Plant pathologist John Scrace summarises the threat from potato viruses to crop yield and quality, and provides guidance on reducing the risk.
As potato growers can testify, it’s not just humans and animals that are at risk from virus diseases. Now is the time to keep a close watch on growing potato crops, particulary those being grown for seed. Pocket Diagnostic tests for potato viruses are a useful tool in the rogueing of crops.
PVYO, PVYN, PVYNTN, PVX, PVA, PVV, PVS, PLRV, etc. etc. – the list of potato viruses (not to mention the symptoms they cause) seems endless, and it is certainly beyond the scope of a brief article to cover them all. So here are a few key facts about potato viruses and the threat they pose:
Potential yield loss will depend on both the virus and the potato variety it affects. Potato virus Y (PVY) and Potato leaf roll virus (PLRV) (often known as the ‘severe’ viruses) are usually regarded as the biggest threats. Infection of a plant of a susceptible variety could cause up to 80% yield loss. In a field situation, the overall yield loss is usually much less than this as a) not all plants will be affected and b) there can be a compensatory yield increase from healthy plants taking advantage of the space left by their stunted, virus-affected neighbours.
There’s no doubt that potato viruses can make a big dent in profits, however. Even ‘mild’ viruses such as PVA and PVX can sometimes cause significant yield loss in a susceptible variety.
The method of virus transmission also depends on the virus. Many of the viruses, e.g. PVY, PLRV and PVA are aphid-transmitted. PVX is mechanically transmitted, and easily spread by plant-to-plant contact or by machinery or people moving through the crop.
Use of insecticides can help prevent the spread of PLRV, as it takes several hours of feeding for an aphid to acquire or transmit the virus. Insecticides are less effective against viruses such as PVY and PVA which are picked up and transmitted very quickly.
Symptoms will vary according to the virus/virus strain, the potato variety and the stage of growth at which infection occurred. In some cases (e.g. with PVYO) there may be obvious stunting, with clearly visible leaf distortion and mosaic. Other viruses such as PVYN, PVX or PVA may produce much milder symptoms (or even none at all), yet could still be affecting yield.
Potato leaf roll virus lives up to its name, but be careful not to confuse it with leaf-rolling due to drought stress or diseases such as blackleg and stem canker. The lower leaves of plants grown from PLRV-affected seed are not only rolled but also still fully turgid and ‘crisp’, whereas rolled leaves of plants affected by drought, pests or diseases are often soft or wilted.
With many of the viruses, ‘first year’ symptoms of infection (when growing plants in the field first become infected) are different from ‘second year’ symptoms (shown by a plant grown from virus-affected seed).
Mixed infections with different viruses will not only modify symptoms but can also increase yield loss, e.g. ‘crinkle’, caused by mixed infection by PVX and PVA.
Follow these rules:
1. Assess the risk
The threat to a potato crop from viruses will depend on factors such as seed source, geographical location and proximity to other potato crops, weather conditions and aphid activity.
Certified seed has strict virus tolerance levels (e.g. not more than 2% ‘severe’ virus in ‘CC’ seed) whereas home-saved seed stocks can potentially have very high levels of infection. Seed produced in places where the climate is cool and windy (e.g. upland areas) is less likely to be affected, as these conditions deter aphid activity. The risk is also reduced if a crop from which seed may be saved is grown as far away as possible from general ware crops. Check the press regularly for information on aphid migration into crops and the associated virus risk. This year, for instance, in the UK there has been a rapid build-up in aphid numbers despite the cold winter.
2. Know your variety
As mentioned previously, symptoms and yield loss vary not only with the virus but also with the potato variety. Varietal resistance is an important control measure for many of the viruses. Knowing the susceptibility/resistance of a variety to different viruses is also important when it comes to testing of plants or seed stocks (see below). The Potato Council’s British Potato Variety Database has detailed descriptions of varieties and their pest and disease resistance.
3. Test for virus
It is vital when home-saving seed to have the stock tested prior to planting. As a minimum, the seed should be tested for the ‘severe’ viruses PVY and PLRV, but where a variety has a particular susceptibility to other viruses then these may also be tested for (e.g. Estima to PVA and PVV). Various laboratories offer a seed testing service.
The testing of plants with virus symptoms in a growing crop, or the general screening of plants selected from the crop, can give a useful early warning of potential problems should seed be saved from that crop. Pocket Diagnostic tests, in single or multi-virus formats, are available for the following viruses: PVA, PVS, PVV, PVX, PVY and PVYN.
Sources of further information on potato viruses and their symptoms include:
Diseases, Pests and Disorders of Potatoes – a Colour Handbook, by Wale, Platt & Cattlin (Manson Publishing, 2008)
Compendium of Potato Diseases, 2nd edition (American Phytopathological Society, 2004)