Aphis And Virus Control In Potatoes

Press, Volume CIII, Issue 30586, 31 October 1964, Page 9

Aphis And Virus Control In Potatoes

(By Dr. K. C. CLOSE, Senior Scientific Officer, Crop Research Division, D.S I.R.J In the last two years fanners have heard much about aphis and virus control in wheat, but what about aphis and virus control in potatoes? It is well known that farmers in certain areas need to buy every year good certified seed potatoes that come from areas where there are few aphides, such as the Sheffield and Methven districts.

The important question Is, why do seed potatoes break down? The reason farmers cannot grow their own seed is because aphides arej abundant, there is much; spread of virus diseases and the potato seed stocks quickly become infected with viruses. There is a poor yield from infected plants and if there are many then the total yield of the crop is considerably reduced. On a 10-ton crop, every 5 per cent infection with severe virus diseases means a loss of half a ton per acre. Since virus diseases can increase ten-fold in a single season in poor localities, then crops soon become heavily infected and the seed is not fit for replanting. If this spread of viruses could be prevented, then all could save their own seed potatoes. This would mean cheaper and better seed, since it could be properly stored and made ready for replanting.

Now what are the aphisborne virus diseases that are found in potatoes, and are perpetuated from crop to crop only in the tubers. There is leaf roll virus and also severe mosaic.

The main potato varieties are now 11am Hardy, Sebago, Rua, Katahdin (Chippewa) and Aucklander Short Top and in these leaf roll is a big problem. They are rarely infected with severe mosaic, except for Aucklander Short Top which is completely infected with this virus. Varieties which are attacked with both severe mosaic and

leaf roll are Glen Ham, Dakota. Epicure, Arran Banner and several others. Leaf roll virus is what is known as a persistent virus—that is once infected the aphides remain infected. Aphides take a long time (24 to 48 hours) to pick up the virus from a diseased plant and an equally long period to infect a healthy plant. In this time the aphides can be killed with a systemic insecticide and transmission of this virus prevented. Thus the breakdown or running out of seed potatoes of those varieties in which leaf roll is the major problem can be almost completely prevented. Seed saved from treated crops can be used to replant the next year’s crop. Any breakdown that occurs will take place much more slowly. Severe mosaic is also aphisborne but is transmitted very rapidly from plant to plant. In fact the damage is done before the aphides can be killed by an insecticide. On average it takes at least six hours for systemic organophosphorus insecticides to kill an aphis. In that time aphides can carry severe mosaic from plant to plant and the percentage of diseased plants increases rapidly.

Leaf Roll Virus As a general rule, therefore, we can say that insecticides, properly applied, will control leaf roll virus but not severe mosaic. Work done at the Crop Research Division over the last four years shows that potatoes can be protected against aphis and leaf roll virus attack, thus enabling seed potatoes of the most

common varieties to be kept for several seasons. But when and in what districts should insecticides be used for leaf roll control? Young potato plants are most attractive to aphides and are often thickly colonised as soon as they emerge. In a survey last season, the number of aphides on newly emerged plants or young crops varied between 7 per 100 leaves (Rakaia Gorge) to 1500 per 100 leaves (Halswell). In the latter case it is likely that the aphides were damaging the plants enough to reduce yield. Aphides were very abundant in areas below 100 ft above sea level and near to Christchurch. . In Christchurch aphides are abundant on home garden potato crops. It is in those areas that good control of leaf roll virus disease, through using insecticides, is likely to be of most benefit to growers. The survey is not complete and this year crops in the Southbridge, West Melton, Sheffield and Waimate districts are to be inspected.

Emergence As has been said before, potato plants are attacked by aphides as soon as they emerge. At this stage they are most susceptible to infection with leaf roll virus and much spread can occur in bad aphis areas. Transmission occurs from diseased to healthy plants within a crop and is done mainly by winged aphides. Infection rarely goes from crop to crop unless crops are planted close together. (This is the main reason why Aucklander Short Top, which carries severe mosaic, must be grown well away from varieties susceptible to severe mosaic.) To achieve control of leaf roll virus it is important that the plants be protected from aphides as soon as possible. There are two ways of achieving this: (a) by sprays applied at emergence and then at 10-14 day intervals, and (b) by the use of granules applied in the furrow at I planting.