Amanda Gevens, Dept. Chair, Associate Professor & Extension Specialist, UW-Madison Plant Pathology, gevens@wisc.edu, Cell: 608-575-3029
Potato early season disease considerations: Wet and cool soils common in early potato plantings can delay germination and emergence. Such conditions also promote activity of plant pathogens, such as Rhizoctonia solani, a potentially seed-, soil-, or debris-borne fungal pathogen which causes stem or stolon cankers resulting in reduced stands, stunted plants, and/or reduction in tuber number, size, or quality.
Later in the season, Rhizoctonia can also cause black scurf on tubers. Cultural management approaches such as planting when soil temperatures are more consistently above 46°F, planting into well-drained soils, avoiding planting too deep, and avoiding hilling prior to adequate emergence can limit early season stem and stolon canker.
Several other seed-, soil-, and/or debris-borne diseases can also impact the potato crop as temperatures increase, including Fusarium seed piece decay caused by the fungus Fusarium sambucinum, Silver scurf caused by the fungus Helminthosporium solani, and Late blight caused by the oomycete Phytophthora infestans. While optimum temperatures for promoting each of these diseases vary, all require high soil moisture levels.
Fusarium, as a dry rotting pathogen which requires wounds for entry, can affect quality of seed potatoes in storage and lead to further disease concerns when potatoes are moved and warmed for planting. As a seed piece decay pathogen, Fusarium can affect seed immediately after cutting and through to sprouting. If initial and subsequent sprouts continue to be affected by Fusarium, the seed piece loses vigor and stand is reduced.
The Silver scurf pathogen is favored by warmer conditions and is recognized as a weak soil-borne and a stronger seed-borne pathogen. Typically, symptoms are not evident on tubers at harvest, but develop over time in storage. The longer the tubers remain in the ground after vine kill, the greater the risk for development silver scurf. Blemishes on tubers are restricted to the periderm. However, damage to the periderm causes increased water loss and shrink. The pathogen is not known to cause above ground plant symptoms.
Fungicide seed treatments have a place in an integrated pest management plan which includes cultural practices such as planting certified seed, proper handling and sanitation of storage/cutting/curing facilities prior to planting, cultivar resistance, and chemical control. In combination, IPM practices minimize economic losses to disease, minimize environmental effects, limit risk of pesticide residues in the food supply, limit development of fungicide-resistant pathogen strains, and limit development of pathogen strains which may overcome host disease resistance.
Seed cutting and planting events provide opportunities for application of fungicides to reduce negative effects of diseases such as Rhizoctonia, Fusarium, silver scurf, and late blight. While this article specifically addresses seed treatments in potato disease control, several potato fungicides are registered for in-furrow application and are also effective in managing seed- and soil-borne diseases. While seed- applied fungicides can enhance disease control and crop success, be mindful that some of the fungicides are contact only (ie: mancozeb and fludioxonil) and are active by limiting direct infection to the protected seed piece. Systemic fungicides (ie: flutolanil and cymoxanil) are xylem mobilized, moving the fungicide upward and outward (acropetally) for protection beyond the point of contact. Generally, seed-applied fungicides provide, at most, 10-14 days of disease protection. However, some active ingredients can protect seedlings considerably longer when applied at the highest labeled rate.
Typically, seed treatments are applied right after cutting with either a liquid or powder formulation. Taking care to avoid clumping or thick coating of the treatment is important as you can cut off oxygen to the seed piece and limit suberization (and promote soft rot). Good suberization of cut seed pieces is a critical component of potato disease management and should include a 3-4 day, 50-55°F, 90-95% relative humidity period with cut seed piled no deeper than approximately 6 ft to maximize airflow throughout the pile.
Seed treatments in potato have received increased interest and use in recent years due to improvements in active ingredients available, and the return on the investment of early season disease control. As there are no true rescue treatments for underground diseases post-planting, seed treatments can provide effective use pattern with added benefits of relative ease of application, small volumes of fungicide necessary, no spray drift, and no waste or negative impact on non-target sites.
Several fungicides with effective control of multiple diseases are available with registration for application to seed pieces prior to planting – see table below. Always read and follow the pesticide label prior to use.
| Active ingredient FRAC Group | Commercial name | Target diseases |
| cymoxanil 27 | Curzate | Late blight |
| difenoconazole 3 | Difenoconazole, Dyna Shield Defuze, Salient | Fusarium, Rhizoctonia, Silver scurf (must be mixed with fludioxonil) |
| difenoconazole 3 + fludioxonil 12 +
thiamethoxam |
Cruiser Maxx Potato Extreme | Fusarium, Rhizoctonia, Silver scurf |
| difenoconazole 3 + mandipropamid
40 + sedaxane 7 |
Vibrance Ultra Potato | Fusarium, Silver scurf, Rhizoctonia,
Pink rot, Late blight |
| difenoconazole 3 + fludioxonil 12 +
sedaxane 7 + thiamethoxam |
Cruiser Maxx Vibrance Potato | Fusarium, Rhizoctonia, Silver scurf |
| fludioxonil 12 + thiamethaxoam | Cruiser Maxx Potato | Fusarium, Rhizoctonia, Silver scurf |
| fludioxonil 12 | Start Up Fludioxonil, Spirato,
Maxim FS |
Fusarium, Rhizoctonia, Silver scurf |
| flutolanil 7 + mancozeb M3 | Moncoat MZ | Fusarium, Black scurf, Rhizoctonia,
Silver scurf, Late blight |
| mancozeb M3 | Start Up Mancozeb, Potato Seed Treater, Nubark Mancozeb, Fortuna,
Dithane, Koverall |
Fusarium, Rhizoctonia, Silver scurf, Late blight, Common scab |
| mandipropamid 40 | Revus | Late blight, pink rot |
| penflufen 7 + prothioconazole 3 | Emesto Silver | Fusarium, Silver scurf, Rhizoctonia |
| sedaxane 7 | Vibrance | Fusarium, Rhizoctonia, Silver scurf |
In special consideration of late blight control in potato, key components of management include:
- Destroy all potato cull piles (May 20 deadline by DATCP)
- Manage potato volunteers in all fields –volunteers pose great risk for late blight introduction
https://www.canr.msu.edu/psbp/resources/disease-tools/volunteer-survival
- Acquire disease free seed from a reputable certified source –infected seed poses great risk for introduction
- If there is a risk of disease associated with seed, use seed treatment or in-furrow application of effective late blight controlling fungicides (seed treatment is best)
- Apply only proven effective fungicides for control of late blight when disease forecast tool indicates environmental risk and stay on a fungicide spray program (DSVs reach 18)
- For conventional systems, a current list of registered late blight-specific materials can be found in the Commercial Vegetable Production in Wisconsin A3422 publication (further information below)
- For organic systems, copper-containing fungicides have been long-standing effective materials for preventing late blight in susceptible crops. Some newer organic fungicides are also available with promising late blight control (ie: Zonix, EF400).
- Scout regularly and thoroughly for disease in all potato fields
- Re-apply effective fungicides for disease control on a 7 day schedule (recommendation adjusts to a 5 day schedule when late blight is in the area and weather favors disease; recommendation adjusts to a 10 day schedule when late blight is not found in area and weather is hot and very dry)
- If late blight is identified in a field, have a mitigation plan in place for specific site. Depending on days to vine kill, environmental conditions, and extent of infection – plan may vary from complete crop destruction to early vine kill with continued maintenance fungicide sprays. Mitigation plan should limit disease spread within field and from field-to-field.
We will continue to provide Blitecast information via this newsletter and through the vegetable pathology website: https://vegpath.plantpath.wisc.edu/. We will have in-potato-field weather stations in Grand Marsh, Hancock, Plover, and Antigo as in past years, with access to the station data (with DSV and PDay values at: https://vegpath.plantpath.wisc.edu/dsv/. New in recent years, is the Vegetable Disease and Insect Forecasting Network (VDIFNet) site which provides information on DSVs from NOAA weather data across the state of WI, as well as insect phenological data (Dr. Russell Groves, UW-Madison Entomology). The link to the VDIFNet site is: https://agweather.cals.wisc.edu/vdifn/maps
Accessing the 2020 University of Wisconsin Madison Extension Commercial Vegetable Crop Production Management Guide: Our production guide is updated every October with release of a new guide in January. The book can be downloaded for free as a pdf at the link below, or can be purchased online for $12.50. https://learningstore.extension.wisc.edu/products/commercial-vegetable-production- in-wisconsin
Yi Wang, Assistant Professor & Extension Potato and Vegetable Production Specialist, UW-Madison, Dept. of Horticulture, 608-265-4781, Email: wang52@wisc.edu
Well, let’s just cut to the chase. #COVID-19 is substantially affecting everyone’s life. Some stories and comments from my grower folks about this hashtag are:
- We are taking extra precautions and doing a lot of cleaning. – from a red potato grower and packer
- We are really busy now, people eating a bunch of chips and stockpiling. Interesting to observe sales of small bagged chips are going up fast, likely caused by people’s intention to avoid cross contamination during lockdown. – from a chipping potato grower
- We are starting another packing shed to accommodate all the orders at full capacity, but still can’t keep up. – from a fresh potato grower and packer
- Fresh potatoes are selling very fast for retail, but large russets (food service) and processing potato sales are way down. The consequence of this is that a large volume of processing russets now enters the fresh market. To make it worse, planting of processing potatoes is being cut off, as contract volumes are reduced due to the lower demand for French fries and other food service products. – from my articulate postdoc advisor
In summary, fresh market and chipping potato sales are significantly up whereas frozen fry potatoes are hard hit. To make it easier to read, potatoes aren’t like toilet paper or hand sanitizer so hoarding leads to a mess.
Luckily, Mother Nature has not been too moody this year. Planting on large farms has been ongoing for a week or so and other farms are getting started.
Stay safe and healthy farmers. You are our heroes.