Amanda Gevens, Chair, Professor & Extension Vegetable Pathologist, UW-Madison, Dept. of Plant Pathology, 608-575-3029, Email: gevens@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. 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 comes from weather stations that are placed in potato fields in each of the four locations. 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) June 12 |
Potato Physiological Days (P-Days) June 12 |
|
| Grand Marsh | Early | April 2 | May 10 | 14 | 200 |
| Mid | April 10 | May 15 | 14 | 190 | |
| Late | May 1 | May 23 | 8 | 128 | |
| Hancock | Early | April 5 | May 12 | 7 | 215 |
| Mid | April 15 | May 15 | 7 | 207 | |
| Late | May 5 | May 23 | 2 | 145 | |
| Plover | Early | April 7 | May 12 | 13 | 202 |
| Mid | April 20 | May 20 | 10 | 157 | |
| Late | May 7 | May 30 | 5 | 95 | |
| Antigo | Early | April 26 | May 28 | 0 | 107 |
| Mid | May 10 | June 5 | 0 | 69 | |
| Late | May 20 | June 13 | TBD | TBD | |
The persistence of hot and dry weather has kept disease risk low, however, heat in an irrigated crop still produces some risk which is evidenced in the gradually accumulating DSVs for earliest planted potatoes in southern and central Wisconsin. It is likely that by next week we will begin to meet thresholds for onset of early blight and preventative fungicides are necessary. There have been no reports of late blight in tomato or potato in the US yet this season (usablight.org). 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 2021 Commercial Vegetable Production in Wisconsin Guide, Extension Document A3422: https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A3422-2021.pdf
Jed Colquhoun, Professor and Extension Specialist, IPM Program Director, UW-Madison Department of Horticulture, colquhoun@hort.wisc.edu, https://specialtycrops.wisc.edu/
Optimizing herbicide performance during droughty conditions. Many non-irrigated areas of the Upper Midwest are becoming significantly droughty as our weather prematurely enters the dog days of summer. There are several agricultural impacts of these hot and dry conditions, including on weed growth and management. While we can’t make it rain, we can use awareness of how drought affects weeds to adjust plans in a way that optimizes management. Here, we offer a very short summary of weed growth and herbicide performance in a drought and a few tips on how to adjust plans in these conditions.
Weed growth in a drought: Annual weeds are biologically conditioned to germinate and emerge when weather conditions favor survival and reproduction. There are many factors that affect seed dormancy, such as soil temperature, seed age, seed coat mechanical breakdown and soil moisture. Weed seed that is close to the soil surface is most affected by drought and we often see fewer weed seeds break dormancy and germinate in these conditions. And, some weeds that do germinate will die before establishment because of inadequate moisture to support growth.
That’s the good news. Unfortunately, there are two negative aspects of weed biology during a drought that affect successful management. First, in the absence of annual small-seeded weeds that germinate from near the soil surface, the weed spectrum shifts to those that are often harder to control: perennials regrowing from deep and established root systems such as yellow nutsedge and Canada thistle and large- seeded weed species that can germinate and emerge from deeper in the soil where moisture may be more available. Second, emerged weeds become “hardened off” and more difficult to control during a drought. In particular, they will put on a thicker, waxier cuticle on leaf surfaces to protect from water loss.
Herbicide performance during a drought and what we can do to optimize it:
There are two common misconceptions that we need to first address: soil residual herbicides don’t prevent weed seed germination and stressed weeds are actually harder to control than healthy, actively growing weeds. In a very general sense, herbicides act by blocking an active biological process required for normal weed growth, and so that target biological process needs to be happening for the herbicide to work.
For soil residual herbicides that are applied prior to weed germination and emergence (pre-emergent herbicides), the herbicide needs to be near the weed seed in the soil as it germinates and the seed needs to break dormancy. In many cases we rely on soil moisture from moderate rainfall to incorporate pre- emergent herbicides into the target weed germination zone. If this doesn’t happen weed seed can germinate and emerge rapidly through herbicide sitting on the soil surface. When rainfall is scarce or sporadic, it’s worth watching the forecast even more carefully to try to time crop planting and/or pre- emergent herbicide application to the best chance of adequate soil moisture as outlined on the herbicide label. Editorial note: this is obviously easier said than done. I have a beet trial where I planted just before an 80% chance of showers that never happened and has sat with herbicide on the powder dry soil surface for two weeks now. When it does finally rain, I don’t expect much out of the pre-emergent herbicide and have started planning for a more aggressive post-emergent program. This is a time when scouting is more important than ever so that post-emergent control options can be considered and timed relative to unanticipated changes in the weed spectrum.
If the droughty conditions persist during the remainder of the growing season and beyond, we’ll need to consider another consequence: the risk for extended soil residual carryover. In short, the vast majority of herbicides rely on soil microbes for breakdown and these microbes aren’t very active in dry soils. But, let’s cross that bridge if and when we get to it in a follow-up article – just be aware of what might be planted next as you think about your herbicide choices and rotational restrictions.
For post-emergent herbicides, as always be very careful to follow the details of the label to optimize performance. Be sure to use the correct adjuvant to help the herbicide penetrate drought-protected leaf surfaces, follow the weed growth stages carefully, and consider adjusting the rate within the labeled ranges with a more challenging target in mind.
In the meantime, stay cool and hydrated and know that I’ll be right alongside you checking the weather forecast day and night…
Ground cracking in a powder dry Wisconsin mint field on June 7, 2021.
Vegetable Insect Update – Russell L. Groves, Professor and Department Chair, UW-Madison, Department of Entomology, 608-262-3229 (office), (608) 698-2434 (cell), e-mail: rgroves@wisc.edu
Vegetable Entomology Webpage: https://vegento.russell.wisc.edu/
Onion thrips – (https://vegento.russell.wisc.edu/pests/onion-thrips/). Onion thrips overwinter as adults and later instar larvae in legume and grain fields and along weedy field edges. They can often overwinter in cull onions (bulbs) left in the fields from the previous year. Females are parthenogenetic (can reproduce without mating) and lay eggs just beneath the leaf’s surface. Eggs hatch after 5-10 days (temperature dependent), and immatures develop through 4 instar stadia in 15-25 days (again, temperature dependent. Development of the last two immature stages occurs in the soil, without feeding. After the fourth molt, adult female thrips return to the plant. Again, dependent upon prevailing daytime high and nighttime low temperatures, thrips can produce 5-8 generations per year, and outbreaks are most likely to occur in hot, dry weather.
Although we experienced some unseasonably low temperatures in mid-May, the daytime highs (and evening lows) have been back up around avg maximums. Unfortunately, the annual precipitation estimates lag by approximately 7.5” of moisture increasing the risk of onion thrips outbreaks in the region (Fig. 1). Thrips management should be considered ‘early’ before they have a chance to develop large populations under ideal weather conditions. Due to their small size and reclusive habits, onion thrips are difficult to monitor, and it is important to scout for the larvae in the leaf whorls.
Onion thrips are an important annual pest of onion, but they can also attack nearly all garden crops. Serious damage is generally limited to onions, cauliflower, cabbage, cucumber, and greenhouse tomatoes. Monitor plants weekly, and scout plants on field edges initially as thrips are more common at borders in the early part of the season. 1-3 immature thrips per onion leaf is a widely accepted threshold for chemical treatment, and this varies according to the compound considered for use. Feeding damage causes whitish blotches and dry, silvered areas on leaves, decreased pollen set, and, under heavy infestations, brown leaf tips and distorted or undersized bulbs. Both adults and larvae can cause silvery streaking on leaves, which becomes dry and yellow. Immature thrips prefer to feed on the youngest leaves.
Onion thrips are an important annual pest of onion, but they can also attack nearly all garden crops. Serious damage is generally limited to onions, cauliflower, cabbage, cucumber, and greenhouse tomatoes. Monitor plants weekly, and scout plants on field edges initially as thrips are more common at borders in the early part of the season. 1-3 immature thrips per onion leaf is a widely accepted threshold for chemical treatment, and this varies according to the compound considered for use. Feeding damage causes whitish blotches and dry, silvered areas on leaves, decreased pollen set, and, under heavy infestations, brown leaf tips and distorted or undersized bulbs. Both adults and larvae can cause silvery streaking on leaves, which becomes dry and yellow. Immature thrips prefer to feed on the youngest leaves.
Active ingredients that currently have the greatest impact upon immature onion thrips include:
- abamectin (Agri-Mek SC & many registrations), MoA Group 6 (plus pre-mixes), 30-day PHI, ideal with mild penetrating surfactant (NIS), pH < 8, avoid high UV
- cyantraniliprole (Exirel, Minecto Pro), MoA Group 28 (plus pre-mixes), 7-day PHI, ideal a methylated seed oil (MSO) adjuvant, pH < 5, avoid high UV
- methomyl (Lannate LV), MoA Group 1A, 7-day PHI, pH < 0
- spinetoram (Radiant, Delegate), MoA Group 5, 1 day PHI, pH < 0
- spirotetramat (Movento HL), MoA Group 23, 3-day PHI, ideal with mild surfactant, pH < 6.8, avoid high UV
- tolfenpyrad (Torac), MoA 21, 7-day PHI, pH < 7.0
Due to insensitivity among populations of onion thrips, synthetic pyrethroids (MoA Group 3) often do not provide adequate control of developing thrips populations. Consider rotating products as a series of two, successive applications of the same mode of action (MoA) class, and then rotating to a new MoA.
Colorado potato beetle – (https://vegento.russell.wisc.edu/pests/colorado-potato-beetle/). Adults continue to colonize potato fields along field edges through the past week, but many egg masses are now present and hatching into early larval populations. Forecast temperatures into next week will remain above average, and attention should be paid to the rapidly developing populations or larvae. 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. 3).
Applications of novaluron (Rimon), tolfenpyrad (Torac), spinosad (Blackhawk), 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 event will likely occur through the upcoming week at many locations in central Wisconsin, with additional egg masses being deposited this week as well. 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. Our second generation materials often include the anthranilic diamides (several registrations), as they control many lifestages of CPB to include adults, eggs and larvae.