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Warm weather this week provided an opportunity for farmers to make up lost ground after spring fieldwork delays. Afternoon high temperatures were seasonal for late May, and planting of corn, oats, potatoes, and soybean crops continued statewide. Scattered showers and isolated storms developed throughout the week, though most of Wisconsin experienced several days of clear weather for planting and weed control. Soybean planting reached 73 percent complete, nine days behind last year but five days ahead of the five-year average. Planting progress for corn also caught up to the average, and farmers are now shifting priorities to cutting hay and managing weeds.
The warmer weather spurred insect activity, resulting in a noticeable increase in pest pressure in field, fruit and vegetable crops. Damage by alfalfa weevil larvae will become more pronounced next week, especially if predicted rain and cooler temperatures delay harvest of the first crop.
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Alfalfa weevil larva | K. Hamilton DATCP
 
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Surveys to assess alfalfa pest populations and distribution are underway. As expected for early June, alfalfa weevil larvae and pea aphids are currently the most abundant insects in alfalfa sweep net samples. Surveys across Dane, Green, Iowa, Jefferson, Lafayette, and Rock counties in south-central Wisconsin indicate alfalfa weevil larval counts range from 0.1-4.8 per sweep. The average is approximately 1.4 per sweep. Estimates of leaf tip feeding generally vary from 10-20%, though a few scattered fields with higher larval pressure (3.0-4.8 per sweep) are showing above-threshold levels of tip defoliation (40-60%). The peak weevil damage period occurs between 600 and 800 degree days (sine base 48°F) and will begin in the next week. With the first alfalfa harvest only 13% complete, scouting is strongly recommended for unharvested first-crop fields next week and until new growth of the second crop is established.
Pea aphid counts are typical for early June at approximately 1-10 aphids per sweep. A few fields have higher counts of 12-19 aphids per sweep, which is still far below the 100-per-sweep threshold for alfalfa. The appearance of winged aphids in the past two weeks signals that scouting should begin in pea fields, where the threshold is much lower at 35 aphids per sweep or two per plant. For alfalfa, harvesting fields on time is usually the most effective approach to reducing pea aphid populations.
Surveys this week also found potato leafhoppers in 78% of the 46 fields checked. Counts were very low at less than 0.02 per sweep (14 per 100 sweeps). While first-crop alfalfa is generally not susceptible to injury from the migratory leafhoppers arriving in May, populations can build quickly in second-crop alfalfa under favorable conditions. As a reminder, the economic threshold for leafhoppers in alfalfa taller than 12 inches is 2.0 per sweep. |
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Codling moth trap liner | NC State Extension
 
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Spring codling moth flights have resumed, with a sharp increase in activity reported from southern Wisconsin apple orchards. Six of 27 cooperator sites reported very high counts of 14-32 moths this week. Most southern and central monitoring locations (those that do not use mating disruption) have recorded a sustained flight and biofix as of June 2.
Apple orchards with a history of high codling moth pressure that register a large first flight of >10 moths per week are candidates for a first larvicide application at 250 degree days (base 50°) from the biofix. At this time of year, 250 degree days are equal to about 14-21 days, depending on location.
For orchards where the spring codling moth flight has been inconsistent, growers may delay applications until 350 degree days after biofix, when approximately 60% egg hatch will be targeted by one application. With the intermittent flights observed this spring, many orchards will likely wait until 350 degree days to apply a larvicide.
A density of one trap per 2.5 acres, or one per five acres where blocks are uniform in size, shape and topography, is suggested for accurate codling moth monitoring. |
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TRV on bleeding heart | Anette Phibbs DATCP
 
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The Plant Industry Lab has identified tobacco rattle virus (TRV) in a variety of herbaceous ornamental plant samples submitted by nursery inspectors in the past two weeks. The affected hosts and cultivars were: anemone ‘Honoring Jobert,’ astilbe ‘Burgundy Red’ and ‘Fanal,’ bleeding heart ‘Valentine,’ hellebore ‘X Royal Heritage Mix,’ Japanese spurge ‘Green Carpet,’ petunia ‘Crazytunia BlackBerry Jam,’ and speedwell ‘Icicle.’
Tobacco rattle virus is a significant pathogen in the nursery industry due to its wide host range and multiple modes of transmission. Over 400 different plants can be infected with TRV, including popular ornamentals like astilbe and bleeding heart, common weeds such as chickweed and hairy nightshade, and vegetables like beans, potatoes, and spinach. On potatoes, the disease is referred to as corky ring spot. Virus transmission can occur with contaminated tools, plant sap, grafting, seed, and stubby root nematodes (Trichodorus and Paratrichodorus spp.). As the primary vectors of TRV, stubby root nematodes are particularly challenging to manage in agricultural or nursery fields because populations can persist in the soil and cause infection year after year.
Symptoms of TRV vary by host and environmental conditions. Infected plants may exhibit chlorotic mottling, ringspots, chevrons, spots or streaks, wavy lines, and necrotic lesions. In infected potato tubers, the symptoms are necrotic arcs, rings, and corky spots in the tissue. Plants infected with TRV remain infected indefinitely and must be removed and destroyed to eliminate the virus. |
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