Amanda Gevens, Chair, Professor & Extension Vegetable Pathologist, UW-Madison, Dept. of Plant Pathology, 608-575-3029, Email: gevens@wisc.edu
Early blight of potato/tomato. Accumulations of P-days this past week were between 59-66 across the state of Wisconsin. In all locations and all planting dates, potato fields have surpassed the threshold and should receive (and continue to receive) preventative fungicide applications for early blight management. Hotter days generate roughly 10 P-days per day if you are looking ahead to likely accumulations and preventative fungicides.
Late blight of potato/tomato. Accumulations of Blitecast DSVs were extremely low this past week in the 7 sites detailed in our table, below, ranging from 0 to 1. The usablight.org website (https://usablight.org/map/) indicates a new report of tomato late blight from Henderson County North Carolina on 8/9/23 (genotype not yet known). No new reports from NY or Canada. So far, all characterizations of the late blight pathogen identified in North America this growing season have resulted in the US-23 type. Fungicides for the management of late blight in tomato and potato crops are provided: https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin. A specific list of fungicides for potato late blight in Wisconsin was also offered in a special report shared via email on July 28.
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 2023. 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 is from weather stations that are placed in potato fields in each of the four locations, as available. Data from an alternative modeling source: https://agweather.cals.wisc.edu/vdifn will be used to supplement as needed for missing data points and for additional locations (indicated with *). Data are available in graphical and raw formats for multiple locations at: https://vegpath.plantpath.wisc.edu/dsv/.
| Planting Date | 50% Emergence Date | Disease Severity Values (DSVs) through 8/12/2023 | Potato Physiological Days (P-Days) through 8/12/2023 | ||
| Spring Green* | Early | Apr 3 | May 9 | 9 | 750 |
| Mid | Apr 17 | May 12 | 9 | 728 | |
| Late | May 10 | May 23 | 9 | 660 | |
| Arlington* | Early | Apr 5 | May 10 | 7 | 758 |
| Mid | Apr 20 | May 15 | 7 | 721 | |
| Late | May12 | May 25 | 7 | 661 | |
| Grand Marsh | Early | Apr 5 | May 10 | 6 | 719 |
| Mid | Apr 20 | May 15 | 6 | 686 | |
| Late | May 12 | May 25 | 6 | 634 | |
| Hancock | Early | Apr 10 | May 17 | 7 | 687 |
| Mid | Apr 22 | May 19 | 7 | 681 | |
| Late | May 14 | May 28 | 7 | 632 | |
| Plover | Early | Apr 14 | May 19 | 11 | 672 |
| Mid | Apr 24 | May 20 | 11 | 667 | |
| Late | May 19 | May 29 | 11 | 618 | |
| Antigo | Early | May 1 | May 28 | 11 | 596 |
| Mid | May 15 | June 3 | 11 | 545 | |
| Late | June 7 | June 23 | 11 | 410 | |
| Rhinelander* | Early | May 7 | June 1 | 4 | 560 |
| Mid | May 18 | June 5 | 4 | 525 | |
| Late | June 9 | June 24 | 4 | 402 | |
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. 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 2023 Commercial Vegetable Production in Wisconsin Guide, Extension Document A3422. https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin
Cucurbit Downy Mildew. The Cucurbit Downy Mildew forecasting webpage (https://cdm.ipmpipe.org/) is not forecasting the movement of the pathogen, but the group is offering reporting of findings of cucurbit downy mildew from the US (see current map below showing red counties with new reports from https://cdm.ipmpipe.org/. Dr. Mary Hausbeck reported cucumber downy mildew in commercial fields in 9 Michigan Counties as of 8/9/2023. To date, there have been no reports of downy mildew here in WI. If reports arise, we should be considering preventative treatment of cucumber and melon crops here in due to the likelihood of the disease resulting from clade 2 downy mildew.
Late blight ‘look-alike’ noted in central Wisconsin potato. While Phytophthora nicotianae, a cousin to the late blight pathogen Phytophthora infestans, is typically a soilborne pathogen infecting roots and tubers, lesions can sometimes form on the foliage of tomato and potato (its host range includes citrus, tobacco and ornamentals). The presence of this ‘nicotianae blight’ indicates leaf wetness and higher temperatures and the lesions are often seen in potatoes along the irrigation pivot tracks. Phytophthora nicotianae can form overwintering soilborne structures known as oospores and chlamydospores. Foliar infections look menacingly like late blight, but the lesions do not exhibit sporulation (pictures below). It’s important to be aware of this disease and to have it diagnosed to confirm the underlying pathogen. Late blight is typically favored under cooler temperatures than P. nicotianae and does sporulate profusely, making spread much more challenging to control. Foliar fungicide programs which protect against late blight can also control ‘nicotianae blight’. More information can be found in a nice web article by Dr. Jean Ristaino, Amanda Saville, Inga Meadows, and Mary Lorscheider from North Carolina State University at: https://plantpathology.ces.ncsu.edu/2018/06/phytophthora-nicotianae-causing-severe-disease-on-potato-and-tomato-in-north-carolina/
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/
Emergence of adult CPB from the first full generation is now nearly complete in much of central and even northern Wisconsin. Unlike their overwintered parents, this next generation (2nd generation) of adults are very active feeders and can quickly defoliate unprotected foliage. Second generation adults normally appear in mid- to later July and if numbers are large, severe defoliation of the crop can ensue. Generally, second generation adults will produce another generation of larvae. Looking ahead to the 14 day forecast weather conditions, these adults will undoubtedly produce a complete, second generation and the subsequent adults (3rd generation) will likely complete development by mid September. Typically, there are only two discrete generations of beetles per year in South-Central Wisconsin and only a single generation in Northern Wisconsin. Again this year, we are likely to see a full 3rd generation complete development in much of central Wisconsin.
The magnitude of the 2nd (and even 3rd) generation often results from the control successes (challenges ) experienced in controlling the 1st generation throughout June. If many larvae escaped control, then numbers of 2nd generation adults can be very challenging to manage. On the other hand, if larvae were very adequately managed with well-timed, at-plant or foliar sprays, then the 2nd generation adults can be easier to manage.
Colorado Potato Beetle – 2nd generation (13 Aug 2023) – https://agweather.cals.wisc.edu/vdifn
Treatment thresholds for control of CPB during the 1st generation are based upon the stages of development. In order to use compounds like spinosad (Blackhawk), spinetoram (Delegate), abamectin (Agri-Mek), novaluron (Rimon), or tolfenpyrad (Torac) correctly, we aim for the peak or midpoint of early larval feeding. At this time in mid to late June, we often have only 1-2% defoliation. But if we wait until defoliation estimates reach 10% or greater, we will be too far behind and will NOT get adequate control.
During mid-July and early August, however, when 2nd generation adults are emerging, actively feeding and laying eggs, we often resort back to the use of defoliation thresholds. Recall that potato plants can tolerate varying levels of defoliation before they suffer yield losses. The level of tolerance depends on the plant’s growth stage. Flowering plants can tolerate the least defoliation, (e.g. only 5-10% of total leaf area). Post-flower potato is able to withstand a slightly higher amount of defoliation, but since this is a critical point for tuber formation and bulking, producers and pest management practitioners should limit the amount of feeding done by CPB not to exceed 5-8%. This estimate should be based upon a field estimate of defoliation, and not simply a hot spot or edge of a field adjacent to a recently killed and neighboring field. Finally, late season feeding on potato that is beginning to senesce is the least critical period for yield loss.
Recall that nearly all foliar-applied compounds should be applied as a series of two, successive applications spaced 7–10 d apart to improve control of staggered life stages. Several of the compounds we propose for 2nd generation use (MoA Group 28 anthranilic diamides) may have less activity on other key potato pests (e.g., potato leafhopper and colonizing aphids); so it is important to scout for secondary pests. The decision to apply any insecticide for this next generation of CPB should be completed for each field based on scouting results and established economic damage. Tools for use against this second generation management options are available in the attached listing.