By Rosalie I. Tennison
In this experiment, Boquel mixed mineral oil with dye and viewed the plant under a UV light to research how mineral oil penetrates the plant. Photo courtesy of Dr. Sebastien Boquel.
Seed potato growers wage a continuous battle against Potato Virus Y (PVY) both economically and physically. The most effective way to control the virus is to plant PVY-free seed, but the challenge for seed growers is keeping the virus from infecting the crop. Spread by aphids, it can take only minutes for a plant to become infected once an aphid lands on it and probes it to see if it is a suitable host.
While planting clean seed is the best way to prevent the spread of the virus, other cultural practices, such as crop rotation, will minimize infection. However, another somewhat expensive but effective means of controlling PVY is to keep the crop coated with mineral oil. A method used mostly by seed growers, spraying the oil reduces the spread of PVY. Scientists don’t know why or how mineral oil works, but they are working to find out.
Dr. Sebastien Boquel, a research scientist from Comité Nord Des Producteurs de Pommes de Terre (an association of seed potato growers from France) posted at Agriculture and Agri-Food Canada (AAFC) in Fredericton, N.B., has launched a study to explain why the oil works and to determine if there is a more economical means of using it. He explains that most research on the use of mineral oil proves that it works, but the reason why the strategy is effective is unknown. It is possible the oil affects the aphids similar to an insecticide or that the oil modifies aphid behaviour, reducing the ability of the insects to transfer the virus into the plant. Another possibility is that the oil reduces the ability of aphids to acquire the virus from an infected plant. It is also speculated that the oil inhibits the movement of the virus into the plant. Boquel believes that, if he can determine how the oil works, he may be able to recommend more economical ways to use the oil while getting the same effectiveness.
“We are trying to determine the mode of action of the oil and explain how it works,” Boquel says. “We know that within one week of spraying the oil, there is PVY control, but we don’t know how it works on the plant to reduce the transmission.”
Boquel is also working to determine how much oil is needed and at what concentration to get good control, which could reduce the costs to seed growers. There has been concern that even though the mineral oil is food grade, it could be toxic for plants. In his research, Boquel deliberately sprayed the crop with high concentrations of oil to try to learn at what point toxicity became an issue.
“We tested the phytotoxicity and proved the oil can be used safely,” he reports. “We sprayed at 160 l/ha in an attempt to deliberately kill the plants and they still produced tubers and did not die.” It is recommended that growers spray mineral oil at 10 l/ha, which has no phytotoxicity at all.
Some growers try to cut back on the amount of oil used to economize. But, Boquel says, not using the oil is not an
option and seed growers need to keep the plants coated throughout the growing season in order to ensure the virus is not transmitted or at least transmitted to a lesser degree. He also considered modifying the amount of oil used at various times of the growing season.
“We studied increasing the amount of spray early in the season and then modulating the concentration across the
remainder of the season,” Boquel explains. “Within the first four weeks, spray at 15 l/ha and the following three weeks spray at 10 l/ha, and then continue to the end of the season spraying five l/ha. The same amount of oil is used during the season but at different concentrations. The high concentration is needed at the beginning of the season because the plants are growing fast and applying a higher concentration might insure good control of the virus.”
Meanwhile, Boquel continues to study mineral oil in an attempt to determine just how it works to prevent the spread of PVY. If he can identify the mechanism that is working to prevent the virus from transferring into the plant or moving to the tuber, it could be possible to formulate other means of using that mechanism in the future that might not require the intense spray schedule. As well, knowing the mode of action of the oil could prompt an update of the current recommendations given to growers, which may result in a more economical way of using it. He expects to have more answers within a couple more years.
“Mineral oil is a very important tool for seed potato growers because they need to have pure seed to sell,” Boquel says. Therefore, understanding how it works could be a major breakthrough in PVY control. Even though mineral oil has been used to minimize PVY infection by aphids since the 1970s in France, identifying its mode of action could give growers the opportunity to use it even more effectively.