Diseases of Potatoes: Fungal

Black Dot
Colletotrichium cocodes

What to look for?
Not a common disease.

Photo: Ormrod

Picture description Sclerotia on stems are black and disk-like and may be confused with Rhizoctonia.

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Management strategy
Disease-free seed and crop rotation are control measures.

Dry Rot
Fusarium sp.

What to look for?
This dry rot can be extensive in stored potatoes in some seasons.

Photo: Evans

Picture description Tuber lesion with white mycelium showing.

Photo: Evans

Picture description Severe tuber infection; Whole tubers may become shells in storage.

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Management strategy
Potatoes should be harvested during cool dry weather and avoid bruising.

Early Blight
Alternaria solani

What to look for?
Early blight can turn green vines brown in a few days in August under farvourable conditions.

Photo: Howard	Picture description Concentric rings from the leaf lesions. A target-like spot.	Photo: Howard	Picture description Infested field (Norchip)
Photo: Howard	Picture description Storage rot of tubers resulting from field infection by Alternaria.	Photo: Evans	Picture description Outbreaks may be controlled with fungicide sprays.

Management strategy
Several fungicides will give good disease control. Weekly application may be necessary in July and August under wan and humid conditions.

Late Blight

What to look for?
Late blight is an extremely destructive disease of potatoes reducing yields by 50% or more as well as causing considerable quality loss.

Photo:Evans

Picture description Late blight can occur directly on potato vines.

Photo: Evans

Picture description Very early leaflet infection

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Management strategy

1.	Fungicides must be applied early and regularly in years when blight appears.
2.	Avoid buying potato seed from areas where late blight normally occurs
3.	Use a blight forcast system for your growing area

Leak
Pythium ultimum

What to look for?

Photo: Evans

Picture description Rotted tubers.

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Management strategy
Particularly destructive in potatoes harvested under warm or even hot soil conditions. Cut or bruised tubers may be easily infected. Severe losses may occur in storage when potatoes are warm and stored at high temperatures under poor ventilation..

Phoma Rot
Phoma sp.

What to look for?
Rare in Canada, more common in the U.S. and common in Europe.

Photo: Evans

Picture description "Thumbmark" lesions on tubers.

Photo: Evans

Picture description Close-up of tuber lesion showing a typical pock mark caused by the Phoma infection.

Management strategy


Pink Rot
Phytophthora erythroseptica

What to look for?
A disease that occurs in potato crops when soil moistrue levels are unusally high. Tubers are infected through stolons, eyes and lentials.

Photo: Evans

Picture description Discolored tuber.

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Management strategy
Healthy tubers planted in well drained soils.

Rhizoctonia
Rhizoctonia solani

What to look for?

Photo: Evans	Picture description Pink upper leaves that look like aster yellows	Photo: Evans	Picture description Perfect stage of Rhizoctonia forms a white mat on the potato stems referred to as the telemorph.
Photo: Evans	Picture description Aerial tubers caused by severe Rhizoctonia infection of the stem. Tubers form above the infection.	Photo: Evans	Picture description Sprout infection by Rhizoctonia may cause emergence failure of the seed piece shoots.
Photo: Evans	Picture description Black scurf caused by the resting stage of the Rhizoctonia fungus does not wash off.	Photo: Evans	Picture description Knobby tubers formed as a consequence of rhizoctonia infection.

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Management strategy
Plant clean scurf-free tubers and follow a 3 to 4 year crop rotation.

Powdery Scab
Spongospora subterranea

What to look for?
This disease is common in the U.S. and parts of eastern Canada. The fungus, which can persist for many years in the soil, is a vector of the mop-top virus disease. More information on Powdery Scab.

Photo: Howard

Picture description Small pock marks form on the tuber that are filled with loose spores - the resting stage of the fungus.

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Management strategy
Russet Burbank, Atlantic and Russet Norkotch are resistant to this disease. Shepody and Norland are highly susceptible.

Seed Piece Decay
Various species

What to look for?
Tuber damage can spread considerably in storage and may be caused by a number of soft rot bacteria species e.g. Erwinia and Pseudomsray sp.

Photo: Howard

Picture description Field damage resulting in a poor stand establishment. Norchip.

Photo: Howard

Picture description Tuber with soft rot

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Management strategy


Silver Scurf
Helminthosporium solani

What to look for?
When we peeled potatoes this disease of little consequence. The disease is most obvious on red and pink skin types.

Photo: Evans

Picture description Tuber infection. Silver coloration occurs in the field and further develops in storage.

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Management strategy
Fungicides provide some control. Disease-free tubers should be planted with a 3 - 4 year rotation. Harvested tubers should be fungicide treated and fried off prior to cool dry storage.

Verticillium Wilt
Verticillium albo-atrum

What to look for?
This disease can persist and build up in soil if potatoes are grown in short rotations.

Photo: Howard

Picture description Field damage can be severe under dry or stressful growing conditions. This fungus is soil-borne.

Photo: Evans

Picture description Vascular necrosis by this fungus in infected potatoes can render them unmarketable.

Management strategy
Maintain high plant vigour with adequate fertility and again follow a 3 - 4 year crop rotation.

Potato Wart
Synchytrium endobioticum

What to look for?
Wart is a rare quarantinable pest found in a few locations in Prince Edward Island, Newfoundland and the U.S.

Photo: British Ministry of Agriculture, Fisheries & Food

Picture description Galls at stem base.

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Management strategy
Resistant cultivars are available but since this is a quarantinable pest all measures to eradicate the nematode must be followed immediately.

More information about bacterial diseases of potatoes can be found at Pest Management - Diseases - Fungal. This is part of the Guide to Commercial Potato Production on the Canadian Prairies.

Photographs and information assembled and prepared for ARD by Dr. Ieaun R. Evans Agri-Trend Agrology Ltd.