Cultivation/Production, North America, Pests and Diseases

Water molds and bacterial diseases of potato tubers

While every growing season typically offers a few hiccups in the weather, some years offer a few more extreme disruptions than normal.  Extreme temperatures, in concert with moisture stress, exacerbate crop stress which may not be evident immediately following an intense weather event – but can reveal itself at time of harvest and beyond.

When test digs turn up evidence of pink eye, enlarged lenticels, or other physiological abnormalities, it’s good to be watchful for early evidence of disease.  These data aid in making the best, prescriptive decisions on late season crop treatment, harvest date, and any necessary special handling, as well as storage and market considerations.  

Some of the most common and problematic potato tuber diseases in storages include diseases caused by fungi such as silver scurf and Fusarium dry rot, and diseases caused by fungus-like or “water mold” pathogens such as pink rot, late blight, and Pythium leak.  Bacterial soft rot should not go without mention. 

Each disease is promoted by slightly different environmental conditions and each has key diagnostic features on tubers. 

Highlighted in this article are the water molds and bacterial diseases of tubers.

Pink rot

Phytophthora erythroseptica, the fungal-like pathogen causing pink rot, often causes tuber symptoms initiating from the stolon end which appear rubbery, yet firm.  The infected areas of tubers are often delimited by a dark line visible through the skin.  Buds, lenticels, and underlying tissue are black and usually exude a clear liquid. 

When cut and exposed to air, pink rot infections turn pink-salmon in color after about 30 minutes.  Pink rot is favored by high soil moisture which promote open lenticels and temperatures around 77°F.  Planting in well-drained fields with no history of pink rot, avoiding wounding at harvest and bin filling, and lowering temperature and humidity in storage can aid in management. 

Phytophthora nicotianae was documented in commercial potatoes in several fields of Wisconsin, as well as other North American growing regions in 2018.  While we have not yet detected this pathogen on tubers during test digs or post-harvest, it can cause tuber symptoms very similar to pink rot. 

Tuber symptoms caused by late blight

Late blight, caused by another fungal-like pathogen Phytophthora infestans, produces tuber symptoms that can be both superficial and visible externally as dark brown to purple lesions and present in the interior as brown, dry, and granular lesions. 

This disease is favored by temperatures from 64 to 75°F and high relative humidity.  Use of protectant fungicides in the production field while foliage is still viable is recommended. The use of fungicides with some systemic activity can aid in management.

Pythium leak

Pythium leak, caused by the fungal-like pathogen Pythium ultimum, produces tuber symptoms that begin as light tan, water-soaked areas around a wound.  As disease progresses, tissues can swell and periderm discolors with a dark line separating diseased tissue from healthy.  Internally, the tissue is spongy and wet and may contain cavities. 

When squeezed, tubers infected by Pythium ultimum exude a watery liquid.  Over time, affected tubers in storage appear as empty, papery skins.  This disease is favored by high temperatures, 77-86°F.  Avoiding harvesting in hot, dry weather and enhancing post-harvest conditions to promote wound healing can help manage Pythium leak. 

Tuber soft rot

Bacterial tuber soft rot can be caused by Pectobacterium spp., and more recently in the U.S., Dickeya spp.  Dickeya dianthicola was confirmed in the eastern U.S. in just 2015.  Seed infection can lead to blackleg symptoms including poor emergence, chlorosis, wilting, stem rot, and darkened slimy, black stems. 

Tuber infection can occur from movement of the bacterial pathogen within a plant infected from seed, or it can occur from late season infection following wounding or damage at- or post-harvest. 

Although disease symptoms are often indistinguishable between Pectobacterium and Dickeya spp., Dickeya spp. appear to require lower inoculum levels in order to start disease, have ability to spread through the plant’s vascular tissue better than Pectobacterium, are more aggressive, and require higher optimal temperatures for disease. 

Symptoms caused by Dickeya spp. tend to develop when temperatures exceed 25ºC (77ºF), while Pectobacterium predominate below 25ºC.  

Tuber soft rot ranges from a slight vascular discoloration to complete decay. Affected tuber tissue is cream to tan in color and is soft and granular. Brown to black pigments often develop at the margins of decayed tissue.

Lesions usually first develop in lenticels, at the site of stolon attachment or in wounds.  Dickeya spp., particularly at temperatures of >27ºC (80ºF), cause more severe rots than P. atrosepticum and are more likely to produce a creamier, cheesy rot.

Management strategies 

While nature can be uncooperative in helping to limit post-harvest disease, there are cultural approaches and chemical tools available to mitigate infection and spread.  Strong disease control programs during the production season are the best post-harvest storage disease control programs. 

Harvest temperatures should optimally fall between 12-18ºC (or roughly 54-64ºF) and soil moisture should be adequate to minimize damage.

Care should be taken to minimize drops and subsequent bruising which can become sites for post-harvest infection. 

Once in storage, maintenance of good air movement, along with temperature and humidity appropriate to the variety, market type, quality status, and disease condition for optimum conditioning are critical.

Amanda Gevens

This article was first published in Global Potato News magazine, February 2019. It was written by Amanda J. Gevens, Extension Vegetable Pathologist, Associate Professor, UW-Madison, Dept. of Plant Pathology, 608-890-3072 (office).
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