Update on University of Minnesota Crop Border Treatment Experiment.

We are still in the process of analyzing data from the crop border experiment.  Results were as expected in that we found that initial colonization of potato by green peach aphid was heavily concentrated on the headlands and field margins abutting fallow.  Only two fields had appreciable numbers of green peach aphid in the field interior, one was a poor stand, the other had drownouts.  Monitor treatments generally provided excellent aphid control.
 

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.

Weather conditions across the Northern Great Plains for the past month generally have not been conductive to development of Potato Late Blight.  In most areas, the potato crop is in good to excellent condition.  However, with high relative humidity beneath crop canopies, especially in irrigated fields, Disease Severity Values have been pushed above threshold at most locations. Humid weather with the possibility of thundershowers is forecast for much of the region over the next several days. Rapidity of change in Disease Severity Values is the key determinant of Late Blight risk.  
 

Potato late blight status reports
Minnesota
North Dakota
Wisconsin
Manitoba 
 

(Abstract of paper to be presented at annual meeting of Potato Association of America, Spokane, WA, August 10-14, 2003)

Two of the most serious diseases of potato in recent years have been late blight, caused by Phytophthora infestans, and mosaic, caused by potato virus Y (PVY). Both of these diseases have been difficult to control. In-field control of late blight requires repeated application of fungicides. Insecticide applications are not effective for PVY control because transmission of this stylet-borne virus occurs before the insecticide can act against the vector. Crop oils have been shown to be effective in reducing spread of stylet-borne viruses such as PVY by 40-60%. Most fungicides are applied as water soluble suspensions, in contrast with crop oils applied as aphicides. Because both of these applications frequently occur at the same time during the growing season, and producers often combine fungicides and insecticide application to save money, it was of interest to us to determine the compatibility of these products. Eight fungicides commonly used for late blight control were applied full season alone or tank-mixed with Aphoil, a crop oil commonly used in our area. The trial was conducted in 2001 and 2002 using the cultivar Norvalley inoculated with late blight prior to the first treatment application. Treatments were applied eight times during the growing season. Combined results showed that the RAUDPC of all fungicide treatments for late blight was not significantly reduced or enhanced by the addition of Aphoil. The efficacy of Dithane was numerically reduced, and the efficacy of Acrobat was numerically enhanced, by the addition of Aphoil. It is concluded that Aphoil can be tank-mixed with commonly used fungicides for late blight control without adversely affecting efficacy of the fungicide.
 

(Abstract of paper to be presented at annual meeting of Potato Association of America, Spokane, WA, August 10-14, 2003)

Since its discovery in North America in 2000, Aphis glycines (Matsumura) has spread to 20 states, representing more than 60 million acres of soybean. Soybean aphids are competent vectors of Alfalfa mosaic virus (Genus Alfamovirus), Soybean mosaic virus (Genus Potyvirus) and Tobacco vein-banding mosaic virus (Genus Potyvirus) a presumed variant of Potato virus Y (PVY, Genus Potyvirus). Soybean aphid is closely related and biologically similar to the efficient PVY vectors, the cotton aphid, Aphis gossypii (Glover), and the buckthorn aphid, Aphis nasturtii (Kaltenbach). Field studies conducted in 2001 and 2002 implicated soybean aphid in the spread of PVY in potato. We conducted greenhouse experiments to determine if soybean aphid could transmit PVYO, PVYN, and PVYN/NTN. Thirteen cages, approximately 1.2 m2, were used. Five cages contained a PVY positive center plant and eight cages contained a PVY negative center plant. Nine ELISA-tested potato plants were placed around each center plant. A soybean plant infested with soybean aphids was placed adjacent to the PVY source plant. After four weeks of exposure to soybean aphid alatae and apterae, all potato plants were tested again for PVY by ELISA and RT-PCR. One-way analysis of variance was performed. Results indicate soybean aphid can transmit PVYO, PVYN, and PVYN/NTN (P < .00002). Fourteen of 45 (31%) test plants caged with a PVY source became infected. Control cages had a small amount of transmission with 2 of 72 test plants infected (3%). We believe transmission in control cages represents plants that initially tested negative with ELISA when in fact they were infected. This is the first report that soybean aphid can transmit PVY.
 

Soybean aphid has been abundant across much of Minnesota this summer.  Populations as high as 2,000 aphids per plant have not been uncommon.  Many acres of soybean have been sprayed with insecticides to control soybean aphid, and some acreage has required treatment a second time.  University of Minnesota entomologists recommend that insecticides for soybean aphid control be applied at a threshold of 250 aphids per plant and still increasing.  Soybean aphids are easily killed with most insecticides labeled for use on the crop.  Pyrethroids, e.g., Warrior and Baythroid, tend to give more persistent control of soybean aphid than do organophosphates. We rarely catch soybean aphid in the Aphid Alert traps, but soybean aphid may represent a threat to seed potato production.  Our research has shown soybean aphid to be an efficient vector of PVY.  To learn more about soybean aphid in Minnesota visit: SW Minnesota IPM Stuff 2003, vol 9.

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