Aphid flight activity across the Northern Great Plains increased considerably during the past week.  The most abundantly represented species were bird cherry-oat aphid, buckthorn aphid, and turnip aphid.  The trap data did not show increases in green peach aphid activity, but colonization of potato by this species is now occurring.  Growers are advised to scout their fields, paying particular attention to field margins adjoining fallow, e.g., headlands or planting skips. 

This summer participating Minnesota and North Dakota potato growers have entered 30 potato fields in a large-scale University of Minnesota demonstration/research project.  The project's objective is to evaluate possible benefits of targeting the year's first insecticide [Monitor (methamidophos)] spray treatment for green peach aphid along field margins.  The first 6 fields were sprayed today.  Twenty four more are scheduled to be sprayed next week.  We have previously shown that initial colonization of potato fields by green peach aphids occurs almost exclusively along field margins that are kept fallow.  For the first 10-14 days following initial invasion of a potato field by winged green peach aphid, more than 90% of colonizing green peach aphids are found within the first 10-20 yards from a fallow border.  This colonizing behavior of green peach aphid provides growers the opportunity to target insecticide applications on those portions of the field where treatment will be most advantageous.  In preliminary studies, potato growers were able to reduce their use of Monitor by 80% by treating only crop margins.  The advantages of using targeted insecticide applications include effective aphid control at less cost, conservation of natural enemies in the untreated center of the field, and reduced selection for insecticide resistance.  Monitor resistant green peach aphids have been reported from some fields in the Pacific Northwest.
 

Additional information on the aphid situation in Manitoba can also be found at www.gov.mb.ca/agriculture/crops/insects/index.html, and the Manitoba Agriculture and Food potato hotline at 1-800-428-6866.

During the past week, weather conditions in the Northern Great Plains were generally not conducive to development of Late Blight.  There is a chance of severe thunderstorms late this afternoon in eastern North Dakota and northwestern Minnesota and unstable weather is expected to continue into the weekend.  In some locations rainfall may be heavy.  The Late Blight Disease Severity Threshold has been reached over much of the region.  If conditions become favorable and inoculum is high, Late Blight could develop within a week.  So far, the only known case of Late Blight this summer occurred near Carberry, Manitoba. 

Potato late blight status reports
Minnesota
North Dakota
Wisconsin
Manitoba
 
 

Minnesota seed potato inspectors have reported current season spread of mosaic (PVY) in some seed potato fields.  Since PVY is transmitted in a non-persistent manner by many aphid vectors, including a number of aphid species that do not colonize potato, insecticides tend to be considerably less effective in reducing PVY incidence than in reducing spread of potato leafroll (PLRV). Mineral oils may be an effective alternative.  A product locally available is Aphoil (Agsco, Grand Forks).

Research has shown that application of nontoxic crop oils can substantially reduce spread of PVY and other non-persistently transmitted plant viruses vectored by aphids.  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 the 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 supports the hypothesis that oils must 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.

Mineral oils are a mixture of aromatic, naphthenic, and paraffinic structures. Paraffinic oils are the most effective at inhibiting non-persistent virus transmission.  Optimal qualities of a mineral crop oil are: viscosity-gravity constant 0.790-0.819, viscosity 66-150 SUS, boiling range 370-420 ºC, paraffin-pourpoint below 0 ºC, and mean molecular weight 340-380.  The unsulfonated residue content of a mineral crop oil should be nearly free of phytotoxic aromatic structures.

To interrupt transmission of non-persistently-transmitted viruses, it is important to achieve thorough coverage and even distribution of oil particles on the leaf surface. The label rate for application of Aphoil is a 2% v/v solution applied in at least 20 gal of water per acre. Applications should be made at high pressure.  Addition of an emulsifier and use of spray nozzles that produce a fine droplet size (e.g., TeeJet® TX-4) is recommended. Mineral oils should not be applied with fungicides containing tin or copper.
 

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