What is Potato Scab? Garden Tips 2023

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what is potato scab

Potatoes are a vegetable crop of enormous commercial importance worldwide. Classified as tubers, not root vegetables like carrots, turnips, beets, and parsnips, you will find them in every supermarket or fruit and veg outlet anywhere. As long as you have the space and a well-drained position for growing potato tubers, it isn’t difficult – unless your potatoes get potato scabs. 

Potato scab is a very common tuber disease that occurs wherever people grow potatoes. It doesn’t usually affect total yields, but it is known to result in very significant economic losses. Even though you can eat scabby potato tubers, they are so unsightly, they definitely aren’t marketable. 

What is a potato scab?

Potato scab, also known as common scab, is a disfiguring disease that attacks potatoes. It causes ugly, scabby lesions that spoil the appearance of otherwise healthy potatoes. 

Despite its appearance, potato scab doesn’t affect the total yield of potatoes grown on a commercial scale. But it is a major problem in terms of quality, which has a huge economic impact. 

There are several different types that all have quite distinctive symptoms. For example, the typical corky tissue that emerges from scab potatoes is sometimes superficial, sometimes raised, and sometimes pitted, with shallow-to-deep holes.

According to the Cornell University College of Agriculture and Life Sciences, these symptoms are all caused by the same pathogen, Streptomyces scabies. 

The reason the scab disease symptoms look different is probably because of several outside factors. These include the resistance of the potato and what potato type has been attacked. 

It also depends on when the infection sets in, what the environmental conditions are, and just how aggressive the pathogen scab potato strain is.   

The dilemma is that there are more than 400 species in the Streptomyces, and according to the experts at the Michigan State University (MSU) Extension, only a fraction of these are plant pathogenic

What is the cause of potato scabs?

As mentioned above, a common scab is caused by one of the most common tuber diseases. It appears to be more common in alkaline soils, which have a pH over 7.0.  

Interestingly, the bacterium, S. scabies, is believed to be related to a kind of bacteria that is used to produce antibiotics to treat human diseases. But that doesn’t help potato farmers or those of us who grow potatoes at home. 

But what is the cause? According to the experts at the North Dakota State University (NDSU) Potato Extension, there are hundreds of bacterial species in the Streptomyces genus. They say that the bacterium is spread by spores, in the soil, in water, or on insects or nematodes. 

It is likely that tuber infection happens in the early development of the potato. But they also say that the earlier the infection, the deeper the scab lesions are likely to be.

The experts at the University of Massachusetts (UMass) Amherst Extension Vegetable Program warn that disease severity is greater in warm, dry climatic conditions. It also tends to be more prevalent in light, gravelly, or sandy solid types. 

They advise maintaining a pH below 5.2 but warn that some less common species of this disease can survive in soils with a pH as low as 4.0. 

How do you treat potato scabs?

how do you treat potato scabs

Unfortunately, there’s not a lot you can do to treat potato scabs. Once your potatoes have the common scab disease, you’re unlikely to discover it until you dig the tubers out of the ground. 

But, as long as the scab disease isn’t too bad, there’s no reason why you shouldn’t eat your potato tubers. Nevertheless, if you initiate a potato scab control program, you will have a head start on protecting future crops from getting this soil-borne disease. 

A few steps you can take are to ensure that your soil isn’t alkaline and to avoid using fresh manure in garden beds where it has been a problem in the past. Rather use manure that has been well-composted, because it will usually be pathogen free.

How do you prevent potatoes from scabbing?

There’s not a lot you can do to stop potatoes from scabbing. The best solution for potato scab control is to start out with scab-free potatoes that have been certified. 

There are also several different varieties that have at the very least moderate resistance to potato scab. These include Norland, Superior, and Russet Burbank. 

Another approach is to plant potatoes in different areas of your garden, rather than in the same spot year after year. According to the Wisconsin Horticulture Division of Extension, scab disease caused by the pH in the soil will probably disappear if your soil pH is below 5.2.

This is more or less in keeping with what the UMass Extension experts say.

The experts at the NDSU maintain that cultural and chemical practices are the best options to prevent potato scabs. pH is one option, another is to increase the moisture of the soil to 80-85% until the tubers are at least 1-1.5 inches in size. 

Chemical management includes good seed treatment. They also mention a soil fumigant that controls several soil-borne diseases that work well to reduce common scabs.

Potato scab can be frustrating for anyone growing potatoes at home, but it can be a negative life-changer for commercial potato growers. Because of this, researchers continue to research better breeding and management practices in an endeavor to decrease this awful soil-borne disease.  

Can you eat potatoes with a scab?

Unless they are severely affected, you can eat potato tubers with scab disease. All you have to do is peel them and dig out any inferior bits.

It’s best to cook and eat affected potatoes as soon as possible. If you can’t, then store them in a dry, cool, dark place. This will minimize the chance of the scabby areas being infected by a soft-rot bacteria that will literally make them rot. 

Conclusion

You might be growing potatoes at home and have never encountered scab potatoes. But if you spot this disease caused by a soil-borne pathogen, the chances are you’re going to be horrified. 

The good news is that however ugly they might look, you can usually eat potatoes that are infected with common scabs. But if you’re planning on going to your local farmers’ market, you’re going to have a problem.

There’s not a lot you can do once you discover that your potatoes have potato scabs. But you can start a potato scab control management plan to prevent it from recurring.

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