The season is almost over, autumn is around the corner and the cool temperatures at night are really starting to set in. Even with all the fun of growing a garden, it is still a lot of work and you are ready to reap the rewards.
Grabbing your trowel and gloves, you head out into the garden again to start this gratifying process and dig out your first beautiful mound of potatoes. Lo and behold, instead of coming out dirty, and glowing with good eating potential, these potatoes look like a witch’s brew.
They are covered in scabs and lesions, still the shape of a potato but the smooth skin is gone, and they look like a warty mess!
What has done this to your potatoes and what should you do now?
In this article, we are going to answer those two questions, to give you some information about what is potato scab, how to identify it, as well as some prevention and management techniques to use. Hopefully, this does not occur year after year.
Potato scab (Streptomyces scabies) is a common tuber disease that has spread throughout the world wherever potatoes are grown.
Another name for it may be ‘Common scab’, as there are other scabs, like ‘Acid scab’ caused by Streptomyces acidiscabies, that are found on potatoes but have much more limited distributions.
Since it affects almost any tuberous plant, keep in mind that it will also infect plants like beets, turnips, carrots, radishes and even rutabaga and parsnips. It is a bacterium-like organism that overwinters in soil and fallen leaves from the year before.
For those of us who like to use more natural fertiliser, it is also good to know that it can be spread through fresh manure since it is strong enough to survive through an animal’s digestive tract.
The organism will enter through cuts or bruises on the stem of the plant, down through the stem and directly into the skin of the potato.
The disease caused by Streptomyces scabies can be confused during identification with the fungus Spongospora subterranea, or Powdery scab, but is identifiably different.
It is also very important to note, that although ugly, potatoes infected with the potato scab are still edible, it is a problem simply because it can greatly decrease their market value.
Simply cut off the outside of the potato and around any chunks that have formed depressions into the inside of the potato.
Differences between potato common scab and powdery scab
The symptoms of potato scab are easy to see on the potato, but as said before, also easy to confuse with powdery scab.
Potato common scab
With Common scab, you will notice raised lesions on the skin, which have a cork-like texture.
As the disease progresses the corky area will become more raised and generally get darker and rougher.
Depending on the direction of the disease, these corky areas can end up covering the entire exterior of the potato by growing and merging together.
At times the potato will also show signs of craters and cracks that develop and dip into the skin of the potato.
These differences in symptoms will depend on how aggressive your particular strain of the disease is, the resistance of the potato, the amount of time of infection as well as favorability of the environmental conditions in your garden.
With Powdery Scab the marks on the potato are not raised and appear less aggressive than with common scab. The scab marks will not appear to be cork-like, and will usually have a thin flap of skin around the outside of the scab.
The differences in the two diseases, Potato scab and Powdery scab, are important to note as knowing your disease will make the fight against it much easier.
There are some big differences between the symptoms of the two diseases that make it easy to tell once the potatoes are harvested.
Potato scab will infect the roots and stolons of the plant, normally with star-shaped lesions and sunken centres.
However, powdery scab is different in that the stolons (part of the roots) may still be infected but will form galls (ball like formations) and the roots will have milky-white galls that are easy, and ugly, to see.
Powdery scab will also develop small pimple-like purple-brown swellings over the surface of the potato along with the scabs, and this will never be observed with potato scab.
The other major difference is that with powdery scab, the skin at the edges of the lesions will begin to loosen and form flaps.
In Potato scab, the lesions will all merge together instead of becoming loose.
Look out for powdery-looking centres to the sunken lesions that may develop on the potatoes as well, as this will be a dead giveaway for Powdery scab.
Treatment and management of potato scab
Although it can be quite difficult to get rid of, as Streptomyces scabies can live in the soil for many years without a host, there are some ways to lessen the likelihood of the disease becoming very prevalent in your garden.
Having an awareness of what you are planting and when will always be very helpful for diseases like this.
Use certified seed potatoes
Check that the seed potatoes that you are planting are certified or you can plant scab-resistant varieties also.
This will give you a one-up before the disease even has a chance.
Have a good rotation
Also, keep your garden on a rotating schedule to decrease the build-up and aggressiveness of the disease.
It is important to keep in mind the other root crops that are easily infected by this disease as well and keep them out of the rotation on that part of your garden along with the potatoes.
Till cover crops in well
When considering your schedule, also know that planting a cover crop, for example alfalfa or canola, and tilling it in before planting the root crop has also been shown to greatly decrease the amount of infection.
Environment – correcting the pH
Favorability of environmental conditions was talked about briefly earlier in the article and when considering these, you should do your best to make your environment as unfavorable as possible.
The bacteria thrives in dry, alkaline soils and will be much less prevalent if the soil pH is lowered below 5.2 by adding products like elemental sulfur.
Avoid very dry conditions
Common scab is harder to manage when the soil is too dry, so closely monitor the moisture in the soil. Ensure the soil does not dry out completely for about 2-6 weeks during the early stages of growth to greatly reduce the aggressiveness of the disease.
You can help to ensure your soil keeps its moisture by tilling it to a very fine tilth. Large untilled clumps of soil in the drill will dry out much quicker than finely-tilled soil.
There you have it, now you know what potato scab is, where to look for it and what to do with it if it does become a presence in your garden.