W. R. Stevenson University of Wisconsin - Madison, Department of Plant Pathology

Potato Late Blight

W. R. Stevenson, UW-Madison Why Do We Worry About Late Blight In A Seed Area ?  Seed potatoes are generally grown in cooler climates more favorable for late blight development  Late blight pathogen is wind borne, long distances  Late blight pathogen is tuber-borne  Late blight is a community disease

W. R. Stevenson, UW-Madison  Concerned customer base  Possible loss of crop certification  Cost of disease control $50 per acre  $250 per acre$50 per acre  $250 per acre  "Triple bottom line" financial viabilityfinancial viability environmental responsibilityenvironmental responsibility responsibility to societyresponsibility to society Why Do We Worry About Late Blight In A Seed Area ?

W. R. Stevenson, UW-Madison  Pathogen arrives in seed area via wind or seed  Infection of susceptible, unprotected plants Before adequate fungicide programsBefore adequate fungicide programs Poor or no fungicide coveragePoor or no fungicide coverage  Long wet and cool period  Disease spreads rapidly  Control costs escalate  Tuber infection  Pathogen survives winter on tubers  Repeat the cycle The Late Blight Cycle

W. R. Stevenson, UW-Madison Tougher seed standards  Currently allow 1% seed infection  Some states force growers to plant only certified seed  Rejection of seed lots when late blight on foliage  Would a field be rejected for 1? 10? 100? 1000? lesions per field??  Can we predict tuber infection on basis of foliage infection?? Breaking The Cycle

W. R. Stevenson, UW-Madison 1% seedpiece infection1% seedpiece infection 10 ft-diameter area killed from 1 infected seedpiece10 ft-diameter area killed from 1 infected seedpiece 10 foot diameter Potato Late Blight A Little Inoculum Goes a Long Way

W. R. Stevenson, UW-Madison Potato Late Blight A Little Inoculum Goes a Long Way 1% seedpiece infection1% seedpiece infection 10 ft-diameter area killed from 1 infected seedpiece10 ft-diameter area killed from 1 infected seedpiece = 25% of the area of the field= 25% of the area of the field

W. R. Stevenson, UW-Madison Tougher seed standards  Currently allow 1% seed infection  Some states force growers to plant only certified seed  Rejection of seed lots when late blight on foliage  Would a field be rejected for 1? 10? 100? 1000? lesions per field??  Can we predict tuber infection on basis of foliage infection?? Breaking The Cycle

W. R. Stevenson, UW-Madison Rules regarding cull piles  WI – mandatory cull pile destruction by May 20 Bury Bury Grind and spread Grind and spread Landfill Landfill Feed to livestock Feed to livestock Breaking The Cycle

W. R. Stevenson, UW-Madison Volunteer potatoes  Mild winters or heavy early fall snows  Early season planting of row crops – peas, beans, corn  Lack of effective herbicides  Are volunteers a public nuisance? Breaking The Cycle

W. R. Stevenson, UW-Madison Seed purchase and handling  Know the seed grower Visit the grower, walk the fields, view the storages Visit the grower, walk the fields, view the storages  Ask for field records – IPM scouts IPM scouts Certification inspectors Certification inspectors Shipping point inspection Shipping point inspection Breaking The Cycle

W. R. Stevenson, UW-Madison If late blight risk associated with seed -  Consider treating seed with Tops/MZ/Curzate = Evolve  Consider treating field with protectant fungicide at emergence If late blight pathogen confirmed on seed -  Consider the consequences of planting infected seed Increased management costs Increased management costs Risk to crop in field and storage Risk to crop in field and storage Community source of late blight Community source of late blight Breaking The Cycle

W. R. Stevenson, UW-Madison At emergence (risk of late blight in seed, volunteers or cull pile)  Consider treatment with protectant fungicide At row close or at 18 severity values  If no late blight, but risk high (18 severity values) Protectant spray with chlorothalonil, mancozeb, metiram… Protectant spray with chlorothalonil, mancozeb, metiram… Repeat at 5-7 day interval depending on crop growth and weather Repeat at 5-7 day interval depending on crop growth and weather Breaking The Cycle

W. R. Stevenson, UW-Madison  If late blight present in area, but not in field Treat with cymoxanil plus chlorothalonil, mancozeb, metiram immediately Treat with cymoxanil plus chlorothalonil, mancozeb, metiram immediately Continue at tight interval for three sprays then protectant alone Continue at tight interval for three sprays then protectant alone Breaking The Cycle

W. R. Stevenson, UW-Madison  If late blight present in field in pockets Disk hot spots with perimeter Disk hot spots with perimeter Treat rest of field and adjacent fields with cymoxanil immediately, and again in 5 days Treat rest of field and adjacent fields with cymoxanil immediately, and again in 5 days Then continue with protectant program Then continue with protectant program Breaking The Cycle

W. R. Stevenson, UW-Madison  Late blight throughout field Destroy field immediately and treat adjacent fields with famoxate/cymoxanil immediately and again in 5 days Destroy field immediately and treat adjacent fields with famoxate/cymoxanil immediately and again in 5 days Continue with protectant program Continue with protectant program Breaking The Cycle

W. R. Stevenson, UW-Madison Mid Season  Focus on protectant program that may include Curzate, Acrobat or Tattoo  Scout fields weekly or even more often  Careful irrigation to avoid excess Breaking The Cycle

W. R. Stevenson, UW-Madison Late Season  Focus on protectant program that may include Acrobat, Tattoo  Apply vine desiccant to protect size of seed potatoes and to allow at least 3 weeks of dead vines before harvest  Careful irrigation to avoid excess Breaking The Cycle

W. R. Stevenson, UW-Madison Effective late blight strategy focuses on:  Keeping the fungal pathogen out of the planting area  Intensive monitoring of every field  Careful monitoring of weather conditions  Careful application of fungicide  Partnership between each and every grower  Prompt response to local outbreaks Summary

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