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Managing Fungal Diseases

Michael Harding and Ron HowardAlberta Agriculture & Forestry RJH Ag Research

Potato Post-Harvest Management WorkshopLacombe, AB March 8, 2017

Overview

DISEASES CAUSED BY FUNGI

Black dot Black scurf

Dry rot Early blight

Late blight Leak

Pink rot Scab

Silver scurf

NOVEL DISEASES OF CONCERN

Zebra chip

Dickeya soft rot2

Black Dot

• Caused by Colletotrichum coccodes

• Colletotrichum survives in the soil on crop residues and on seed tubers

• Black dot is often overlooked because symptoms occur late in the season and can be confused with other diseases, e.g. fusarium and verticillium wilts, rhizoctonia, early blight and silver scurf

Disease Cycle

• Starts with sclerotia

• Invades underground stem tissues

• Airborne spores can infect the foliage

• Infection can progress downward

• New sclerotia are formed

• Sclerotia can overwinter and spread

G. Letendre

Black Dot

Tubers exhibit grayish lesions on the surface that resemble silver scurf

Tuber injury is more severe on thin-skinned than on netted-skinned cultivars Nebraska G92-1090-A

Sclerotiacan form on the surface of infected tubers

Tuber Infection

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Control Measures

• Plant early maturing cultivars

• Plant when soil conditions favor rapid germination and emergence

• Good agronomy (avoid stress)

• Apply protective fungicides to the foliage

Rhizoctonia Canker and Black Scurf

• Caused by Rhizoctoniasolani

• Survives in the soil and on seed

• Attacks all below ground plant parts

• Low soil temperatures in early spring favor disease development

Black Scurf Tuber Symptoms Disease Cycle

• Rhizoctonia persists as mycelia and sclerotia in soil, on crop debris and on tubers

• Sclerotia germinate and infect emerging sprouts, roots, stolons and developing tubers

• Early season infection causes delayed emergence, stunting, wilting and death

• Low soil temperatures favor infection

Control Measures

• Apply registered seed-piece treatments

• Plant when soil conditions favor rapid germination and emergence

• Maintain uniform soil moisture levels

Fusarium Diseases

• There are three main types of fusariumdiseases: seed piece decay, wilt and dry rot

• Fusarium species can attack potatoes both in the field and in storage

• They are able to survive harsh conditions

• They produce spores that can spread long distances

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Disease Cycle

• Many Fusarium species are involved

• Can be both seed- and soil-borne

• Tubers can become infected during cutting and handling and harvesting

• Seed pieces and potato roots can be directly infected from the soil

• Seed-piece decay and dry rot can proceed over a wide range of temps Seed-Piece Decay

Seed-Piece DecayFusarium Wilt

Dry Rot

Control Measures in the Field

• Apply registered seed-piece treatments

• Harvest tubers when skins are well set (top kill)

• Avoid damaging tubers during harvest and subsequent handling

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Control Measures in Storage

• Grade out infected tubers

• Apply post-harvest fungicides

• Encourage suberization of tubers (55-65C)

• Keep storage temperatures as low as possible for the type of potato

• Avoid unnecessary mechanical injury during handling operations

Alternaria Diseases

a. Early blight (Alternaria solani)

• Infects potato and tomato

b. Brown spot (Alternaria alternata)

• Also known as black pit

• Can be both a saprophyte and parasite on a wide variety of plant species

Disease Cycle

Several conditions can predispose plants to infection:

– Leaf wetting

– Wounding and bruising

– Insects and nematodes

– Aging

– Poor nutrition, especially low N

– Other environmental stresses

Early Blight

Brown Spot

Comparing Early Blight to Brown Spot

Early Blight Brown Spot

Pathogen Alternaria solani A. alternata

Aggressiveness Strong/moderate Moderate/weak

N deficiency Strongly favors Slightly favors

Leaf damage Slightly favors Strongly favors

Tuber rot Yes - lesions Yes - pits

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Control Measures

• Reduce plant stress, e.g. use high quality seed and provide optimum fertilization, irrigation and weed control

• Apply protective fungicides

• Avoid bruising tubers during harvesting

• Provide proper conditions for rapid suberization in storage after harvest

Scab

Common scab is caused by Streptomyces scabies, an actinomycete

Powdery scab is caused by Spongosporasubterranea, a protist

These diseases may look similar in the early stages of symptom development and may occur together on the same tuber

Comparing Powdery Scab to Common Scab

Common Scab Powdery Scab

Cause Streptomyces scabies

Spongosporasubterranea

Damage Tubers Roots & tubers

Soil moisture Dry Moist or wet

Temperatures Warm Cool

Virus vector No PMTV

Disease Cycle

Common Scab• Streptomyces species can persist indefinitely

in the soil on organic residues and plant roots• They infect through lenticels on young tubers

Powdery Scab• Spongospora can survive for long periods in

the soil as cystosori (spore balls)• Spores germinate to produce zoospores which

infect young tubers and roots

COMMON SCAB

Powdery Scab Lesionson Potato Tubers

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Powdery ScabRoot Galls

Control Measures – Common Scab

• Avoid planting scabby seed

• Plant resistant or tolerant varieties

• Maintain high soil moisture levels at tuber initiation

• Avoid using soil amendments that increase soil pH

Control Measures – Powdery Scab

• Select tolerant varieties, e.g. Russet Burbank and Ranger Russet

• Avoid susceptible varieties if PS potential is high, e.g. AC Glacier Chip, Dakota Pearl, Shepody, Norland, Yukon Gold

• Control soil moisture at tuber set• Don’t spread infested soil, manure or

compost onto potato fields

Silver Scurf

• Caused by Helminthosporium solani

• Seed- and soil-borne fungus

• Can persist in the soil for many years

• More severe on thin/white-skinned var’s

• Symptoms increase in storage

• Infected potatoes are unappealing to consumers and difficult to peel for processing into French fries and chips

Disease Cycle

• Helminthosporium survives on tubers and in soil

• Infects through lenticels or directly through the skin

• May take several months to express visible symptoms on tubers

• Generally worsens with time in storage

Silver Scurf

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Control Measures

• Harvest tubers when mature with good skin set

• Ventilate storages to keep tuber surfaces dry

• Keep storage temperatures as low as possible

• Apply post-harvest fungicides

OOMYCETE DISEASES

• LATE BLIGHT (Phytophthora infestans)

• PINK ROT (Phytophthora erythroseptica)

• LEAK (Pythium ultimum)

DISEASE CAUSAL AGENT FOLIAGE TUBER AIRBORNE

LATE BLIGHT P. infestans Yes Yes Yes

PINK ROT P. erythroseptica No Yes No

LEAK P. ultimum No yes No

SYMPTOMS MANAGING PINK ROT & LEAK• Avoid harvesting swampy areas.

• Allow tubers to mature, skins to set.

• Avoid mechanical bruising during harvest.

• Apply metalaxyl (Ridomil) during early bulking and/or in-furrow at planting.

• Apply phosphite-based fungicides at tuber initiation (then twice more two weeks apart)

• Harvest when temperature is below 75°F.

• Cure tubers for 3 weeks at 45-50°F.

• Cool potatoes rapidly to 40-45°F for storage.

• Keep humidity low during cooling.

• Force air over tubers continuously during cooling

• Low temperature and humidity will stop leak from growing and dry out infected tubers.

MANAGING LATE BLIGHT

• Use clean, disease-free seed

• Use systemic seed treatment after cutting

• Eliminate cull piles, volunteers and weeds

• Use resistant cultivars where possible

• Be prepared to apply protective fungicides– Use forecasting info and fungicide rotations as

recommended by the PGA

• Destroy all foliage in “hot spots”

• Desiccate crop prior to harvest

• Allow tubers to mature, skins to set.

• Apply phosphite-based fungicides at tuber initiation (then twice more two weeks apart)

• Same storage recommendations as PR and Leak

NEW DISEASES OF CONCERN

• Dickeya soft rot

• Zebra chip

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Dickeya soft rot

• Dickeya spp.

• Can initiate disease with less inoculum

• Better, faster spread through vascular tissue

• Is more aggressive

• Needs a higher optimum temperature to flourish.

Management of Dickeya• AVOIDANCE!

• Use clean seed

• Clean and disinfect cutting and handling equip

• Plant seed that is well suberized

• Plant into well drained soil 10 – 13°C

• Minimize tuber bruising and wounding

• Store tubers with good air flow, humidity and as cool as possible.

Zebra chip

• "Candidatus Liberibacter solanacearum,“

• Vectored by the “potato psyllid”

• Causes dark stripes in tuber flesh and potato chips

Management of Zebra chip

• Scout for and test psyllids for Lso

• Remove and destroy infected plants

QUESTIONS?