Update 6 – June 4, 2022

Amanda Gevens, Chair, Professor & Extension Vegetable Pathologist, UW-Madison, Dept. of Plant Pathology, 608-575-3029, Email:  gevens@wisc.edu, Lab website:  https://vegpath.plantpath.wisc.edu/

Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations.  Many thanks to Ben Bradford, UW-Madison Entomology; Stephen Jordan, UW-Madison Plant Pathology; and our grower collaborator weather station hosts for supporting this disease management effort again in 2022.  A Potato Physiological Day or P-Day value of ≥300 indicates the threshold for early blight risk and triggers preventative fungicide application.  A Disease Severity Value or DSV of ≥18 indicates the threshold for late blight risk and triggers preventative fungicide application.  Red text in table indicates threshold has been met or surpassed.  TBD indicates that data are To Be Determined as time progresses.  Weather data used in these calculations will come from weather stations that are placed in potato fields in each of the four locations, once available.  Data from an alternative modeling source: https://agweather.cals.wisc.edu/vdifn will be used to supplement as needed for missing data points.  We currently have our Grand Marsh, Hancock, and Plover weather stations up and running.  We will add the Antigo station soon.  Data are available in graphical and raw formats for each weather station at:  https://vegpath.plantpath.wisc.edu/dsv/.

Location Planting Date 50% Emergence Date Disease Severity Values (DSVs) 6/4/2022 Potato Physiological Days (P-Days) 6/4/2022
Grand Marsh Early Apr 5 May 10 1 177
Mid Apr 20 May 15 1 136
Late May 12 May 25 1 77
Hancock Early Apr 7 May 12 1 153
Mid Apr 22 May 17 1 119
Late May 14 May 26 0 75
Plover Early Apr 7 May 15 1 109
Mid Apr 24 May 20 1 100
Late May 18 May 27 0 64
Antigo Early May 1 Jun 3 0 10
Mid May 15 TBD TBD TBD

In addition to the potato field weather stations, we have the UW Vegetable Disease and Insect Forecasting Network tool to explore P-Days and DSVs across the state (https://agweather.cals.wisc.edu/vdifn).  This tool utilizes NOAA weather data (stations are not situated within potato fields).  In using this tool, be sure to enter your model selections and parameters, then hit the blue submit button at the bottom of the parameter boxes.  Once thresholds are met for risk of early blight and/or late blight, fungicides are recommended for optimum disease control.  Fungicide details can be found in the 2022 Commercial Vegetable Production in Wisconsin Guide, Extension Document A3422, linked here: https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin.


Vegetable Insect Update – Russell L. Groves, Professor and Department Chair, UW-Madison, Department of Entomology, (608) 698-2434 (mobile), e-mail rgroves@wisc.edu. Vegetable Entomology Webpage: https://vegento.russell.wisc.edu/

Colorado potato beetle (CPB) – (https://vegento.russell.wisc.edu/pests/colorado-potato-beetle/). Continue to scout populations of CPB adults especially as nearly all potato plants have emerged and most hilling operations are nearly complete in central Wisconsin. In southern Wisconsin, adults have now colonized much of the field and many egg masses have hatched into 1st instar larval stages (Fig. 1). Potato plants can tolerate varying levels of defoliation before they will suffer yield losses. The level of tolerance depends on the plant’s growth stage. Flowering plants can tolerate the least defoliation, only 5-10% of total leaf area. Post-flower potato is able to withstand a slightly greater amount of defoliation, but since this is a critical point for tuber formation and bulking growers should limit the amount of feeding done by CPB. Growth stages ahead and behind these two points are more tolerant of defoliation.

Figure 1. Egg masses and early instar larvae of the Colorado potato beetle.

Now is the time to begin looking for small larvae (1st and 2nd instars) on the crop. To do this effectively, focus the majority of scouting for larvae on the tops of plants. In potato, small larvae will typically move to the newly expanding foliage at the crown of the plant. Larvae are very small at this stage and may not be directly apparent unless the emerging foliage is carefully parted. First instar larvae may be easily confused with dirt or debris until closer inspection.

For most CPB chemical management tools, timing application occurs with the appearance of first instar larvae in the field. Early instar larvae are the most susceptible life stage for chemical management, applications should be timed with the midpoint of egg hatch. The first application should be followed up in 5-7 days with a second application of the same compound depending on the formulation and label restrictions. Refer to the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for a list of registered insecticides and management recommendations.

Forecast temperatures into next week will remain at or below average, and thus developing larval populations may be slowed somewhat.  Recall that our insecticide programs are built around product rotations across generations, and 2-3 successive applications of the same, reduced-risk (RR) mode of action (MoA) within a generation (Fig. 2).

Figure 2. Application timing of reduced-risk (RR) modes of action (MoA) for the control of CPB.

Applications of novaluron (Rimon), tolfenpyrad (Torac), spinetoram (Radiant, Delegate), or abamectin (Agri-Mek) should be applied when nearly 50-75% of egg masses have hatched, and a few 2nd instar larvae are present from the earliest hatched egg masses.  This milestone may occur at the end of the upcoming week at locations in southern Wisconsin, with egg masses continually being deposited this coming week in southern regions.  Recall, these 1st generation larvicides often require 2-3 subsequent re-applications spaced on a 7-10 day interval to achieve sufficient control of this damaging generation.  In central Wisconsin, CPB adults are still colonizing fields, and mating and egg laying is underway along field perimeters.  Initial egg hatch was just underway later this week in field perimeters.  With cooler and more moderate daytime high temperatures forecast for the coming week, it may not be time to initiate first larval sprays until the following week. Careful scouting will reveal the exact timing!

For detailed information on insecticide options for CPB management in Wisconsin, refer to this document.


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