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By: Amy Grant
Potatoes may be infected with many different viruses that can reduce tuber quality and yield. Mosaic virus of potatoes is one such disease that actually has multiple strains. Potato mosaic virus is divided into three categories. Symptoms of the different mosaic virus of potatoes may be similar, so the actual type usually can’t be identified by symptoms alone and is often just referred to as mosaic virus in potatoes. Still, it is important to be able to recognize the signs of potato mosaic and learn how to treat potatoes with mosaic virus.
Types of Potato Mosaic Virus
As mentioned, there are different mosaic viruses that afflict potatoes, each with similar symptoms. Positive identification requires the use of indicator plant or laboratory examination. With that in mind, diagnosis can be made by mosaic patterns on foliage, stunting, leaf deformities and tuber malformations.
The three types of recognized mosaic virus in potatoes are Latent (Potato virus X), Mild (Potato virus A), Rugose or Common mosaic (Potato virus Y).
Signs of Potato Mosaic
Latent mosaic, or Potato virus X, may produce no visible symptoms depending upon the strain but yields of infected tubers may be reduced. Other strains of Latent mosaic show light leaf crinkling. When combined with Potato virus A or Y, crinkling or browning of leaves may also be present.
In an infection of Potato virus A (mild mosaic), the plants have light crinkling, as well as mild yellow mottling. Leaf margins may be wavy and appear rough with sunken veins. Severity of symptoms depends on the strain, cultivar and weather conditions.
Potato virus Y (Rugose mosaic) is the most severe of the viruses. Signs include mottling or yellowing of leaflets and crinkling that is sometimes accompanied by leaf drop. Underside leaf veins often have necrotic areas showing as black streaking. Plants may be stunted. High temperatures exacerbate the severity of the symptoms. Again, symptoms vary hugely with both potato cultivar and virus strain.
Managing Potatoes with Mosaic Virus
Potato virus X can be found in all varieties of potato unless certified virus free tubers are used. This virus is spread mechanically by machinery, irrigation equipment, root to root or sprout to sprout contact, and via other gardening tools. Both viruses A and Y are carried in tubers but are also transmitted by several species of aphids. All of these viruses overwinter in potato tubers.
There is no method for eradication of the disease once the plant is infected. It should be removed and destroyed.
To prevent infection, use only seed certified free from viruses or that have low incidence of infected tubers. Always keep garden tools as clean as possible, practice crop rotation, keep the area around the plants weed free, and control aphids.
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Potato Blight Cause, Identification. Prevention, Treatment Potato Blight
Potato blight is the worst problem that the potato grower faces. Once it arrives it can devastate a crop in a day or two and when the infection moves down from the foliage to the potato tubers, cause them to rot as well.
Potatoes infected with late blight are shrunken on the outside, corky and rotted inside They also stink and once smelt, never forgotten.
Most famously the potato blight was, if not the only cause, certainly the major contributor to the Irish Famine of the 1840’s.
The blight had started in Europe and eventually reached Ireland where the potato was a staple food. Worse still, the Irish all grew the same very susceptible variety (Irish Lumper). Not only did they lose crops in the ground but crops in store were lost as well.
There’s a lesson for growers today in the Irish Famine. Lack of genetic diversity and mono-culture increases the risk of devastation. Sadly farmers ignore it at our peril.
Viruses and viroids are the smallest organisms. They are incomplete organisms in the sense that they contain genetic material, but lack the ability to reproduce. Instead, to reproduce, they must invade the cell of another organism and use its cellular machinery to make new viruses. A typical virus is a small strand of RNA or DNA encapsulated within a protein shell. The shell protects the genetic material and takes part in delivering the RNA or DNA into the cell of a host organism. There are viruses that infect all forms of cellular life, including plants. A viroid is an even simpler organism, consisting of nothing but a piece of RNA. Viroids are only known to infect plants.
There are many important viral pathogens of plants. When a virus infects a plant, the plant diverts some of its energy to manufacturing viruses instead of the products that the plant cell normally produces. This means that the plant has less energy to devote to its normal activities, so growth and yield tend to be reduced. There may also be more complex interactions with the plant. For example, the virus may cause the plant to produce RNA products or proteins that directly interefere with the normal functions of the plant. Not all viruses are pathogenic, in the sense that they cause symptoms. Some are well tolerated by plants and may actually add value. There are some ornamental plants, for example, that produce prized flower colors or patterns only when infected by a virus. Generally though, viruses are harmful to crop plants, because we are most interested in obtaining a high yield of edible parts and yield will generally be reduced when the plant has to compete for resources with the virus.
Viruses produce a wide range of symptoms. Many viruses are latent, essentially undetectable, although they often reduce yield in a slow and insidious fashion over a period of years. Other viruses have clear and obvious symptoms like mosaic patterns or mottling of the leaves, yellowing, necrotic spots or veins, all the way up to complete defoliation and death of the plant. Some viruses get worse over time, while others can coexist with the plant indefinitely. Plants can be infected with more than one virus at a time and cases of multiple infection usually have worse symptoms.
Viruses can transmit horizontally, plant to plant through direct contact or through pests that feed on the plant, or they can transmit vertically, through the pollen and seeds of a plant. Most plant viruses transmit horizontally. Viruses are a particular problem in potatoes because the plants are reproduced clonally, by replanting tubers. Once a plant is infected with a virus, the tubers are normally all infected. That means the plants of the next generation will all be infected and will all produce infected tubers. Before the advent of tissue culture techniques to remove viruses from potatoes, all varieties eventually “ran out,” due to virus burden. They simply became uneconomical to grow anymore and were abandoned. Because most viruses are transmitted horizontally, true seed of the potato is usually virus free – the pollen and seeds are not infected. This is not always the case and this is covered in more detail below.
Most potato varieties that are not grown commercially are infected with viruses. It is somewhat expensive to remove the viruses from a potato variety and then to keep a clean source for future crops. Because of this, most heirloom varieties have been infected with viruses for decades and they often have poor performance and lose the ability to flower. Commercial potatoes are grown in certification programs, where the plants are sampled for major viruses and only allowed to be sold for seed when the percentage of virus infected plants is low.
Potato Virus Y (PVY) – Symptoms and Diagnosis
PVY symptoms vary with strain of the virus, cultivar and environmental conditions. PVY symptoms include yellow, light green and dark green “mosaic” patterns on leaves, leaf drop, brown or black (necrotic) line patterns often on veins or shoots, necrotic lesions on leaves and stems, rugosity (wrinkling), yellow flecking, stunted growth, death of growing points, tuber cracking and tuber necrosis. Tuber necrosis caused by PVY is called potato tuber necrotic ringspot disease (PTNRD). Affected tubers show raised rings of darker brown or reddened skin that can progress to sunken necrotic areas. Necrosis beneath the rings may extend into the tuber flesh.
Some emerging strains of PVY can cause tuber necrosis in some cultivars and most emerging strains cause mild foliar symptoms in most cultivars. The photos below illustrate some typical types of symptoms seen on foliage and tubers. More photos can be found on the PVY symptom pagewith links to photos of symptoms on many different North American cultivars. This comprehensive gallery of symptoms by cultivar can also be found here PVY Photo Gallery.
Diagnosis of PVY with lab tests
A visual diagnosis based on symptoms can be confirmed with lab testing. Information on PVY diagnostic tests of foliar or tuber tissue can be found here Diagnostic Lab Testing.
To minimise PVY spread in potato crops an integrated disease management approach is necessary, however the most important management practice is to use seed lots with zero levels of PVY. The addition of other practices in combination will provide added control.
- Plant seed potatoes with zero level of PVY. WA certified and registered seed potatoes with Rating 1 and Rating 2 have a zero tolerance for PVY.
- Use PVY resistant potato varieties when available. Commercial varieties used in WA that are resistant to PVY are Royal Blue, FL1867 and FL2195.
- Plant a non-host border crop around the potato crop about four weeks before planting, for example, wheat, oats, sorghum. This acts as a cleansing barrier for aphids. If PVY is entering the crop from an outside source then infective aphids may feed on the barrier crop, lose the virus and will no longer be infective when they land on the potato crop.
- Plant new crops upwind from older crops – there is less infection upwind from infection sources as aphids can be blown along with the wind.
- Remove any potato plants showing virus symptoms – removing virus sources within the crop may help to slow down the spread of the virus to nearby plants especially early in the season.
- Employ good hygiene practices. Use 1:4 dilution of household bleach or 1% Virkon to wash equipment and machinery.
- Avoid moving machinery, equipment and workers from old crops to new ones to minimise spread from older to young crops.
- Remove and destroy old potato crops immediately after the final harvest to minimise virus spread to new crops.
- Destroy volunteer potato plants and weeds before planting to reduce any potential virus and aphid sources for new crops.
Insecticides are ineffective
It is important to note that although PVY is spread mainly by aphids, insecticides are ineffective as a control measure because they do not work fast enough to prevent aphids from feeding on and infecting a healthy plant before it is killed.
Mineral oils have been used overseas to limit PVY spread. These oils interfere with aphid feeding and virus transmission. It is important to note phytotoxic damage has been reported from their use.
This work was supported with funding from the Potato Producers' Committee of the Agricultural Produce Commission with in-kind support from the Department of Primary Industries and Regional Development.