by B. Rosie Lerner
Purdue Extension Consumer Horticulturist‘Unusual growths’ usually caused by insectsQ: 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? — David Syler, Angola, Ind.
A: Oak trees have an amazing array of odd formations called galls that grow on the leaves and/or twigs. 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.
Leaf galls are usually more of a cosmetic problem rather than a health crisis.
Photo submitted by David Syler
Q: Some years our potatoes and beets have a pitting or scabbing on the surface. It makes them very unappealing to eat. I think they are OK on the inside. What causes this? — Jim Frey
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 three to four years will also help reduce the source of inoculum.