Aphid Alert 1998, Inaugural Issue, July 16

Research Initiative To Address Virus Problems Confronting Seed Potato Industry

Ted Radcliffe and David Ragsdale
Department of Entomology, University of Minnesota
reprinted from Valley Potato Grower July 1998

The 1998 Minnesota State Legislature provided funding to the University of Minnesota College of Agricultural, Food, and Environmental Sciences to address urgent issues challenging the State's agricultural sector (Special Initiatives and Rapid Response Projects). One of the first projects selected for support addresses aphid transmitted viruses in seed potatoes. Project leaders are entomologists Dave Ragsdale and Ted Radcliffe and plant virologist Ben Lockhart.

Background

Until very recently, Minnesota and North Dakota produced nearly 40% of the U.S. supply of seed potatoes, enough to plant a half-million acres of potatoes. Since 1995, aphids and aphid-transmitted viruses have unexpectedly become the major threat to continued economic viability of the seed potato industry in this area.

As with all epidemics, there are several mitigating factors. One key factor we have identified is that recent changes in fungicide use have contributed to greatly increased green peach aphid, Myzus persicae, pressure. The predominance of a metalaxyl (Ridomil)-resistant strain (US8, A2 mating type) of Phytophtora infestans, causal agent of potato late blight, has precipitated major changes in fungicide use patterns in commercial and seed potato (both in chemistries used and frequency of application).

Unfortunately, more intensive fungicide application has proven devastatingly disruptive to a group of beneficial fungi that are key mortality factors regulating the populations of green peach aphid. Many commercial growers are now finding it necessary to spray aphids in late season to prevent direct yield losses. For seed potato producers, the problem is particularly serious because even a low incidence of aphid transmitted-viruses, such as potato leafroll or potato virus Y, can cause rejection of seed lots for certification. In 1997, the combination of abundant aphid populations, high levels of virus inoculum in the seed crop, a wet spring which delayed planting, and a warmer than normal fall that allowed growers to delay harvest until late September, produced conditions conducive for a viral epidemic. Seed lot rejections in winter trials reached proportions unprecedented in the history of the seed potato certification programs of the two states.

Ultimately, survival of the potato industry is dependent upon developing effective management of green peach aphid and PLRV. There are effective management tactics available, but lack of a biological monitoring network leaves researchers and growers without the information needed to make meaningful management decisions.

Response Activities Planned for 1998

We will attempt to monitor aphid flight activity on a regional basis. Both suction traps and pan traps will be operated at seven locations. Sites will be provided by Pieper Farms, Williams; Justin D. Dagen, Karlstad; Larson Farms, Climax; Brian Halverson & Sons, Baker; Robert L. Anderson, Little Falls; Brad Nilson, Hoople; and Jim Jorde Farms, Cando. The hypothesis is that suction traps detect long distance flight whereas pan traps detect local movement, i.e., winged aphids developing on plants within the potato field. If suction traps actually are effective in detecting long distance flight activity, a permanent trapping network could be established to provide warning that virus spread can be anticipated. In some other seed producing regions, e.g., Scotland, such networks are used to recommend or mandate vine kill. Robert Suranyi, entomology graduate student, will coordinate this activity and identify the aphids captured. Robert will work out of the Minnesota Seed Potato Certification Program laboratory in E. Grand Forks.

State seed potato certification personnel will monitor aphid numbers on potato plants when making their field inspections. This will provide information on the dynamics of within field increase of green peach aphid numbers. They will also record information on the proximity of the seed potato fields to commercial potato fields and other crops that could harbor aphids that play a role in transmission of potato viruses, e.g., cereal aphids which do not survive on potato, but can move PVY. Dr. Ian MacRae, entomologist at the Northwest Agricultural Experiment Station, Crookston, will direct the thesis research of a graduate student who will analyze spatial data associated with seed potato production in the Red River Valley using a Geographical Information System (GIS) via remote sensing. To this end, the longitude and latitude coordinates will be recorded for every seed potato lot entered into the seed certification program. We will also use data on virus incidence obtained by the seed potato certification programs of the two states, i.e., field inspections and winter-testing of samples taken at harvest. The goal of this component of the research is to determine the isolation required maintaining virus-free seed when growing in the vicinity of commercial potato growers.

Results of the aphid surveys, i.e., winged aphids from trapping and non-winged from foliage sampling, will be reported on a near real-time basis to seed potato growers in both states by newsletter, Aphid Alert, and via the WWW (ipmworld.umn.edu/alert.htm). As in 1991-93, Aphid Alert will be edited by Carlyle Holen, Northwest Agricultural Experiment Station. We will be planting clean seed plots at three Minnesota agricultural experiment stations distant from commercial production, i.e., Morris, Waseca, and Lamberton.

We will intentionally manage these potato plots in ways that favor aphids, e.g., surrounded by fallow borders and treated with pesticides that enhance aphid numbers. We do this because our objective is to identify a location suitable for potato increase that is sufficiently isolated from sources of virus inoculum so that seed potatoes generated from tissue culture can be safely increased for 1-2 years. Such a site might ultimately be developed as an elite seed farm as has been done in some other states.

Dr. Lockhart is cooperating on the latter project. He will also compare efficiency of virus indexing by ELISA, immunocapture-PCR, and visual inspection, and investigate cultivar differences as sources of virus inoculum and biological variability among potato leafroll isolates. In other studies, he will monitor virus spread within representative fields and determine whether multiple infection, i.e., leafroll and potato virus Y, makes the plants more effective virus reservoirs.