The Spread of Virus Diseases in the Potato Crop

Abstract

The following is taken from the authors' summary. The rapid degeneration of potatoes brought from seed-growing districts to the English lowlands is due to the spread through the stock of the Aphid-transmitted virus diseases, rugose mosaic and leaf-roll. This study of the epidemiology of these diseases is based on counts of Aphid populations in the field and on records of the spread of the diseases in potato crops in the east of England during 1940-45, supplemented by data obtained in other parts of England and Wales. The maincrop variety Majestic was used in most of the work. Periodical sampling of 63 crops of Majestic showed that on the average three-quarters of the full yield of tubers was obtained by the middle of August, but in the wetter seasons of 1941 and 1944, about a quarter of it was added in September. The weight of "seed " size tubers averaged 4.1 tons per acre at its maximum in July or August and declined to 3.2 tons at the end of the season. In the variety Majestic, the tubers are formed during July and subsequent differences in individual rate of growth leads to their differentiation into " ware ", " seed ", and " chats " by the end of the season. The final yield per acre of seed tubers can be increased by close spacing and by planting large setts, but under present conditions in the ware-producing districts, it is more profitable to allow a crop to mature at normal spacing than to attempt by early lifting to control virus or by close spacing to increase the proportion of seed. The four Aphids that occur regularly on potato crops are Myzus persieae, Sulz., Aphis rhatnni, Boy., Macrosiphum solanifolii [Macrosiphum euphorbiae], Ashm., and M. (Aulacorthum) solani, Kalt. Fruiting peaches out of doors and in unheated glasshouses were the most important plants on which Myzus persicae overwintered in the egg stage. In some seasons it can persist throughout the year on cruciferous crops, but its survival on these plants was greatly reduced in south-east England in the severe winters of 1940-41, 1941-42 and 1944-45. After these winters, its numbers on peach were small and it developed late, and infestations on potatoes were also relatively late in developing. Overwintering on sugar-beet seed crops was also recorded. Aphis rhamni apparently overwinters only in the egg stage on Rhamnus, but heavy infestations sometimes developed on potatoes in the Lincolnshire fens, where this plant is rare. The spring migration of M. persicae from peach or crucifers to potatoes took place each year during May and sometimes continued into June, when early potatoes are well advanced and maincrop varieties just above ground. The maximum population on the foliage was usually reached towards the end of July. Numbers then declined, owing to migration to other food-plants and the activities of parasites and predators, and reached a minimum during the first half of August; they sometimes showed a second, but smaller, rise in September. In 1941, the population did not reach its maximum until about the end of August, and this was associated with unusually late spread of leaf-roll in potato crops. In 1942, populations of M. persicae were very small. The spring migrants of A. rhamni reach the potato crop in June or early July, and peak infestations occur in August. Populations on potato were high in 1940 and 1941, and very high in 1942, when M, persicae was scarce. A. rhamni was almost absent from potatoes in 1943-45. Populations of Macrosiphum solanifolii [Macrosiphum euphorbiae] and M. solani followed approximately the same course as those of Myzus persicae, but heavy infestations occurred on potatoes only in 1945. There is no evidence that either of these two species plays any appreciable part in transmitting rugose mosaic or leaf-roll in the field. On the whole, both diseases spread most in fields with large numbers of M. persicae, but wide discrepancies were found, indicating that factors other than the presence of large populations of apterae on the plants or flights by dispersal migrants as shown by trap records, must play a part in determining the amount of virus transmission. There is evidence that much spread is effected by winged spring migrants, Apterae may also play a part at or near the peak of the infestation, especially where interlacing foliage facilitates movement from one plant to another. The winged dispersal migrants of M. persicae in summer are believed to be of relatively little importance. Periodical sampling on a standard plan gave information on the season and extent of spread of the diseases at a number of centres in different years. In recognised seed-growing districts, such as North Wales, Dartmoor and Northumberland, there was very little increase in diseases, but the amount of spread in lowland districts varied greatly. It was generally slight in 1942, when M. persicae was scarce on all food-plants. In lowland districts, the increase of rugose mosaic on a standard plot varied from one quarter to nearly six times that of leaf-roll. Most infection occurred early in the season, and only about 10 per cent. of the crops sampled showed a substantial increase of rugose mosaic after the first week in August, whereas the figure for leaf-roll was 30 per cent. Burning the haulms or lifting early enough to avoid virus infection of the tubers would involve a substantial loss of ware. The probability of a plant becoming infected is greatest when it is next to an infected plant and generally decreases rapidly at increasing distances from an isolated infected plant and less rapidly from a strip or an area of diseased plants. In healthy crops adjoining diseased ones, a gradient of infection cannot usually be detected at distances greater than about 10-20 rows from the diseased plants. The amount of infection from sources more distant than an adjoining crop is usually slight in comparison with the spread from diseased plants present in the field. Removing secondarily diseased plants from Majestic stocks during July usually did not appreciably reduce the spread of either disease. Roguing in mid-June, tried in one season only, reduced rugose mosaic to one-third of the amount in unrogued plots, but had no effect on the spread of leaf-roll. The failure of roguing is presumably due to the early date at which virus transmission takes place. It is estimated that if plants showing primary symptoms of virus infection were also removed during repeated roguing, the percentage of diseased tubers would be halved, and that further improvement would result if the two plants adjacent to each infector were also removed as a routine measure. The effect of fumigating large plots with nicotine vapour was studied in 1942-44. A single treatment early in July was usually successful in destroying the Aphids, but had little effect on the spread of the virus diseases, presumably because of transmission before July by spring migrants. In 1944-46, the survival of self-set potatoes on English arable fields averaged 2, 000-7, 000 per acre in the first crop after potatoes and fell to 500-800 per acre in the third. Three years was the average interval between potato crops in the fields studied. Rugose mosaic and leaf-roll do not usually increase in self-set plants, and those from a stock planted with healthy seed are not as a rule an important factor in the degeneration of potato stocks, but those from diseased stocks can cause an abnormally large increase in virus disease. Reasonably good seed could probably be produced in the English lowlands during an emergency by drastic roguing and early lifting, but this would normally be uneconomic. The most practical method of increasing the life of seed stocks appears to be to improve still further the health of stocks raised in seed-growing districts, to grow them in relative isolation and, most important of all, to plant them on land free from self-set potatoes.