Aphid Alert 2002, No. 9, August 30
Aphid Situation in Week Ending August 30
Aphid flight activity remained high across the Northern Great Plains in the week ending August 30. Captures of green peach aphid per trap were slightly higher than in the previous week in Manitoba, but were down by a third in Minnesota and North Dakota. This regional difference may reflect the earlier maturing of green peach aphid host plants in more southern locations. Turnip aphid continued to be the most abundant aphid in Minnesota and North Dakota traps, representing 55% of the week's captures, but numbers were down from over 80% the week before. The decline of turnip aphid is associated with the maturation and harvest of canola. Bird cherry-oat aphid numbers remained high this week even though much of the small grain acreage has been harvested. Cumulative numbers of bird cherry-oat aphid are higher than in any of the seven previous years of the Aphid Alert network. The exceptional abundance of this aphid is of particular concern because bird cherry-oat aphid is an efficient vector of PVY and was implicated as a primary contributor to previous PVY epidemics in the northern great plains. The recent wet weather experienced in much of the northern great plains has provided favorable conditions not only for late blight, but also for fungal aphid pathogens. Mycoses (fungal infection) appear to be prevalent in green peach aphid on potato and colonizing populations are generally in decline. Potato fields with green peach aphid infestations are now producing large numbers of winged aphids. This greatly increases the risk of virus being spread from production fields to seed potatoes. Seed potato fields should be vine-killed as soon as economically and practically feasible to minimize risk of virus spread. If the crop must be permitted to grow, mineral oil sprays are the most effective means of minimizing PVY spread.
Weather Conditions and Late Blight
Heavy rains blanketed much of northern great plains during the past week. Crops were damaged by water logged fields and even standing water in many locations in southern Manitoba, eastern North Dakota and northwestern Minnesota. Counties in northeastern North Dakota that had largely escaped damage from earlier storms were hit hard this time. Six or more inches of rain was not uncommon, and Park River, N.D., received 12. In addition to direct damage, the soggy fields provide highly favorable conditions for development of potato late blight. The past week brought the year's first report of late blight in northwestern Minnesota. Growers should continue fungicide applications and scout fields for the presence of late blight. Late season late blight and rain can cause serious tuber infection and decay in storage. It is important to continue fungicide application until vine kill to limit tuber infection. Data show that late season application of the fungicides Gavel and Omega provide the best protection from late blight tuber infection. Dead vines do not support late blight, but green vines and partially dead vines should continue to be sprayed with fungicide to protect against late blight.
Potato late blight status reports
This is the ninth issue of Aphid Alert 2002. This newsletter is intended to alert seed potato producers in the Northern Great Plains to flight activity by aphid species that are known to be potential vectors of potato viruses. We report results weekly on the WWW, by e-mail to subscribers, and by surface mail to all Minnesota and North Dakota seed potato growers. The hard copy and e-mail versions of Aphid Alert report aphid capture data available as of the date they are mailed. The WWW version is updated as additional data becomes available. To become an e-mail subscriber, send us an e-mail message with the word "subscribe" in subject line. If you have no interest in receiving this newsletter by e-mail, please reply with the word "unsubscribe" in the subject line. Some e-mail subscribers may not wish to receive messages containing graphics. If so, reply with the the words "no graphics" in the subject line.
Minnesota locations: mean aphid captures per trap during the week ending August 30
North Dakota locations: mean aphid captures per trap during the week ending August 30
Manitoba locations: mean aphid captures per trap during week ending August 30 (see Manitoba Agriculture and Food Website)
Note: aphid species ID has yet to be completed for Glenboro and Kemnay.
Wisconsin, South Dakota and Nebraska locations: mean aphid captures per trap during the week ending August 30
Figure: Trap locations in the Aphid Alert network in 2002
Figure: Cumulative captures of green peach aphid (per trap), 1992-1994, and 1998-2001. Three distinctly different seasonal patterns of green peach aphid abundance have been observed. In 1998 and 1999 green peach aphid were abundant with total captures approximately an order of magnitude greater (10X) than that of 1992, 1993 and 1994, and two orders of magnitude greater (100X) than that of 1993 and 2001. For the Minnesota and North Dakota seed potato industry, low green peach aphid pressure in 1994 coincided with the end of a multi-year PVY epidemic and the low green peach aphid pressure of 2001 coincided with the end of a multi-year epidemic of PLRV. During the week ending August 30, green peach aphid flight activity increased 80% over the previous week, but overall, 2002 has been a year of only moderate green peach aphid pressure.
Figure: Cumulative captures of bird cherry-oat aphid (per trap), 1992-1994, and 1998-2001. This aphid comes off wheat and other cereals. The species is typically abundant in the Northern Great Plains. In our area, green peach aphid and bird cherry-oat aphid appear to be the two most important vectors of PVY. Lowest abundance of bird cherry-oat aphid during the years the Aphid Alert network has operated was in 1994, which, coupled with low green peach aphid pressure, coincided with the end of a multi-year epidemic of PVY. Bird cherry-oat aphid flight activity has been higher in 2002 than in previous years of the Aphid Alert network.
Figure: Cumulative captures of bird cherry-oat aphid and green peach aphid (per trap), 1992-1994, and 1998-2001. These two aphids have been implicated as the primary vectors of PVY in the Northern Great Plains (see Aphid Alert 2002, no. 2). Other potential vectors of PVY, e.g., greenbug and turnip aphid are also abundant this year.