Aphid Alert 1999, No. 6a, July 20
Crop borders and mineral oils: two tactics for management of PVY in seed potatoes
by Robert Suranyi
Department of Entomology
University of Minnesota
PVY (= mosaic, potato virus Y) is transmitted by aphids in a non-persistent manner. Non-persistent transmission is characterized by presence of virus particles on the mouthparts and in the foregut of the aphid. Transmission of PVY is a complex biological process and not simply contamination of mouthparts with virus particles. Acquisition and inoculation of PVY requires only seconds. Aphids rapidly lose their virus charge after brief feeding on healthy plants and must again feed on a PVY infected plant to continue to transmit PVY. Although the green peach aphid is the most efficient PVY vector, other aphids species such as those associated with sunflower, canola, and small grains are far more abundant and play a significant role in the spread of PVY in our region. As the cereals, canola and various weed species begin to mature and senesce in mid-summer, the aphids associated with these plants produce winged adults which can migrate in search of suitable alternative hosts. Since aphids must sample ("taste") the epidermal tissue of a plant to determine its suitability as a host, many individuals of aphid species incapable of reproducing on plants will sap sample potato foliage and can in the process spread PVY.
Insecticides are much less effective in reducing the incidence PVY than in reducing the spread of PLRV (potato leafroll) which is transmitted in a persistent manner. Controlling PVY spread relies on measures that utilize the biological characteristics of the aphid vector-PVY pathosystem. Knowledge and exploitation of aphid flight behavior and virus transmission led to the development of the concept of crop borders. Since, aphids are especially attracted to green-dark interfaces of field edges during their search for suitable host plant, aphids tend to land in greatest numbers in field edges. Crop borders planted with non-virus host plants, such as soybeans, can provide a landing site where aphids can "clean" their mouthparts from non-persistent viruses prior to their immigration to potato. Crop borders have been shown to reduce PVY spread in potato by up to 60% from sources outside of field.
During the growing season non-persistent virus spread can also be reduced with the use of mineral oils. Mineral oils have successfully been used to reduce virus spread in Florida on peppers, squash, and tomatoes, and oils are also routine components of the IPM program of the Hungarian paprika production. Mineral oils reduced PVY transmission efficiency by 73% in peppers in simulated field situation when aphids acquired PVY from untreated plants and then probed on oil treated plants. Oil treatment of the source plant or both the source and test plant proved even more effective in reducing virus transmission efficiency. It is conceivable that inhibitory amount of oil particles are carried over on aphid mouthparts during subsequent feeding events (Simons et al. 1977, Powell 1991). The exact mode of which mineral oils interfere with virus transmission is unknown but it appears to be a complex contact-based action. To be effective complete coverage of the leaf surface with oil is essential and oil needs to be reapplied frequently (e.g., weekly) to cover new growth. The universal inhibitory effect of mineral oils on non-persistent virus transmission appears to support the hypothesis that oils somehow interfere with some basic underlining process of the virus-aphid mouthpart interaction. However, mineral oils also appear to interfere with normal leaf exploratory processes of aphids and increase pre-probing time on oil coated leaves (Simons et al. 1977, Powell 1991).
Mineral oils are a mixture of aromatic, naphthenic, and paraffinic structures. Their effectiveness in inhibiting virus transmission is dependent upon their physical and chemical characteristics, such as viscosity-gravity constant (VGC), kinematic viscosity (SUS), paraffine-pourpoint, unsulfonated residue (USR), distribution of carbon atoms, specific weight (Sp.W.), refractive index (n20D), molecular weight (MW), and boiling range. Based on these properties mineral oils are grouped into paraffinic, naphthenic, and aromatic categories. The most effective oils inhibiting non-persistent virus transmission are the paraffinic mineral oils which are characterized by the following: VGC 0.790-0.819, viscosity between 66-150 SUS, boiling range of 370-420 ºC, paraffin-pourpoint below 0 ºC, and a mean molecular weight of 340-380. The unsulfonated residue content should be between 95 and 100 indicating a near complete absence of phytotoxic aromatic structures. (DE Wijs 1980).
We have been informed that mineral oil with the appropriate physical-chemical characteristics is available from local source (Agsco, Grand Forks, ND). To achieve control of non-persistent virus transmission spray application characteristics are critical when applying mineral oils. Since mineral oils are contact in their mode of action, therefore complete coverage and even distribution of oil particles on the leaf surface is essential. The recommended concentration for spray application of mineral oils is 1 to 2% solution (30 to 60 gal solution /acre). High application pressure (200-400 psi), addition of an emulsifier, and spray nozzles (e.g., TeeJet® TX-4) help to achieve the required small droplets of oil particles for controlling non-persistent virus spread (Zitter and Simons 1980). However, caution should be taken when using mineral oils, especially at the higher concentration because of the possibility of damaging the foliage.
Photograph showing behavior of mineral oils on the leaf surface (from the product label of a mineral oil product, Vektafid A®, made by Rogator Kft., Nagyszénás, Hungary).
Transmission of PLRV (potato learoll virus) is very different from that of PVY. Potato leafroll virus is phloem-restricted transmitted by aphids in a persistent manner. PLRV can only be acquired and transmitted by potato colonizing aphids. To acquire the virus an aphid probe an infected plant long enough to reach the phloem. Typically, this requires sustained feeding for periods of 20 minutes or more. Once the virus is taken up by the insect the infective particles must penetrate the gut and migrate to the salivary glands. Transmission of the virus by the newly infected aphid can only occur once the virus particles are present in the saliva. This does not occur until 24 hours or more after acquisition. This relatively long lapse from acquisition until the insect becomes competent to transmit the virus means that there is sufficient time for insecticides to kill the aphid before it become capable of transmission. However, if a winged aphid arrives in the potato field with virus particles in its salivary glands, transmission may occur if the insect probes the phloem. Insecticides are not likely to kill quickly enough to prevent this. In our region, green peach aphid, Myzus persicae (Sulzer) is recognized as the most efficient vector of PLRV. Other potato-colonizing vector species include buckthorn aphid and and potato aphid.
- De Wijs, J. J. 1980. The characteristics of mineral oils in relation to their inhibitory activity on aphid transmission of potato virus Y. Neth. J. Pl. Path. 86:291-300.
- Powell, G. 1992. The effect of mineral oil on stylet activities and potato virus Y transmission by aphids. Entomol. Exp. Appl. 63: 237-242.
- Simons, J. N., D. L. McLean, and M. G. Kinsey. 1977. Effects of mineral oil on probing behavior and transmission of stylet-borne viruses by Myzus persicae. J. Econ. Entomol. 70:309-315.
- Zitter. T. A. and J. N. Simons. 1980. Management of viruses by alteration of vector efficiency and by cultural practices. Ann. Rev. Phytopathol. 18:289-310.