Some Thoughts on Common Potato Scab

Posted: March 13, 2012

I have often said to growers and potato breeders that I believed common scab was a serious problem in potatoes, because like it or not, potatoes are in a beauty contest at the local supermarket. Yes, we know you can peel off the skin with the scab on it and you are fine and your mash potatoes look beautiful. It is just that scab ruins the outward appearance of a beautiful, bright round white potato.


I was reading an article by Lauchlin W. Titus in the Maine Potato Grower Winter 2012 newsletter and thought I would share some of the information contained in the article. It came from information gleaned from the International Potato Common Scab Conference.


A survey of Canadian potato growers rated common scab of potato as their third priority disease of concern -- with late blight first and bacterial ring rot second. Common scab it was said was of concern because it is poorly understood and seldom managed. It is certainly true that what works in one field may not work in another field. There is no one factor that works in Maine, Pennsylvania, or the world.


Common scab of potatoes is caused by several species of the genus Streptomyces. The most common species is S. scabies, but there are other species that cause the problem. The problem is these species exist in all agricultural soils of the world and are found in forest soils and other non-agricultural areas.


Streptomyces spp. are filamentous spore and toxin producing bacteria. The toxin that causes the common scab symptoms is called thaxtomin. It disrupts the development of cell walls and results in scab lesions. The balance of Streptomyces spp. in a soil may be such that the scab causing species are suppressed. Factors that disrupt this balance may cause a field that never produced scabby potatoes to suddenly produce potatoes with so much scab that the field may have to be abandoned to potato production. The reverse can be true also.


Soils that are compacted, have poor soil structure, and are low in organic matter, tend to have high incidence of scab. Some of the practices that we will look at may work largely due to the improvements that have been made to the soil quality and health.


Some of the practices that reduce common scab are use of resistant varieties, crop rotations with various cover crops, and certain nutrient and fertility practices, and fumigation. No one of these work alone, but a holistic approach that incorporates several of them is probably the best way to manage for common scab.


For the potato breeders, breeding for resistance to common scab is a high priority, but can be elusive. It does offer one of the strongest defenses against common scab for growers.


As far as crop rotations, there has been a lot of research on the use of brassica crops or brassica cover crops (mustard, canola, rape, broccoli) prior to potatoes that tended to show a reduced incidence of common scab more so than other materials. Sorghum x sudan looked pretty good also. They probably "work" for different reasons though. The brassicas work as a result of the bio-fumigant activity of the breakdown of the crop. The sorghum x sudan produces a lot of sugars and other components that are readily digested by soil microbes, thus feeding the good species to the detriment of the bad species of Streptomyces spp. Rye, both winter rye and annual rye, did not work to suppress common scab.


Lime application is tricky. Most agreed that adequate lime spreading to provide calcium and magnesium nutrition is important, but high levels of lime spreading just prior to potatoes may cause some short-term soil chemistry changes that can result in increased scab incidence. Remember, scab is a result of cell wall disruption, so adequate calcium to build strong cell walls may be helpful. Gypsum can also be used to provide calcium nutrition. Some additional ideas on nutrients and fertilizers that may be useful: Have adequate soluble phosphorous in your fertilizer. Foliar applied phosphorous in one trial reduced scab by 20 percent. Use an ammonium source of nitrogen. Have adequate magnesium. Do not have excessive potassium. The ratio of potassium to magnesium percent of base saturation should be less than .5 and a range of .3-.4 seems to provide both increased yield and scab reduction. Adequate manganese is important as well. High carbon residues (small grain straw or corn stover) can bind up manganese in the short term and this micronutrient may need to be considered for inclusion in the fertilizer material.


Common scab is indeed a difficult disease to manage.