06-03-10

Question and Answer

Q. Some years our potatoes and beets have a pitting or scabbing on the surface. It makes them very unappealing to want to eat them. I think they are OK on the inside. What causes this?

A. While it is difficult to diagnose with certainty without seeing samples, there is a disease commonly called potato scab that affects both potatoes and beets. Other root crops, such as radish, rutabaga, turnip, carrot and parsnips, are also affected. 

The disease causes irregularly shaped, raised scabs or cork-like blemishes on the surface on the outer surface and, as you've suggested, you can peel away the affected portion. Scab is caused by a soilborne microorganism called Streptomyces scabies. The pathogen is usually initially introduced into uninfested soils by infected seed potato pieces. This is why it is so important to use certified disease-free seed potatoes to start your crop. Once the disease is established in an area, the scab pathogen will survive indefinitely on infested crop residue in the soil.

Unlike most other diseases, potato scab is worse in dry soil, so maintaining adequate soil moisture can reduce infection. Dry soil and a soil pH of 5.5-7.5 favor scab infection. Maintenance of adequate soil moisture during tuber set and reducing soil pH to 5.2 or below will reduce the severity of infection. Use acid-forming sources of fertilizer, such as ammonium sulfate, and avoid alkaline-forming sources, such as fresh barnyard manure, calcium nitrate and potassium nitrate. Rotating the garden out of susceptible crops for 3-4 years will also help reduce the source of inoculum.

Q. We have two beautiful young red oak trees, each about 15 feet tall. I observed unusual "growths" on one; a large sphere is attached to the leaves and a smaller sphere was attached to a branch. Can you identify? (see photo)

A. Oak trees have an amazing array of odd formations called galls that grow on the leaves and/or twigs called galls. Leaf galls are usually more of a cosmetic problem rather than a health crisis. The galls themselves are mostly made up of plant tissue, usually as an attempt to recover from insect or disease injury. Galls can be quite small, just a fraction of an inch, or can be as large as several inches long, depending on the plant and cause of injury. Some of the most common landscape plants that develop galls include oaks, maples, hackberries and roses.

Most galls occur on leaf tissue and are caused by insects. Adult insects lay eggs inside the leaf tissue, and either the adult or the developing young insects secretes a growth stimulating substance. Each insect causes a very characteristic gall. Most leaf galls are nothing to be concerned about from a plant health standpoint, although they may be unsightly. However, once the gall appears, the look of the current growth cannot be remedied.

Pruning out affected growth is about all that can be done once the galls appear. Pesticides to prevent insects or diseases from attacking the plants must be applied before injury occurs and the growth stimulating substances occur. For leaf gall-forming insects, insecticides must be applied during the brief week-or-so period that leaves unfold and fully expand in spring. Unfortunately, other pest cycles are less understood, and chemical controls may prove inadequate. And spraying large mature trees is not practical or advisable.

Fortunately, most gall-causers are host-specific, meaning that they each have a preferred plant species. So galls that occur on oak trees, for instance, will not spread to other types of plants in the yard.

For more information on galls of ornamental plants, see Purdue E publication E-56 Galls on Shade Trees and Shrubs http://extension.entm.purdue.edu/publications/E-56.pdf

Q. Can you tell me what this plant is? It starts out as a shoot with crinkly leaves and quickly turns into a bush with sticker branches. It is taking over everywhere - ditches, fields, woods. If I cut off a shoot, three more come up. If I spray, the leaves come off, but it quickly re-leafs. Is there a way to contain it? (see photo)

A. I did not observe the stickers you mention, but the foliage looks like it could be autumn olive, Eleagnus umbellata, a spreading shrub that often forms thickets. The young foliage and stems are covered with silvery scales, and the bottom of the leaves retains the silvery appearance as the foliage matures. Flowers are white to pale yellow and are fragrant, followed by red fruit that are eaten by wildlife (thus spreading new plants!) The species can be quite invasive, and it has been declared a noxious weed in some states. I'm afraid repeated cutting is probably your best strategy. You could try cutting it back and treating the cut trunk with a stump killer-type herbicide, but you would need to be extremely cautious about how and when you apply to avoid damage to desirable species growing nearby. As always, read and follow the label instructions before you apply any pesticide.