Why Some of the Most Dangerous Potato Diseases are Successful

Bacterial soft rot (Pectobacterium carotovorum)
Photo by Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org

All but a few of the plants and animals we’re familiar with have one thing in common: they require oxygen in the atmosphere (or the water) to exist. Potatoes, people, dogs, cats, fish, corn, wheat, and nearly everything else, including most species of bacteria and fungi, need oxygen.

We refer to these oxygen-dependent lifeforms as being “aerobic.”

Less familiar are lifeforms that cannot grow in the presence of oxygen. Referred to as “anaerobic,” most of these organisms are bacteria, though a few rare, multi-cellular forms do exist.

Probably the most familiar of the anaerobes are members of the bacterial genus Clostridium, including C. tetani, the organism that causes tetanus, and C. botulinum, which produces botulinum toxin, a poison that can cause paralysis and other serious complications in humans. Incidentally, tiny doses of this toxin (sold under the brand name Botox and others) are used for cosmetic purposes.

The Pathogens That Are OK With or Without Oxygen

There is also a group of organisms that straddle the fence and are able to live in the presence or absence of oxygen, their metabolism converting to some form of fermentation under low oxygen conditions. We refer to these types as “facultative anaerobes.”

Easily the most familiar of these is Saccharomyces cerevisiae, the common yeast used for brewing beer and raising bread. Other familiar facultative anaerobes are bacteria of the genus Lactobacillus, used in the production of a large number of important foodstuffs like yogurt, sauerkraut, pickles, and sourdough bread, among others.

So, what does this have to do with potatoes? Soft rot bacteria, arguably the most important potato pathogen known, are facultative anaerobes. Bacteria of the genera Pectobacterium (formerly Erwinia), and Dickeya (responsible for causing seed piece decay), blackleg, stem soft rot, as well as extensive storage losses, are members of this group.

Like it or not, potato tubers are regularly subjected to anaerobic conditions in the field and in storage. Portions of a potato field that are over watered, such as low spots, center tower areas, or even areas that are just plain subjected to too much rain can provide anaerobic conditions in the soil. In fact, the most frequent cause of soft rot seed decay and blackleg is probably soils that become waterlogged at the wrong time.

In spite of all these capabilities, soft rot bacteria are unable to create their own entry points and require a wound of some sort to gain entrance to the tuber. Nicks, cuts, and bruises are frequently invaded, and natural weak points like lenticels can offer access to the tuber, especially under waterlogged conditions.

The wounds most often exploited, however, are provided by different potato disease organisms. Previous infection by late blight, pink rot, Pythium leak, and Fusarium dry rot, etc., can render tubers vulnerable to soft rot infection.

The tendency to invade preexisting wounds makes the soft rot organisms excellent secondary invaders, and in this role they are often responsible for severe losses. Frost damage is another frequently encountered type of foliar and tuber damage that readily leads to soft rot infection.

The ability to exist between worlds, with or without oxygen, affords a unique niche for this group of extremely important pathogens, making soft rot by far the most destructive storage disease of potatoes.

Careful management of ventilation and temperature are the only effective methods of control for soft rot in storage. High volumes of air will dry-down infected tubers and temperatures below 50˚F are unfavorable for soft rot development. In the field, about the only effective management techniques are to properly manage other pathogens, avoid over-watering and take steps to minimize mechanical damage.

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