INTRODUCTION

Development and Use of ThisManagement PlanThis action plan was developed in cooperationwith the Idaho potato industry in consultationwith late blight experts in the Pacific Northwestand other areas of the United States. It is hopedthis information will help the Idaho potato indus-try reduce potential yield and quality lossescaused by late blight. This action plan shouldalso help in making informed decisions about dis-ease prevention, fungicide spray programs, andoptimizing disease management costs. Additionalinformation about late blight is contained inPacific Northwest publication PNW 555,Managing Late Blight on Irrigated Potatoes in thePacific Northwest, available at local University ofIdaho Extension offices, or on the web athttp://info.ag.uidaho.edu/pdf/PNW/PNW0555.pdf.

Occurrence in IdahoLate blight is the most important disease of pota-toes on a worldwide basis. Prior to 1995, only oneisolated case of late blight had been reported inIdaho. Late blight appeared in numerous fieldsthroughout southwest Idaho in July 1995, andlater spread to south central Idaho. Since then,the disease has occurred in all commercial potatoproduction areas of the state and has beenresponsible for severe damage during some years.

Spread of Late BlightPotatoes may be exposed to late blight during thegrowing season from inoculum produced oninfected cull piles, volunteer potato plants, orplants developing from infected seed. Tomatotransplants in home gardens may also be a

source of late blight. Under the right conditions(see Avoid conditions that favor late blight, p.3),spores from infected plants can be carried formiles in moist air, such as in thunderstorms, andinfect healthy plants, thus spreading the disease.Currently, no labeled chemicals will cure the dis-ease once it becomes established in a plant.Because of this, it is critically important thateveryone in the potato industry develops a “lateblight prevention attitude.” Effective best man-agement practices (BMP) are based on preventionand include cultural and chemical managementpractices that reduce the potential for occurrence,spread, and loss from late blight.

CIS 1132

University of Idaho Extension • Idaho Agricultural Experiment Station

Late Blight ManagementAction Plan for Potatoes

by Jeff Miller, Phil Nolte, Nora Olsen, Terry Miller, Bill Bohl, and Mike Thornton

Terms Used in Discussing Late BlightInoculum – Structures of late blight pathogencapable of infecting plants. Examples includewindborne sporangia and waterbornezoospores.

Lesion – Area on a leaf, stem, or tuber show-ing symptoms of late blight.

Pathogen – A microscopic organism thatcauses disease in higher plants, such as pota-toes and tomatoes.

Spore – A pathogen structure containing oneor more cells capable of reproducing, germi-nating, and causing infection.

Strain – A population of late blight pathogenswith similar traits. A late blight strain is desig-nated using two letters to designate the countrywhere it was first identified (such as US) fol-lowed by the number of identification (e.g. US-8represents the 8th strain identified in the UnitedStates). Strains have also been called genotypesby some.

CULTURAL CONTROLBEFORE PLANTINGLearn to recognize late blight symptoms –Early detection is critical for late blight manage-ment, so it is essential for all personnel involvedin farming operations to be able to recognize andidentify late blight symptoms. Both the plant(Figures 1-1, 1-2, 1-3) and tubers (Figure 2) pro-duce distinct symptoms.

Understand conditions favoring develop-ment – The late blight pathogen is more likely tospread, and disease is more likely to develop whenrepeated evening thundershowers occur and/orduring periods of high humidity when tempera-tures are between 55 and 80°F. Even in theabsence of rainfall, sprinkler irrigation may pro-vide conditions that favor late blight development.

Eliminate sources of inoculum – The initialsources of late blight inoculum are likely to beplants produced from infected tubers in cull piles,volunteers infected the previous year that havesurvived the winter, and plants that develop frominfected seed tubers. In addition, infected potatoand tomato plants grown in home gardens,greenhouses, and nurseries (even those grownoutside and transported to Idaho) can serve assources for late blight inoculum.

Because of the potential danger of these inoculumsources, Idaho State Department of Agricultureregulations mandate daily treatment of wastepotatoes and other non-usable material from allpotato operations, including seed cutting opera-tions, in a manner that renders foliage unable toallow pathogen growth. These regulations are ineffect from April 15 to September 20 in regionswest of Raft River, and from May 15 to September20 in areas east of Raft River.

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Figure 1_1. Late blight lesion on a potato leaf. Grayor brown lesions surrounded by a light green (chlorotic)area are very characteristic of late blight. Lesions thatlook like this are about one week old.

Figure 1_2. Leaf and stem lesions are typical ofpotato late blight.

Figure 1_3. In advanced stages of late blight, leaflesions will be sparse, as most leaves will be completelydestroyed by the pathogen. Stem lesions are commonlyseen at this stage of the epidemic.

ISDA-Mandated Daily TreatmentBecause of the potential danger of inoculumsources listed above, Idaho StateDepartment of Agriculture (ISDA) regula-tions mandate daily treatment of wastepotatoes and other non-usable materialfrom all potato operations, including seedcutting operations, in a manner that ren-ders foliage unable to allow pathogengrowth. These regulations are in effect from:

• April 15 to September 20 in regions west ofRaft River, and

• May 15 to September 20 in areas east ofRaft River.

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Methods for disposing of cull and non-usablematerial include freezing, chopping, feeding tolivestock (non-treated potato tubers), composting,burial, or herbicide spraying. See University ofIdaho Cull and Waste Potato Management athttp://info.ag.uidaho.edu/pdf/CIS/CIS0814.pdf formore detailed information.

Controlling volunteers, late blight – Inregions where soil temperatures at 4 to 6 inchesdeep have not reached 20 to 25°F during the win-ter, volunteer potato plants may be very com-mon.

• Herbicides. Use cultivation and labeledbroadleaf herbicides where possible to suppressgrowth of volunteer potatoes in rotation crops.Check with your local UI Extension office forherbicides available to use for controlling volun-teer potatoes.

• Avoid planting back to back. Avoid plantingpotatoes in the same field for two or more con-secutive growing seasons. Back-to-back growingmakes control of volunteers impossible.

• Down wind fields. Potato fields directly down-wind from fields that had late blight the previ-ous season may be at a higher risk for lateblight because of the potential for spores tomove from infected volunteers.

• Year-out plantings. Planting “eliminator” or“year-out” seed is risky due to the possibility ofhaving late-blight-infected tubers, especially iflate blight was present during the previous year.

• Certified seed. Purchase only certified seed fromseed operations with which you are familiar.The occurrence of late blight in a region does

not mean seed tubers are infected, but there isan increased risk.

• Examine seed fields. Examine seed fields dur-ing the growing season and/or check inspectionrecords and shipping point inspection reportsfor information about the seed lots that you areconsidering for purchase.

Avoid conditions that favor late blight –Weather conditions strongly influence the inci-dence and severity of late blight. Cool (55 to 80°F),rainy weather, high relative humidity (near 100%),and heavy dew formation favor infection, diseaseprogress, and spore production. Although weatherconditions are beyond grower control, careful irri-gation management can help reduce the extent ofperiods favorable for disease development.

• Good water infiltration/drainage. Select fieldswith good water infiltration and drainage char-acteristics for planting potatoes. If water collectsin some areas of the field, consider makingmodifications to eliminate standing water. Lowspots in a field, areas near the pivot point of acenter pivot system, irrigation overlaps, or areasnext to windbreaks where shade allows plantsto stay wet for longer periods, favor late blightdevelopment.

• Vulnerable areas. Areas of fields next to wind-breaks, houses, or power lines may be difficultto cover with fungicides applied by air, andthese poorly protected locations are often wherelate blight first occurs. Avoid planting potatoesin areas of fields where plants cannot be ade-quately protected with fungicide, or areas thatare at a higher risk of infection.

• Ground applications. Ground applications offungicides should be considered in areas notadequately protected by aerial spraying. Anoption for center pivot systems is to installvalves on the first few nozzles of the centertower that can be turned off to avoid keepingthis area excessively wet. Or, better yet, leavethe first 100 feet near the center of the pivotunplanted.

• Avoid 12-hour sets. Growers who use handlinesor wheel-lines artificially encourage late blightdevelopment by irrigating using 12-hour sets.This situation ensures more than 12 hours of leafwetness, far more than the needed 8 to 10 hours,for infection to occur. Reducing set times or begin-ning early in the morning and allowing foliage todry before evening may help reduce infection.

• Excessive nitrogen. Excessive nitrogen applica-tions promote heavy vine growth and extend

Figure 2. The potato tuber (center) shows infectionthrough an eye.The tuber slices on either side depictthe granular, brown dry decay associated with lateblight.

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the period during which relative humiditywithin the canopy favors spore production andleaf infection. Develop a nitrogen managementplan that promotes optimum plant growth andyields without stimulating excessive vine growth(see Potato Production Systems, chapter 8 [J. C.Stark and S. L. Love, eds., University of IdahoExtension]).

Grow less susceptible varieties - Most com-monly grown potato varieties in Idaho are consid-ered susceptible to late blight (Table 1). A notableexception is Defender, a recently released varietywith resistance to both foliar and tuber infection.Early-maturing varieties, such as RussetNorkotah, Shepody, and Frontier Russet seemmore prone to yield losses because disease defolia-tion caused by late blight progresses rapidly, anddiseased leaves are not replaced by new growth.Shepody and Ranger Russet are more susceptibleto tuber infection. It is important to note that sus-ceptibility to foliar infection is not always directlyrelated to the level of tuber infection (e.g.Umatilla Russet—see Table 1).

Table 1. Relative late blight susceptibility rating ofseveral potato varieties.

Variety Foliar Rating Tuber Rating

Alturas MS MS

Atlantic MR --

Bannock Russet S HS

CalWhite S S

Chipeta S S

Defender R R

Gem Russet S MS

GemStar Russet S S

Hilite HS HS

IdaRose S S

Ivory Crisp S S

Ranger Russet S HS

Russet Burbank S MS

Russet Norkotah HS HS

Shepody S HS

Summit Russet MS MR

Umatilla Russet S MR

Western Russet S MS

R = ResistantMR = Moderately resistantMS = Moderately susceptibleS = SusceptibleHS = Highly susceptible-- Data unavailable

CULTURAL CONTROL DURING PLANTINGRecognize that seed handling can be animportant factor in disease spread – Severalexperiments conducted over the last 30 to 40years have indicated that most of the seed tubersor seed pieces infected with late blight will readilydecay from soft rot after planting and are self-eliminating. Yet, seed transmission is an impor-tant means of initiating late blight into an areaand particularly to a field. Late blight spores canspread from infected tubers to healthy tubers dur-ing handling, cutting, and planting operations.These healthy seed pieces can then develop lateblight once planted in the field.

Do not mix seed during cutting and plant-ing – Keep seed lots separate to avoid mixinguninfected lots with seed lots potentially infectedwith late blight. Mixing healthy seed withinfected seed will increase the chances of spread-ing late blight over a larger area. Keep a writtenrecord of where each seed lot is planted. Duringthe cutting operation, eliminate and save sus-pected seed pieces that show a rust brown, firmdecay typical of late blight (Figure 2). Send suspi-cious tubers to a laboratory for positive identifica-tion. When possible, store seed at less than 45°F,then warm to 50-55°F and cut just before tubersbegin to sprout. Also, to minimize spread betweenseed lots, always clean and disinfect equipmentbefore cutting a new seed lot. See also the sectionunder chemical control for seed piece treatmentsthat reduce late blight spread.

Do not plant problem areas – Do not plantareas that you suspect may remain wet forextended periods or will be difficult to spray such asnear trees or power lines. Consider not planting thearea under the first few nozzles of a center pivot sys-tem. This will be less than an acre. Research in theColumbia Basin of Washington showed that signifi-cantly more tuber blight occurred under the first100 feet from the center of the pivot. This area istypically overwatered due to the geometry and noz-zle sizes used on most center pivots.

EARLY SEASONDo not let cull potatoes accumulate –Eliminating cull potatoes early in the season is crit-ical because they could potentially carry the lateblight pathogen. Potato pieces resulting from seedcutting operations or cull potatoes left after loadingor unloading at storage facilities may support theproduction of late blight spores whether or not thepieces are sprouting. Dispose of them properly.

• Because freezing temperatures generally do notoccur in the spring, burial or feeding non-treatedpotatoes to livestock may be appropriate meth-ods for disposing of cull and waste material.

• Do not allow cull piles to build up at feedingareas.

• Check with the Idaho State Department ofAgriculture or local county officials for informa-tion on how to properly dispose of cull potatoesin your area.

• Potatoes may sprout and produce plants evenwhen buried to depths greater than two feet, soit is important to continually monitor disposalsites to prevent volunteer plants from develop-ing.

• Remember the Idaho State Department ofAgriculture regulations in effect from April 15 toSeptember 20 in regions west of Raft River, andfrom May 15 to September 20 in areas east ofRaft River requiring cull potatoes to be renderednon-viable.

Control alternate hosts – Hairy nightshadecan be infected with the late blight pathogen andmay contribute to disease spread under some con-ditions. Apply effective pre- or post-emergenceherbicides, such as Eptam, Matrix, or Outlook,during early season to reduce hairy nightshadepopulations. Although other weed species are nothosts of late blight, they can contribute to condi-tions that favor the disease by restricting airmovement within the canopy. Heavy weed infes-tations also prevent adequate coverage of potatofoliage during fungicide applications.Information available from the University ofIdaho can help plan an effective weed controlprogram.

Scout fields regularly – Closely monitor thegrowing potato crop and submit suspected lateblight samples to the University of Idaho for iden-tification. Also,

• Wet. Concentrate scouting in areas of fields thattend to stay wet for long periods, such as centerpivot wheel tracks, irrigation overlap areas, andlow areas where water collects.

• Center pivots. Especially scout plants under thefirst tower of center pivots. This area is almostconstantly wet, and therefore conducive for lateblight development.

• Windward side. The windward sides of fieldsare usually infected from windborne spores socheck these areas first.

• Obstacles. Carefully scout areas that may haveescaped a fungicide application because ofpower lines, trees, or other obstacles.

• Volunteers. Look for early indications of lateblight on volunteer potatoes, especially in fieldsthat had late blight the previous year.

Watch late blight forecasts – Two late blightforecasts have been developed at the University ofIdaho.

The first utilizes the amount of rainfall in Apriland May and the number of favorable hourswhen the temperature is between 50 and 80°Falong with relative humidity values above 80 per-cent. These variables are used to calculate theprobability that late blight will be present.

The second forecast calculates the number ofhours potato leaves are moist and the averagetemperature during that time. This forecast gener-ates a “severity value” between 0 and 4 for eachday. A running total of severity values is main-tained, and late blight is expected one to twoweeks after a total of 17 severity values has accu-mulated.

Find forecasts. Information on these forecasts ispublished on

• The University of Idaho toll-free hotline (800-791-7195),

• The Pacific Northwest Pest Alert—www.PNWPestAlert.net, and

• The University of Idaho Potato Pathology webpage—www.ag.udiaho.edu/potatopath.

MIDSEASONIrrigation management – Leaves and/or stemsneed to remain wet for about 8 to 10 hours forlate blight spores to germinate and infect a plant.The longer leaves or stems remain wet, thegreater the risk of pathogen infection.

Irrigating during or immediately after periods ofcool, rainy weather will increase the possibility oflate blight infection. Irrigation practices orweather conditions that increase the time leavesare wet will increase the potential for late blight.

Carefully examine irrigation management prac-tices for modifications that will reduce conditionsfor late blight development (wet leaves) but willstill maintain plant health and tuber quality. Iflarge wet spots form in localized areas of a field,repair irrigation system leaks or turn off or plugnozzles in these areas to allow drying.

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Sanitation practices – People entering fieldsshould wear boots and clothing that can be disin-fected between fields with products such asdiluted household bleach mixed 1 part bleach to9 parts water. Although late blight is more likelyto spread from field to field by moist air than bycontact with people, it is unwise to take risks. Analternative is to wear disposable boots and pantsthat can be changed between fields, or clothingthat can be reused after washing and drying.

Destroy hot spots –When late blight infesta-tions are found early in small patches in fields, itcan be beneficial to disk, burn with a propaneburner, or spray these patches with a fast-actingdesiccant to remove these local sources of inocu-lum. The area to be killed needs to extend at least100 feet beyond any visible symptoms.

While this can be effective, remember that visiblelate blight lesions show up 3 to 5 days after leavesbecome infected. If conditions were favorable fordisease spread during those 3 to 5 days, killing aninfected area after symptoms appeared may nothave been done soon enough to prevent furtherspread of the pathogen. These areas should becarefully monitored for further disease develop-ment and also marked and inspected prior to har-vest for presence of late blight in the tubers. Thelate blight pathogen is capable of producing asmany as 100,000 spores on a lesion as large asquarter.

LATE SEASONAvoid excessive irrigation – Potato tubersbecome infected and disease develops whenspores are washed down from infected leaves andstems through cracks in the soil surface. Sporesmay directly contact tubers exposed through soilcracks, or may “swim” short distances throughthe soil to infect shallow tubers. In either case,excessive irrigation and soil cracks may increasetuber infection.

Monitor and regulate fertility – Late seasonfertilizer applications help maintain green vinesand promote tuber bulking. However, rememberthat green and vigorous vines are susceptible tolate blight and can also be difficult to kill withdesiccants. Also,

• Green vines may also harbor late blight sporesthat can infect tubers during harvesting.

• Skinned tubers may be more susceptible toinfection because damaged areas remain moistfor a longer period.

At the end of the season, petiole nitrate levelsshould normally drop to levels that encouragevine senescence.

Scout fields regularly – Even small amounts offoliage infection may lead to significant tuberinfection if conditions are favorable for infectionduring harvest. Continue scouting on a weeklyschedule through vine kill.

Infected areas should be marked and harvestedlast so infected tubers have time to decompose, orconsider not harvesting these areas. If the areasare not harvested, lay the tubers on top of theground so they are more likely to freeze duringwinter. If the infected area is harvested, tubersfrom these areas should be placed near the stor-age door so that these potatoes can be removedand marketed immediately if tuber decaybecomes evident.

Prevent next year’s volunteers – Volunteerpotatoes are difficult to control so consider usinga preharvest sprout suppressant. Preharvestsprout suppressants must be applied while vinesare green and actively growing, and must bemade at least two weeks before vine kill. Maleic hydrazide (Royal MH-30) may provide 70to 80 percent control of next year’s volunteerswhen applied at the proper time under good envi-ronmental conditions at the full labeled rate.Maleic hydrazide should not be applied to seedpotatoes, and has been reported to cause somefoliar and tuber injury when applied to stressedcrops or if overlap occurs during application.Check the label for specific use recommendations.

Kill vines completely – The late blightpathogen cannot survive and produce sporeswithout a living host—green foliage or stems, ortuber tissue. During harvest, infected vines mixedwith tubers in cool, wet conditions may lead totuber infections that are not visible until later instorage.

Kill vines at least 2 to 3 weeks prior to the antici-pated harvest date. This interval minimizes thechance of tubers getting contaminated with lateblight spores during harvest and allows previ-ously infected tubers to decompose in the field.Mechanical, chemical, or natural (frost) methodsmay be used to desiccate vines.

No data are available that indicate one methodof vine kill is better than another as long as vinesare completely killed. Vine rolling or flailing maybe helpful to expose the soil and lower canopy todrying in fields with heavy vine growth. Somevine-killing methods are very sensitive to weather

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conditions. Watch weather forecasts, and if neces-sary, kill vines early if wet conditions are forecast.

CULTURAL CONTROL DURINGHARVEST AND STORAGEIdentify level of tuber infection – Sort tubersduring harvest, removing as many decayedtubers as possible. Identifying tubers infected withthe late blight pathogen can be difficult, espe-cially if tubers are covered with soil. Submit sus-pect tuber samples to trained personnel forconfirmation.

Potato lots with more than 3 percent late blightinfection may be very difficult to store. Even verysmall infection levels can lead to serious losses inmarginal storage facilities, so it is important toknow what level of infection is present.

Harvest carefully – Avoid harvesting duringwet conditions. Tubers can become infected atharvest even with minimal foliar blight in a fieldif conditions are wet. Harvesting during rain isparticularly dangerous. Harvesting when skinsare mature, and minimizing skinning, cuts, andshatter bruises will reduce the likelihood of tuberinfection.

Any break in the tuber skin provides an idealplace for the late blight and other disease-causingorganisms to gain entry into the tubers. Althoughthe late blight pathogen does not need a woundto infect tubers, cut, skinned, and shatter-bruisedtubers are more likely to become infected becausethe protecting skin is damaged and these areasremain wet for an extended time period, givingthe late blight spores time to infect the tuber.

Carefully monitor and regulate storageconditions – The ability to provide high volumesof air flow throughout the pile for cooling anddrying tubers is critical during the early storageperiod. Remove vines, loose soil, and anythingelse that may interfere with air distribution in thepile.

If foliar late blight was present in the field prior toharvest, it is important to ventilate the storage asquickly as possible with a high volume of air. Assoon as the first 2 to 3 air ducts are covered, beginsupplying air to the pile to dry wet potatoes andto equilibrate the pile temperature.

It may be necessary to continuously run the fanswith reduced or no humidity until tubers are com-pletely dry. This drying time should usuallyrequire no more than 72 hours. Expect increasedpressure bruise and shrinkage losses in potatoes

subjected to these storage conditions, especially ifthey are not marketed early.

Due to the negative impacts of reducing humid-ity, this difficult decision should involve account-ing for the percentage of infected tubers, thewetness of the rot, and market-end use of theremaining healthy tubers.

Begin observing potatoes in storage immediatelyfor rot development (hot spots). If hot spotsdevelop, supply additional air to those areas ofthe cellar, and plan on removing the potatoes assoon as possible. Holding potatoes at lower tem-peratures (below 45°F) may lessen development ofdisease in storage, but this can have a significantimpact on the market use of the potatoes. Know the end use of the tubers prior to manipu-lating storage temperatures.

Minimize volunteer potatoes – Small tubersleft in the field are potential volunteers the nextyear, and, if infected, may produce late blight-infected plants. The number of tubers left in thefield may be reduced by using a narrower pitchchain on the harvester. However, this may alsoincrease the soil and vine load in the harvester,which must then be removed before placing pota-toes into storage so as not to hamper air circula-tion. Use shallow tillage practices that leavetubers on the surface or within the top two inchesof soil to encourage freezing during the winter.

CHEMICAL CONTROL

Seed Piece TreatmentsSeveral experiments conducted over the last 30 to40 years have indicated that most of the seedtubers or seed pieces infected with late blight willreadily decay from soft rot after planting, makingthem self-eliminating. Yet, seed transmission is animportant means of initiating late blight in afield and to an area. Late blight spores can spreadfrom infected tubers to healthy tubers duringhandling, cutting, and planting operations. Thesehealthy seed pieces can then develop late blightafter planting.

Seed piece fungicide treatments that have activityagainst late blight have been shown to be effec-tive in minimizing seed-to-seed pathogen spread.Use a seed piece treatment that is labeled for con-trol of late blight. Evolve is a combination ofthiophanate-methyl, mancozeb, and cymoxanil(the active ingredient in Curzate) and is the mosteffective seed treatment in preventing tuber-to-tuber spread of the late blight pathogen in seedcutting operations.

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Growers who suspect or know they will receiveseed that contains some late blight are stronglyencouraged first not to purchase the seed. But ifyou do purchase the seed, use seed piece treat-ments that have activity against late blight.Growers using seed lots with known or stronglysuspected late blight infestation should also placethe affected fields on a regular fungicide sprayprogram early in the growing season. Keep inmind that no seed piece treatment can “rescue” abadly infested seed lot.

Fungicide Types and SelectionFor a list of fungicides labeled for use on potatoesfor controlling late blight, go towww.ag.uidaho.edu/potatopath. The fungicideselected is not as important as application cover-age and timing as discussed below. However, theuse of copper or tin fungicides alone is not recom-mended for controlling late blight.

Fungicide ResistanceUniversity of Idaho Extension personnel track anddocument the late blight strains that occur inIdaho. Strains of the late blight pathogen areidentified by the country where the strain wasfirst identified, and by the chronological order ofdescription in that country. For example, the US-8strain is the eighth strain characterized in theUnited States.

Any other strains of the late blight pathogen thatresemble the US-8 strain are given the US-8 desig-nation regardless of where in the world they arefound. Most late blight strains found in Idahosince 1995 have been the US-8 and US-11 strains.

Both of these strains are resistant to the fungicidemefenoxam (Ridomil). Therefore, mefenoxam-based products, such as Ridomil Gold Bravo,Ridomil Gold MZ, and Ridomil Gold Copper arenot recommended for use in controlling lateblight.

Mefenoxam-based products are very effective forcontrolling pink rot in some areas of the state andfor controlling Pythium leak when appliedaccording to label directions. (Many fields ineastern Idaho have mefenoxam-resistant isolatesof the pink rot pathogen.)

The companion fungicides in Ridomil Gold pre-packs, such as Bravo (chlorothalonil) or man-cozeb, are protectant fungicides effective forcontrolling late blight. When applying RidomilGold pre-packs for control of pink rot andPythium leak, consider the companion fungicide

in the pre-pack to be part of your protectant sprayprogram for late blight.

Remember that chlorothalonil, triphenyltinhydroxide and the EBDC fungicides have limitson the amount of active ingredient that can beapplied per acre each season. Carefully followlabel directions to be certain that you are notapplying more active ingredient per seasonthan is allowed.

Alternating fungicide classes should be part ofany program and will help prevent these use lim-its from being approached. Fungicides with a nar-row mode of action should be tank-mixed with abroad spectrum fungicide. Carefully follow labeldirections to be certain fungicides are appliedin the appropriate manner.

Consider These Factors WhenSelecting an Application Method To ensure thorough and complete fungicide cover-age in the potato canopy, a fungicide may needto be applied up to two weeks prior to potatoesbeing exposed to late blight.

Fungicide label directions – Product choice influ-ences application method because some productshave limitations on how they may be applied.Fungicides may be applied by ground, aircraft, orchemigation. Check the label for applicationrestrictions and how the fungicide may beapplied, and follow the label directions.

Field size, shape, tillage practices, and obsta-cles – Choosing an application method dependson many factors including field size and shape,location of obstacles, irrigation system, andtillage practices. For example, fields with powerlines, houses, or tall trees along the edges maynot be good candidates for aerial applications.Likewise, fields with basin tillage pose some prob-lems for ground application. Chemigation is mostapplicable with center pivot irrigation.

Timing/availability of equipment – Anotherfactor to consider is whether a fungicide can beapplied in a timely manner. A successful lateblight management program must include hav-ing a fungicide applied to the potato crop beforelate blight spores enter a field. If you plan onusing air or ground application methods, be cer-tain the equipment will be available whenneeded.

Need for redistribution – To ensure thoroughand complete fungicide coverage of the potatocanopy, fungicides may need to be applied up to

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2 weeks prior to potatoes being exposed to lateblight. This 2-week period allows fungicides to beredistributed throughout the canopy by subse-quent irrigations. Application method, frequency,and amount of water applied all influence fungi-cide redistribution in the canopy. More redistribu-tion is required following air application, less forground, and least is required for chemigation.However, all methods of application work whenused properly.

Equipment must provide complete coverageand be timely – No matter which applicationmethod is used, the crop must be completely cov-ered with no skips or areas untreated, and fungi-cides must be applied at the appropriate intervalfor the disease pressure in your area.

Application MethodsGround application – Applications using aground sprayer tend to be very effective in con-trolling late blight because the water volumesand pressures used provide good leaf coverageand penetration of fungicide into the canopy.Research at the University of Wisconsin foundthat hollow cone and extended range flat fannozzles were superior to flood-jet nozzles.Recalibrate the sprayer often, and replace nozzlesthat are under or over applying by more than 10percent of the boom average. Raise the boomheight as the crop grows to maintain the properoverlap in the spray pattern.

Disadvantages. The main disadvantages ofground applications are the time required tocover large acreage, incompatibility with certainirrigation systems, and basin tillage. Multipletrips through the field with a ground sprayer willalso increase soil compaction, especially on heavysoils. During 1996, the University of Idaho docu-mented a 1 to 3 percent reduction in yield due tosprayer traffic in two fields in the Treasure Valley.This yield effect should be factored into the cost ofground application when comparing with air andsprinkler application methods. Ground applica-tion also damages reservoir tillage basins causingwater to run down rows creating ponds or gulliesin some areas.

Air application – Air applications deposit most ofthe fungicide on top of the crop canopy, and thematerial must then be redistributed into the lowercanopy by irrigation water or precipitation to pro-vide protection. A minimum of 5 gallons of waterper acre is required for adequate coverage.Research has shown that 5 gallons of water peracre is as effective as using 10 gallons. It is impor-

tant to remember that nozzle calibration is just asimportant for air applications as it is for groundapplications.

Disadvantages. One of the biggest problems withaerial application is untreated strips caused byinadequate overlap between passes. If possible, afield could be sprayed in an east-west directionone time and north-south the next. Some aircraftnow have GPS systems, which can help eliminatemissed strips. Availability of planes for timelyapplications has occasionally been a problem inparts of Idaho. Work with the aerial applicator toidentify field areas that cannot be treated due toobstacles (trees, power lines, houses, etc.). Treatthese areas with a ground applicator to ensureproper coverage.

Chemigation – An advantage of using a sprinklersystem for applying a fungicide is the system isalready in place so you do not need to wait forequipment. A 1998 survey in Idaho showed thatpotato fields where late blight was controlled withchemigation had disease severity ratings slightlylower than fields where ground or aerial applica-tion methods were used. The survey does not,however, consider the number of fungicide appli-cations or products used. An important point tonote, though, is that all methods adequately con-trolled late blight.

For any type of irrigation system, make sure thefield is the same size as the system (i.e. no pota-toes outside the water coverage area). The unifor-mity of fungicide application is very dependenton the uniformity of water distribution. Cornersof fields irrigated with center pivot systems usu-ally do not receive adequate fungicide coverageand should be sprayed with a ground applicator.For solid-set or set-and-move systems, inject thefungicide during the last portion of the irrigationset, or make a separate application between irri-gations. Make sure the fungicide has been evenlyapplied throughout the line and has been flushedout of the end nozzles before shutting off the sys-tem.

Disadvantages. One of the main disadvantagesof using a center pivot to apply fungicides is thehuge volume of water applied with the chemical.This results in relatively lower fungicide residuesin the upper canopy compared to aerial applica-tion. Therefore, it is important to adjust the revo-lution time to the fastest setting in order to applythe least amount of water to the foliage. Alwaysuse the highest labeled rate when applying a fun-gicide through a sprinkler system.

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Research from the Columbia Basin of Oregon andWashington indicates that an alternating pro-gram of aerial application and chemigationapplication is an effective method of establishinghigher residues and obtaining product redistribu-tion at a reduced cost over using air alone.

When to Apply FungicideInitial applications – For optimum protection,potato fields should be sprayed with a protectantfungicide before row closure (plants touchingbetween rows) followed by a second applicationin 7 to 10 days. Weather can be unsettled early inthe season when vine growth is rapid and suscep-tible to late blight. The need for additional appli-cations will depend on weather conditions andoccurrence of late blight. These early applicationstreat the stems and lower canopy where lateblight often begins once the vines close the rowsand conditions within the canopy become morefavorable for late blight.

Early fungicide applications are recommendedbecause of the chances of infections starting fromlocal sources such as volunteers, cull piles, or seedand because controlling late blight early in theseason will prevent weekly applications through-out the season if a field becomes infected early.

Additional Applications – Rigorously scout for lateblight and pay careful attention to weather condi-tions favoring late blight development, i.e. weatherthat is cooler and wetter than normal. After thefirst two fungicide applications, if late blight isfound in the area or weather conditions are con-ducive for disease development, then continue tospray a protectant fungicide on a regular schedule.

A preventative attitude is very important whendealing with late blight. Attempting to “rescue” afield after late blight occurs is likely to fail. Thereare no “curative” fungicides (including the limitedsystemic products such as Curzate, Previcur, andAcrobat) that will effectively stop a late blight epi-demic. It is vital to stay ahead of the late blightpathogen with a protectant fungicide spray pro-gram. A regular fungicide program can providethe necessary control of late blight (Figure 3).

Recommendations on spray application fre-quency will be issued regularly throughout thegrowing season. Protectant fungicides must beapplied at intervals that maintain coverage onnew leaves, especially when environmental con-ditions are conducive for late blight development.

Late season applications – Continue fungicideapplications at intervals based on weather condi-

tions and recommendations. Protectant fungicidesmay need to be applied even after vine desicca-tion until all green vines are completely dead iflate blight was present in the region or field.Research from the Columbia Basin has shownthat late season applications of mancozeb can beeffective in reducing tuber blight when foliage isinfected. Applying a fungicide as a tank mix witha vine-killing agent is not recommended. Earlyvine killing may be the best option for fields without-of-control late blight infestations.

Recommendations to reduce late blight tuber rot– Season-long control of the late blight pathogenon plants (leaves and stems) is extremely impor-tant for reducing tuber infection. Tuber blight canoccur even when only small percentages of foliarblight are observed in the field. Follow the recom-mendations above to reduce chances of tuberinfection during harvest.

Post-Harvest – Disinfestants such as chlorinedioxide, hydrogen peroxide/peroxyacetic acidmixtures (HPPA), and ozone are labeled for post-harvest control of late blight. However, Universityof Idaho research has not shown consistent resultsusing these products. Phosphonates (phosphorousacid or phosphite-based products) have beenshown to be effective in keeping healthy tubersfrom being infected at harvest. Application ofpost-harvest disinfestants or fungicides should bedone using 0.5 gallons of water per ton of tubers,and tubers should be rolling when they passunder the spray bar to ensure adequate coverageof tuber surfaces.

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Figure 3. Research plots show potato plants pro-tected by weekly fungicide applications (left, front) com-pared to plants not receiving any fungicide applications(right, front). Weekly fungicide applications can provideprotection against late blight when spray programs arestarted before late blight is present.

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MORE HELP WITH LATE BLIGHT IN POTATOES

Hotlines with current status

• University of Idaho hotline: 800-791-7195

• Washington State University hotline: 800-984-7400

• Oregon State University hotline: 800-705-3377

Web help

• Pacific Northwest Pest Alert: www.PNWPestAlert.net

• University of Idaho Potato Pathology web:www.ag.uidaho.edu/potatopath

• Idaho Center of Potato Research:www.ag.uidaho.edu/potato

• Fungicides labeled for use on potatoes to control lateblight: www.ag.uidaho.edu/potatopath

Publications

CIS 814, Cull and Waste Potato Management: http://info.ag.uidaho.edu/pdf/CIS/CIS0814.pdf

CIS 1130, Managing Fungicide Resistance:http://info.ag.uidaho.edu/pdf/CIS/CIS1130.pdf

CIS 1131, Diagnosis & Management of Potato StorageDiseases: http://info.ag.uidaho.edu/pdf/CIS/CIS1131.pdf

PNW 555, Managing Late Blight on Irrigated Potatoes inthe Pacific Northwest:http://info.ag.uidaho.edu/pdf/PNW/PNW0555.pdf

Potato Production Systems, Jeffrey C. Stark and Stephen L.Love, eds. 2003. University of Idaho Extension

Published September 2006 $6.00

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with theU.S. Department of Agriculture, Charlotte V. Eberlein, Director of University of Idaho Extension, University of Idaho, Moscow, Idaho 83844. TheUniversity of Idaho provides equal opportunity in education and employment on the basis of race, color, national origin, religion, sex, sexual orien-tation, age, disability, or status as a disabled veteran or Vietnam-era veteran, as required by state and federal laws.

About the authorsJeff Miller is a UI faculty researcher at the UIAberdeen Research & Extension (R&E) Center,208-397-4181, jsmiller@uidaho.edu. Phil Nolte isthe UI extension seed potato specialist at theIdaho Falls R&E Center, 208-529-8376,pnolte@uidaho.edu. Nora Olsen is a potato spe-cialist at the Twin Falls R&E Center, 208-736-3600, norao@uidaho.edu. Terry Miller is withMiller Research, Inc., a private pest managementresearch company in Minidoka, 208-531-5124,terry@millerresearch.com. Bill Bohl is theBingham County UI Extension educator andpotato specialist, 208-785-8060,wbohl@uidaho.edu. Mike Thornton is superin-tendent of the Southwest Idaho R&E Center atParma, 208-722-6701, miket@uidaho.edu.

AcknowledgementsFunding for this publication was provided in partby the Idaho Potato Commission and theUSDA/FCIC Risk Management Agency.

PESTICIDES DISCLAIMERALWAYS read and follow the instructions printedon the pesticide label. The pesticide recommenda-tions in this UI publication do not substitute forinstructions on the label. Due to constantlychanging pesticide laws and labels, some pesti-cides may have been cancelled or had certainuses prohibited. Use pesticides with care. Do notuse a pesticide unless both the pest and the plant,animal, or other application site are specificallylisted on the label. Store pesticides in their origi-nal containers and keep them out of the reach ofchildren, pets, and livestock. Trade names areused to simplify the information; no endorsementor discrimination is intended.