Potato viruses are transmitted by aphids in two basic ways. The virus is either non-persistent (stylet-borne) or persistent and circulative (meaning it is ingested and persists in the aphid throughout its life).
Mineral oil sprays are employed by certified seed potato producers to reduce transmission of aphid-transmitted viruses. Mineral oil products are relatively inexpensive but require a minimum of weekly applications through most of the season, which increase their cost. The application of mineral oil to potato foliage provides a barrier on the leaf surface, which the aphid’s stylet must pass through to penetrate the leaf. During that process the virus particles may be physically removed from the surface of the stylet and fail to penetrate the leaf. Because the physical barrier is a major mechanism of the transmission reduction, frequent applications are required to cover new foliage as plants grow.
The most common persistent-circulative virus affecting potatoes is potato leaf roll. Common non-persistent or stylet-borne viruses are potato virus Y (PVY), potato virus A, potato virus S and potato virus M. Because the aphid has to feed for more than several minutes to transmit persistent-circulative viruses, control by insecticides is highly effective, whereas stylet-borne viruses are transmitted as soon as the aphid stylet penetrates the epidermal cells of the leaf—too soon for any insecticide to kill the aphid and prevent virus transmission. For most stylet-borne viruses, the virus is acquired immediately and can stay viable on the stylet for up to two hours.
More than 50 aphid species can transmit potato viruses. Some, such as the green peach or potato aphid, colonize potato fields and be involved in transmission of both persistent and non-persistent viruses. Others species, such as the bird cherry-oat aphid, will not colonize potato but are common in grain crops. Non-potato colonizing aphids migrate as winged adults to potato fields from alfalfa or small grains as those crops are cut or mature. They move through the potato planting, attempt to feed and move on, thus becoming major factors in spreading non-persistent viruses such as PVY. Thus, removing in-field sources of virus by rogueing is important. Migrating aphids typically will land on the edges of vegetation. Therefore, border crops with no breaks between the border crop and potatoes can serve as aphid stylet “cleaning stations.” Once the aphid probes, the virus is removed from the stylet.
While we still have the effective use of neonicotinoid insecticides and use of low-infection seed from certification programs, control of persistent viruses is straightforward. Use of a neoticoinoid planting time treatment on certified seed, followed by use of recommended non-neonicotinoid insecticides until vine kill, is recommended. However, the management of stylet-borne or non-persistent viruses is much more difficult.
Control of stylet-borne viruses like PVY starts with the use of infection-free seed, which is the primary inoculum source. While the use of insecticides or aphid feeding deterrent materials may reduce virus spread by potato-colonizing aphids, they have proven to be relatively ineffective for management of stylet-borne viruses spread by non-colonizing aphids, the major cause of PVY spread.
Research has shown that the use of mineral oils from emergence through vine kill provides about a 40 to 85 percent reduction in PVY spread compared to untreated or full-season insecticide protection. In our research, level of control improved another 10 to 20 percent when PVY-infected plants were rogued out and a no-gap insecticide program (neonicotinoid at planting, followed 60 days later by feeding-deterrent insecticides) was used.
We also studied the use of induced resistance with Certis USA’s biological inducer BmJ microbial plant activator.
While induced resistance was effective, it was not as effective as the mineral oil no-gap insecticide program in reducing PVY infection. Integration with mineral oils did not improve control over the mineral oil program. In studies by other researchers with mineral oil, the addition of insecticides improved control marginally or not at all over the oil itself. This might be explained by the mix of potato-colonizing and non-colonizing aphids acting as vectors in particular trials
Mineral oil sprays do not kill aphids but work by reducing acquisition of stylet-borne viruses. They do not, however, reduce transmission of circulative viruses like leaf roll. They reduce the time virus particles can be retained on the stylet and influence aphid behaviors, such as extended time on foliage without probing, deterred feeding, and reduced virus acquisition. Because oils work at the site where aphids probe with their stylets, it is critical that coverage be uniform on the leaf and stem. This requires small droplets, high volumes and frequent application (every four to seven days) to protect expanding leaves and new growth, where aphids prefer to feed. Longer periods of 10 to 14 days can be used when rapid potato growth stops. Effective control is achieved from both ground and aerial applications.
Recent research in Canada has shown that oils are absorbed into the leaf quite rapidly but do not move from the site of absorption to other parts of the leaf or to other plant parts, and that the concentration of oil in the potato leaves was proportional to PVY control. Mineral oils should not be applied when plants are wet, as it will cause the oil droplets to bead up, thereby reducing coverage. Mineral oils can be applied to field borders to create aphid stylet cleaning stations just like a non-virus host crop border where migrant aphids alight. Application of mineral oils is most important in late June to mid-July when alfalfa is cut or grain ripens and non-colonizing aphids migrate from those fields. Applications should continue until vines are completely dead and should occur at shorter intervals when the crop canopy is growing rapidly.
There is limited evidence of phytotoxicity of co-application of oils with chlorothaonil, copper-based fungicides and TPTH. Mancozeb-based fungicides appear to be safe. Some researchers have reported phytoxicity with oils, but we have not observed this even when temperatures have reached 100 degrees.
There are many petroleum-based high paraffinic mineral oils on the market. Commercial names include Aphoil, Ultra-Fine, JMS Stylet Oil, PureSpray, Glacier Spray Fluid, organic leaf oil, Sunco Sunspray, Superior 70, Seasons Spray oil and Vazyl-Y. These materials generally only show “caution” on the labels but are toxic to fish. Since the droplets do not evaporate like water, spray drift is a hazard. Be sure to follow the label, since some products contain adjuvants that allow emulsification of the oil with water and others do not.
Mineral oils can be an important tool in reducing transmission of stylet-borne viruses such as PVY, but it should be remembered that oils will work only if used in an integrated program, if coverage of leaves and stems is complete, and if new growth is continuously protected.