Late blight strikes in potatoes;
Pest threats changing course
The confirmation of late blight in potatoes on July 31 in both Adams and Barron counties prompted the University of Wisconsin-Extension Service to issue an advisory urging the immediate fungicide treatment of all potatoes and a step-up in the monitoring for the plant disease.
As indicated in the early August edition of Wisconsin Pest Bulletin (WPB) published weekly by the Department of Agriculture, Trade, and Consumer Protection, potato growers who find infected plants need to destroy rather than merely discard them.
The WPB pointed out that the corners of fields and areas near trees are the most likely to be infected with the disease whose early symptom is a graying or apparent water-logging of the leaves before the plant dies.
The advisory to treat all potatoes immediately was based on the probable geographical pattern of dispersal of the infectious spores, the WPB explained. Tomatoes are also vulnerable to late blight.
Among other crop pests, the WPB identified the two-spotted spider mite as a major problem in soybeans throughout most of the state.
For the past month, fields in the southern and south central areas have borne the most pressure from the pest, many acres have been treated with an insecticide, repeat applications might be required on some fields, and cucumbers, squash, and other vegetables can also be infested, it reported.
Soybeans in west central counties are also being hit with whitefly populations of up to 31 per leaf, resulting in the potential of yield losses because of the loss of plant sap and the creation of sooty mold, the WPB noted.
With green cloverworm larvae being caught in southern and central counties, scouting is warranted because treatments were necessary at some sites in 2010 and 2011, the WPB observed.
After very low soybean aphid populations were typical for the first half of the crop's growing season, densities have built to moderate levels in a few fields.
Two Buffalo County fields have been the exception as individual plants had 420-630 aphids each and all of the examined plants in those fields had averages of 38-56 aphids.
For that reason, the WPB advises monitoring for another week in soybean fields, which have not yet reached the 5.5 growth stage.
In another significant reversal of insect pressure, after several weeks of emphasizing that the populations were quite high this year, the WPB reported reductions of corn rootworm beetles compared to 2011 in the annual surveys already completed in south central and central counties.
Twenty of the 99 surveyed fields were found to have populations above the average of .75 per plant that is likely to result in economic losses on corn.
As of Aug.1, the corn rootworm beetle count in the south central counties averaged one per plant compared to 1.4 a year ago while the year to year count in central counties was down from .8 to .5 per plant. Preliminary counts for this year are .4 per plant in northwestern counties and .3 in north central counties.
While the treatment window has closed in all areas for corn infested with the year's second generation of European corn borers, the WPB pointed out that 121 moths were caught in a blacklight trap near Sparta during the reporting week.
Based on that, it suggested that snap beans, sweet corn, peppers, potatoes, and tomatoes could be infested by the borer's larvae.
Although corn earworm infestations in sweet corn have been light so far, trap catches of 136 and 67 at sites near Ripon and 52 at Westport during the reporting week plus larvae of more than one inch lengths on corn in Buffalo, Dane, Eau Claire, Trempealeau, and Waushara counties suggest that more severe outbreaks are possible, the WPB indicated.
Locally heavy populations of the Japanese beetle were reported in Chippewa, Dane, Dodge, Dunn, and Eau Claire counties in late July and early August.
In corn that is still silking, the WPB indicated that a treatment might be necessary if there are three or more beetles per corn plant and if silks are cut to one-half inch before pollination reaches 50 percent.
Abundant populations of potato leafhopper nymphs and adults continue to be caught in net sweeps, the WPB noted.
In western counties such as Buffalo, Juneau, Monroe, Pepin, and Trempealeau, counts averaged 2.3 per sweep with some fields registering 4.5-5.2. The WPB advised monitoring the densities at least through August.
Weeds in soybeans are also a lingering concern - one complicated at this stage of the growing season by the pre-harvest intervals required with some herbicides and the potential of active ingredient carryover to subsequent crops, the WPB observed.
Despite the drought, which plagued most of the state until recent weeks, weeds that were nonetheless growing vigorously in some soybean fields were the giant ragweed, common lambsquarters, pigweed, velvetleaf, and volunteer corn.
For apple orchards, threats persist from apple maggot flies until early September, catches of spotted tentiform leafminer moths were as high as 730 to 900 at sites from Racine to Marathon counties, very high catches (topped by 43 at Oneida) of codling moths are recorded, and scouting should continue for oblique banded leafminers, the WPB stated.