Planting potatoes in clean soil, above ground limits scab

A Stockton gardener writes to ask what causes scabby potatoes and what can be done about it.

The disease is caused by a soil microbe, Streptomyces scabies, belonging to a specialized group known either as branching bacteria or actinomycetes. Infections, while little more than skin deep, can cause severe economic damage to potato farmers. Home potato growers can put up with a certain amount of scab, but it does make for a disappointing crop when the tubers all come up looking so unsightly.

Like so many soil born pathogens (disease causers), Streptomyces can be quite persistent once it has been introduced into your garden soil. Indeed, it may occur there naturally, as well; however, most experts feel that the prime source of initial infection is the seed potato, itself. So, it is highly important to use only clean, certified seed, and don’t be afraid to visually inspect each tuber for any signs of corky, scabby lesions on the skin.

Potato plants are unusually tolerant of acidic (low pH) soil, which is fortunate, indeed, since the scab microorganism is not. Farmers have long known that lime or wood ashes should never be put on soil where potatoes are to be grown.

There is a strain of Streptomyces that has popped up lately which is able to cause scab — so-called acid scab — at lower pH than normal, but we won’t concern ourselves with that, here.

Other than planting clean seed in unlimed ground, there are really only two other ways to escape scab once it has become a problem. The first is to grow your spuds in freshly turned soil, physically separate from contaminated ground. The second is to not grow the spuds in the ground at all but rather upon the ground. Fresh compost, sludge, Cycle-Gro or even leaf mold can be laid in rows on top of the soil and seed potatoes planted directly in the organic matter. Leaves, hay or straw can then be used to “hill” the potatoes, once the vines are up over a foot. I’ve produced some spectacularly clean taters this way. I’ve also had huge problems with mice or some small rodent nibbling at the tubers growing in the loose organic layer.

Beets, turnips, and carrots may also develop scab from the same microbe. Removing all crop residues helps limit the carryover of the disease from year to year.

A Sullivan reader wants to know what to do with her raspberry rows now that the berries have all been picked.

Step one is to prune out all the canes that fruited this year, as they are destined to die this winter, anyway. This will give the new canes added room and sunlight for the serious job they must do this autumn: forming next year’s flower buds. Also, pruning out the spent canes helps to limit the various fungal diseases that may attack the crop.

Step two is to think, thin, thin. Most people let their raspberries grow into an unmanageable thicket. The ideal number of canes per foot of row is four to eight and the row, itself, should be kept to a width of 12-16 inches, measured at ground level. A lawnmower set high is a good way to discourage suckers from coming up where they are not wanted.

Step three is to train the remaining canes on a wire or other support so that next year’s bumper crop will be held high off the ground. Finally, one may prune the canes to a height of 40 to 50 inches later this fall, when the leaves have dropped. This job can also be left until next spring.

Never fertilize raspberries in the autumn as they are quite susceptible to winterkill if overstimulated.

Michael Zuck of Bangor is a horticulturist and the NEWS garden columnist. Send inquiries to him at 2106 Essex St., Bangor, Maine 04401.