Aphid Alert 2003, No. 2, June 27

Potato insect update for the Northern Great Plains, week ending June 27

Last week's prolonged (45 h) low level jet (wind event) and accompanying thunderstorms provided perfect conditions for the long distance transport of aphids and leafhoppers from the south and their deposition across the Northern Great Plains.

Adult potato leafhoppers are abundant in the region and early stage nymphs (mostly first instar) can now be found feeding on the under side of potato leaves. Potato leafhoppers have 5 nymphal instars. Insecticidal treatments should be applied before the nymphs reach fourth and fifth instars. The recommended threshold for treatment to control potato leafhopper is very low: 10-15 late instar nymphs per 100 true leaves (as opposed to leaflets) will cause economic damage. Potato leafhoppers are easily killed by almost any persistent insecticide. Dimethoate, Asana, and Baythroid (Leverage) are examples of products that work very well. Application of these products at rates as low as 1/4 label rate provide good control of potato leafhopper nymphs and tend not to flare aphid outbreaks.

Potato aphids are very abundant in some fields, e.g., densities approaching 1 per leaf were observed near Climax, MN. The most abundant winged aphid captured in suction traps in Manitoba last week was buckthorn aphid. Green peach aphid numbers are still very low, but potato colonization has been detected. Typically, we capture few winged green peach aphids in our traps before the end of July. The initial colonizers probably are the progeny of long distance migrants that presumably arrived virus-free. Of course, these aphids can acquire and move virus if they encounter infected potatoes.

Colorado potato beetle eggs are now hatching in northern Minnesota and eastern North Dakota. In warm weather these insects can complete a larval instar in as little as 2 days. They have only 4 instars and the last two are voracious feeders. Growers should monitor their fields closely and be prepared to spray next week. Insecticides applied when Colorado potato beetle are just hatching or in their early instars are much more effective than insecticides applied when the beetle larvae are in their third or fourth instars. Insecticide resistance can limit product choice. If a nicotinyl insecticide, e.g., Admire or Platinum, was applied at planting, it shouldn't be necessary to spray for 1st generation Colorado potato beetle control. However, if foliar sprays are subsequently used on the crop, you should chose a different class of insecticide.

Status of the Aphid Alert Network

In 2003, the Aphid Alert Network will operate aphid traps at Baker, Climax, Gully, Karlstad, and Williams in Minnesota, and at Hoople, Grand Forks, and Walhalla in North Dakota. One suction trap and two green-tile pan traps have been installed at each location. These traps are now in operation. The first samples from Minnesota and North Dakota will be collected, sorted and the results reported next week. Manitoba aphid captures during the past week, provided by McLaren et al., are reported below.

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.

Manitoba Aphid Data, week ending June 26

table showing Manitoba aphid data for the week ending June 26, 2003

Late Blight Warning

A late blight alert has been issued for: Perham, MN (June 26, 2003), Brooten, MN (June 25, 2003), Morris, MN (June 23, 2003). If you are near these sites, the weather has been favorable for the spread of late blight. It is strongly recommended that you scout your fields on a regular basis and begin a regimented fungicide spray program to prevent the development and possible spread of late blight in your area.

Potato late blight status reports

Nitrogen Management Following the Storms

by Carl Rosen, 612-624-7711 or crosen@soil.umn.edu

Many potato-growing areas received in excess of 10 inches of rain over the past week, raising the concern about how much nitrate leached during that time. Most sandy soils can hold between 1 and 3 inches of water in the top 18 inches, which is generally considered to be the effective rooting zone for potatoes. When water inputs are greater than the water-holding capacity of the root-zone, nitrate leaching will occur. The amount of nitrate that actually leached during the past week depends on many factors including the amount of nitrate that was in the profile at the time of the rainfall, which in turn depends on the rate/timing of N application and plant uptake of N. For example, in a study conducted at Becker a few years ago using excessive irrigation to simulate the effects of leaching rainfall, we estimated that about 60 lb/A nitrate-N leached following a 3.5-inch irrigation when 250 lb N/A (as urea) was applied. In comparison, about 50 lb/A nitrate-N leached when 125 lb N/A (as urea) was applied. The excessive irrigation was applied 30 days after emergence and 10 days following hilling. Strategies for managing nitrogen following these excessive storms will depend on the type of potato being grown. Suggestions for managing N are as follows:

For early season potatoes such as Red Norland (kill date before July 15) – most N has already been taken up by the plant. Additional N at this time will generally not be beneficial and may actually delay maturity. No corrective action is suggested.

For midseason potatoes such as Goldrush (kill date between July 15 and Sept.1) – additional N application may be beneficial. Apply 20 to 30 lb N/A through the irrigation system within the next week and follow up with petiole analysis about 5 to 7 days later. Maintain petiole nitrate-N between 1.0-1.5% during tuber bulking. Apply additional N as needed using petiole analysis as a guide.

For late season potatoes such as Russet Burbank (kill date after Sept. 1) – about 50% of the N has already been taken up by the crop assuming an April planting date. Apply 20 to 30 lb N/A within the next week and follow up with petiole analysis 5 to 7 days later. Maintain petiole nitrate-N between 1.1 and 1.6% during tuber bulking. Additional N will likely be needed for late maturing varieties and should be applied using petiole analysis as a guide.

Subscriber Alert

This is the second issue of Aphid Alert for the 2003 growing season. This newsletter is intended to alert seed potato producers in the Northern Great Plains to flight activity by aphid species 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 will be updated as additional data becomes available. To become an e-mail subscriber to Aphid Alert 2003, send us an e-mail message with the word "subscribe" in subject line. Note that current subscribers need not resubscribe. 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.