Disease of the Month - Various (Potato) Tuber Rots

  Hort Snacks - October 2018
Download 722K pdf file ("HortSnacks-Oct2018.pdf")PDF
     Subscribe to our free E-Newsletter, "Agri-News" (formerly RTW This Week)Agri-News
This Week
     Hort Snacks HomeHort Snacks Home
 Bacterial Ring Rot (BRR) | Bacterial Soft Rot | Fusarium Dry Rot | Pink Rot | Pythium Leak

Bacterial Ring Rot (BRR)

Causal Organism: Clavibacter michiganensis subsp. sepedonicus

Crops Affected:

Disease Cycle:

  • Spread primarily through cutting implements and dirty equipment
  • Bacteria moves through vascular system and forms “ooze” which is full of inoculum and can spread on equipment, etc.
  • Vascular system of potato begins to break down and may cause stems to collapse
  • Entire tuber may break down through secondary infection by bacterial soft rot
  • Does not survive in soil from one season to another
  • Leaves may roll upward with necrosis starting at margins; some lightening and chlorosis of interveinal tissue
  • In some varieties a “dwarf rosette” can form because of internodes not growing properly
  • Milky “ooze” can be squeezed from stem
  • Infected tubers often cracked with characteristic “ring rot” of creamy coloured, cheesy exudate
  • Non-symptomatic tubers can carry disease in a “latent infection”
Cross section of potato tuber infected by BRR showing discoloration of vascular ring and adjacent tissue, and bacterial ooze
Symptoms of BRR on a potato plant: advanced stage of infection, showing wilt, rolling of leaf margins, mottling and necrotic tissue
Central Science Laboratory, Harpenden Archive, British Crown, Bugwood.org
J.D. Janse, Plant Protection Service, Bugwood.org

  • Buy and use only certified seed
  • Sanitize all farm equipment including cutting knives, transport trucks, planters, harvesters, storage buildings, etc.
  • Avoid use of cut seed when possible
  • If using own seed, test a representative sample at an accredited laboratory
  • Ensure that if cull piles have not frozen entirely they are buried and plants growing from these are dealt with
  • As a very serious pest, all seed potatoes grown on an infected farm will be de-certified, the land may not be used for potatoes for two years and all equipment must be inspected by the CFIA for cleanliness prior to subsequent plantings
Bacterial Soft Rot (a.k.a. Soft rot, Erwinia soft rot)

Causal Organism: Pectobacterium carotovora subsp. carotovora; P. carotovora subsp. atroseptica; other species (Bacillus, Clostridium, Flavobacterium, Pseudomonas) - formerly called Erwinia

Crops Affected:
potato and a wide range of root, fruit and leafy vegetables (e.g. carrot, cabbage, onion, tomato)

Disease Cycle:

  • Bacterial pathogen
  • Survives for several months in the soil
  • Can be dispersed by irrigation water or other water movement
  • A wound is required in order for tissues to be invaded
    • may be from mechanical, environmental or other pathogen or insect pest damage
    • may be from wound, bruise or other entry point
  • Immature or overripe tissues are more susceptible than mature tissues
  • May spread between plants during storage, transportation or in wash water
Conditions that Favour Disease Development
  • Disease development is favoured by high temperatures and moist conditions (soil moisture, high humidity)
  • Poor ventilation and high humidity or wet tubers in packaging can promote the disease development
  • Frozen tubers or harvesting under wet conditions can increase disease incidence
  • Often forms as a secondary rot to other diseases
  • Typically restricted to tubers, roots, or fruit tissues rather than actively growing tissues
  • Initial symptoms may include small, water soaked lesions or sunken brown lesions around a wound or entry point
  • Lesions may quickly enlarge and coalesce, with tissues becoming watery and mushy.
  • Infected tuber tissues are cream-coloured and become brown, slimy and stinky
  • Diseased tissues are sharply delineated from healthy tissues
  • Infected tissues may ooze or leak out onto other healthy product
  • Grow crops in well-drained soils
  • Maintain an adequate crop rotation
  • Harvest only mature products
  • Minimize bruising and wounding during harvest and all post-harvest activities
  • Control other diseases and insects that create wounds or weak points
  • Store crops at as cool a temperature as possible (without causing other issues)
  • Ensure wash water is properly disinfected (chlorine or ozone) and is changed frequently
    • Avoid soaking for long periods
  • Only package cooled product
    • Dry product before packaging
  • Disinfect equipment and storages periodically if soft rot is a consistent issue
Fusarium Dry Rot

Causal Organism: Fusarium sambucinum, F. solani var. coeruleum and F. avenaceum

Crops Affected:

Disease Cycle:

  • Inoculum found in contaminated soil or in infected tubers (main source)
  • Requires wound or entry points for infection of tubers
  • After infection, disease develops within infected tubers throughout the storage period
  • Does not spread between tubers in storage
  • Slightly darkened, shallow lesions become apparent within a month of infection
  • Infected tissues become sunken and somewhat wrinkled, with concentric rings of discoloured tissue radiating from the initial point of infection
  • Rotted tissues are dry, with cavities lined with mycelium and spores
  • Tubers may become completely shrivelled as rot advances and tissues dry out
Early (wet) stages of Fusarium dry rot, progressing towards internal cavity rotting
Photos by Robert Spencer
Fusarium dry rot of potatoes
Photo by MAFRI
  • Avoid wounding tubers at any stage of production
  • Adjust all equipment adequately to minimize wounding – during planting, harvest and post-harvest handling
  • Maintain good sanitation during pre-planting (seed preparation, planting, etc.) and post-harvest activities
  • Ensure adequate skin set prior to harvest (top killing)
  • Ensure that there is an adequate curing period to promote wound healing prior to long-term storage
  • Fungicide applications at bin loading are relatively ineffective, as contact between infected areas and control products is difficult to achieve – although it may help to protect wounded tubers initially until wounding healing can occur
    • Fungicide resistance is also very prevalent in Western Canada
  • Maintaining a low storage temperature can help to slow the development of this disease in storage (adjust according to type of potatoes being grown)
Pink Rot

Phytophthora erythroseptica

Crops Affected:

Disease Cycle:
  • Soil borne fungal pathogen that affects tubers
  • Pathogen is suggested to be resistant to drought and cold temperatures
  • Infections occur before or at harvest
  • Pathogen penetrates tubers through stolons (most often), but may also infect through eyes, lenticels or through wounds made during harvest or post-harvest handling
  • Infection by late blight can open door to infection with pink rot
    • Infection with both diseases increases amount of rot in tubers
  • May spread slowly in storage, however, the pink rot pathogen is more aggressive in tubers than the late blight pathogen
  • Causes a spongy, soft rot in harvested tubers
  • Wilt symptoms may occur in plants in the field
  • External surfaces may have a purplish-black appearance
    • The edge of the rot is delineated by a dark line
  • Infected skin rubs off easily
  • Internally, infected tissues are cream coloured or light brown, but turn salmon pink when exposed to air (within 20 minutes) and then turn black (within 1 hour)
  • Internal rots progress in a nearly straight line between healthy and diseased tissues
    • Rots often start from the stolon end
  • Rotted tissues give off an ammonia smell
  • Infected tubers exude a clear liquid when squeezed
  • Tubers remain intact but have a rubbery texture
Conditions Favouring Disease Development:
  • High / excessive soil moisture
    • Low areas in fields, over-irrigation, poor drainage
  • Disease susceptibility of different varieties is variable
    • Red-skinned > Russet-types > white-skinned
  • Use healthy seed tubers
  • Plant / grow potatoes only in well-drained soils
  • Remove diseased plants
  • Cull diseased tubers before storage
  • Protective fungicides can be used at specific stages (in furrow or at tuberization) to reduce pink rot infection
Pink rot – internal & external tuber symptoms
Pythium leak in potato tubers
Photo by Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org
Photo by OMAFRA

Pythium Leak in Potatoes; Shell rot

Causal Organism(s): Pythium spp. (2 main species)

Crops Affected:

Disease Cycle:
  • Fungal pathogen
  • Lives as a saprophyte (organism that lives on dead or decaying organic matter) or as long-lived oospores in the soil for indefinite periods of time
  • Doesn’t attack healthy tubers directly, but penetrates through cuts, bruises and other wounds that typically occur at harvest
    • Very rapid development leads to serious losses in storage, during shipping and in the market
  • May also affect cut seed and emerging plants
  • Extensive, grey to brown-coloured, water-soaked lesions occur on the surface around wounded areas on tubers
    • Often occur near the stem end
  • Infected tissues may be cream coloured, but turn brown when exposed to air
  • The rot will have a dark line at the margin
  • Rotted tissues will turn black over time and is granular in texture
  • Clear, brownish liquid will release from rotted tissues when squeezed
  • Water may drip or run from diseased tissues
    • More obvious in tubers in plastic in high temperature retail storage conditions
  • The entire inside of a tuber may rot, leaving a thin shell surrounding the rotted areas
  • Infected tubers may exhibit a sweet odor
Conditions Favouring Disease Development
  • Immature tubers (incomplete skin set) harvested under warm, moist soil conditions
  • Harvesting during hot weather or exposure of tubers to heat after harvest
  • Allow the crop to mature fully in the field prior to harvest
    • Encourage complete skin set
  • Minimize mechanical injury during the post-harvest process (harvest, grading, storage)
  • Harvest during cool conditions (cooler part of day)
  • For tubers harvested during warm conditions
    • Cool immediately
    • Increase air ventilation
  • Keep tubers out of sun after harvest
  • Ship and store tubers properly (including retail stages), avoiding high humidity and high temperatures
  • Avoid planting in poorly drained areas
  • Space plants to encourage good air ventilation
  • Avoid over fertilization which leads to lush, succulent growth
  • Overhead irrigate with care or avoid when risk of Pythium is high
Share via AddThis.com
For more information about the content of this document, contact Robert Spencer.
This information published to the web on September 24, 2018.