Common Scab Of Potato: Management Begins Early
Some of the problems that show up in potatoes at harvest actually begin very early in the season. Such is the case with the tuber blemish disease, common scab. Within a couple of weeks after potatoes emerge, the tips of the newly produced stolons will begin to enlarge. These swellings are destined to become tubers but shortly after the swellings begin to form, the new tubers become susceptible to a surface defect known as common scab. This disease is caused by Streptomyces scabies and at least two other related species S. turgidascabies and S. acidiscabies.
The causal organisms are members of the Actinomycetes, a group of soil-borne bacteria that are noted for their production of antibiotics. Streptomycin, for instance, is a commonly used antibiotic that was originally derived from a species of Streptomyces. S. scabies, also produces biologically active materials, notable among them a toxin called thaxtomin which is partially responsible for the disease that the pathogen causes.
Off Balance Tubers
Common scab skin blemishes usually have a reddish-brown corky appearance. This appearance is due to the fact that the causal organism exists in a sort of uneasy equilibrium with the potato tuber. The organism probably gets a toehold on the tuber through the lenticels but insect wounds or other types of mechanical damage can also be exploited. Once it has become established, the organism grows between the cells of the tuber periderm and secretes toxic materials, like the aforementioned thaxtomin, that kill some of the underlying tuber cells. The organism then utilizes the dead cells for nourishment. The secretions also stimulate the underlying living cells to form layer upon layer of wound periderm, which consists of dead cells at maturity, which are, in turn, used as a source of food.
As long as the tuber is increasing in size, this hyperactive wound periderm formation continues to take place. The grossly thickened areas of wound periderm, characteristic of the disease, are the reason for the reddish-brown corky appearance of the lesions. Fortunately, the infection process ceases when the tuber stops growing and the blemishes do not become progressively worse in storage. Also on the plus side, common scab lesions generally do not provide openings for invasion by other rotting organisms in storage.
Common scab lesions are usually raised above the level of the normal periderm but lesions wherein the organism appears to have burrowed into the tuber do occur on occasion. This condition is called “pitted scab” and it can be quite severe, especially if the crop is being produced under conditions conducive to this type of symptom formation or if the variety of potato is particularly susceptible.
Controlling Scab
What about management? As mentioned in the introduction, early season decisions are the key. Management is probably best accomplished through the use of resistant varieties but there are several cultural practices that can be effective in reducing or eliminating common scab. Avoid the use of manure for fertilizer, especially in the spring, just before planting. The increase in organic matter associated with manure application is very favorable for scab development.
Lowering soil pH or choosing a planting site with a low soil pH can also be an effective tactic. Common scab ceases to develop when pH gets below about 5.2, and planting potatoes, especially very susceptible varieties, in low pH soil is one of the recommended management procedures for common scab. One of the most effective management techniques for common scab is to maintain good soil moisture during tuber initiation and for at least a month after this process has begun. Overly dry soil conditions during this critical period can lead to severe common scab infestations.
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