Long before the first outbreak of late blight, it became apparent in 18th century Europe that potato crops degenerated from one year to the next. The reasons for this were unclear, but initially the disease was given the name of “curl” because infected potato plant leaves tended to curl or roll. Subsequent crops planted with tubers harvested from infected crops produced increasingly lower yields, hence the idea that potato cultivars would degenerate over time. It was also observed that potatoes infected with curl never recovered. In his influential book The History and Social Influence of the Potato, Redcliffe Salaman reported that the problem had become so severe by the 1770s that some feared potato cultivation would disappear altogether in several areas of Western Europe.
As was later the case with late blight disease, a debate ensued within learned societies about the cause of the degeneration. Some believed the disease had something to do with poor climatic and/or soil conditions in those areas where potatoes were grown. Others were convinced that the problem was due to plant stress and weakness related to old age or fatigue. This in turn led to the development of many new cultivars that initially displayed very little curl; after a few years, however, they also would degenerate.
The situation became so bad in some areas of Europe that prizes were offered to find a cure for curl. In Belgium, the Royal Academy of Brussels offered an award of 1200 francs in 1775. Three years later, the Agricultural Society of Manchester offered a silver cup worth eight guineas for a curl solution. Yet, it took another century and a half before the mystery of curl was finally solved.
Composite of Viruses
Today, we know that what was described as “curl” was in fact often a complex of several virus diseases including potato leafroll, potato virus Y and also, possibly, potato viruses A and X. In 1905, German scientist Otto Appel coined the term “Blattrollkrankheit” (leafroll disease), thereby distinguishing it from the curl disease complex, which at that time in Germany was known as “Kräuselkrankheit.” The English acronym PLRV (potato leafroll virus) was later applied; however, giving the disease a name did not solve the problem.
In 1916, three Dutch researchers discovered that potato leafroll was an infectious disease that could be transmitted by grafting. Because they couldn’t identify any microscopic organisms that might cause this disease, the scientists suggested it was caused by a virus. Others remained unconvinced, since such a virus — if it did exist — could not be detected by microscopes at the time. It was also unclear how viruses could be transmitted from one plant to another.
The unravelling of this disease — from the initial postulation of a virus as the cause to its eventual verification of the theory — is similar to the recent significant breakthrough in physics, when the existence of a subatomic particle known as Higgs boson, which was first predicted 50 years ago, was finally confirmed.
It took another few years for the transmission mystery to be solved. In 1920, Dutch scientist Oortwijn Botjes proved that PLRV was transmitted from sick to healthy potato plants by aphids (especially green peach aphids) feeding on the plants. This finding also explained why potato crops in some areas degenerated so much faster than others. For example, seed potatoes derived from crops grown at higher elevations or in cooler, more northern locations were much healthier than those grown in warmer, southern regions. The reason for this difference was now clear: areas of higher elevation and cooler northern regions had fewer aphids.
Nevertheless, it was several more decades before the virus theory for potato leafroll was generally accepted. It took the invention of the electron microscope in 1930s to prove the existence of a virus. The unravelling of this disease — from the initial postulation of a virus as the cause to its eventual verification of the theory — is similar to the recent significant breakthrough in physics, when the existence of a subatomic particle known as Higgs boson, which was first predicted 50 years ago, was finally confirmed.
Role of Potato Virus Y
As previously mentioned, the disease known as “curl” was typically caused by a composite of viruses. Even after it was determined that leafroll was triggered by a specific virus, several other viruses which were part of the curl complex remained unidentified until 1931. That’s when British scientist Kenneth M. Smith identified potato virus Y as another major virus contributing to curl symptoms in potato plants.
In contrast to PLRV, which causes similar symptoms in most potato varieties, PVY can produce a wide range of symptoms in different varieties. A further complication is that this virus has several strains, each showing different symptoms. The host range of PVY includes tobacco, peppers and other plants related to the cultivated potato, as well as some wild potato relatives.
Another major difference between PVY and PLRV is that PLRV is carried in the intestinal tract of aphids; once an aphid is infected with PLRV, it stays infected for life and will continue to infect healthy plants. In other words, PLRV is a persistent virus. PVY, on the other hand, is a non-persistent virus because the aphid only carries the disease on its stylet or mouth — it is rid of the virus once it’s been transmitted.
In the early 1960s, Roy Bradley was working at the Canada Department of Agriculture Research Station in Fredericton, N.B., when he discovered that a coating of mineral oil on potato leaves could effectively inhibit the transmission of PVY. This monumental discovery is the basis for the application of mineral oil around the world on many crops, including potatoes, peppers and tobacco, which are affected by stylet-borne viruses.
These discoveries about the cause and transmission of PLRV and other potato viruses such as PVY have had a major impact on potato production, leading to effective control measures which are essentially two-fold:
- Controlling the aphid carrier of the virus
- Removing the source of infection (plants that are already infected)
As a result, these two principals are now embedded in the seed potato certification systems of virtually all seed potato-producing countries.
