while they feed.
Tuber transmitted viruses (PVS, PVX). There are no chemical control measures for these viruses. Avoid unnecessary handling of plants. Avoid contact between disease-free tubers and those that are potentially carrying the disease. The disease can also be spread by handling the plants and by tools such as planters and knives. Make sure that hand tools are cleaned frequently while working, and that equipment is cleaned thoroughly between different areas. For PVS infection, plants must be infected early in the season for the disease to occur, since most cultivars are naturally resistant as mature plants.
By Pamela S. Mercure, IPM Program Assistant, University of Connecticut
References
Bagnall, R.H. 1981. Potato
Virus S. pp. 75-77 in Compendium of Potato Diseases. W. J. Hooker,
ed. APS Press, St. Paul, MN.
de Bokx, J.A. 1981. Potato Virus A. pp. 71-72 in Compendium of
Potato Diseases. W. J. Hooker, ed. APS Press, St. Paul, MN.
de Bokx, J.A. 1981. Potato Virus Y. pp. 70-71 in Compendium of
Potato Diseases. W. J. Hooker, ed. APS Press, St. Paul, MN.
Munro, J. 1981. Potato Virus X. pp. 72-74 in Compendium of Potato
Diseases. W. J. Hooker, ed. APS Press, St.Paul, MN.
Peters, D. and R.A.C. Jones. 1981. Potato Leafroll Virus. pp.
68-70 in Compendium of Potato Diseases. W. J. Hooker, ed. APS
Press, St. Paul, MN.
McKinlay, R.G., Spaull, A.M., Straub, R.W. 1992. Pests of Solanaceous
crops. Chapter 8 in Vegetable Crop Pests, R.G. McKinlay, ed. CRC
Press, Inc, Boca Raton, FL.
Rich, A.E. 1983. Potato Diseases. Academic Press. New York.
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