Last year in Michigan the incidence and severity of common scab was worse than had been previously reported for several years. Several varieties were badly affected.
In this short article some of the possible reasons for this occurrence will be described. The common scab pathogen (streptomyces scabies) causes a range of symptoms on the surface of potato tubers including superficial russet lesions, pitted lesions or most commonly erumpent lesions which have a corky appearance and are raised above the surface of the tuber skin. Pitted lesions can be smooth or may also have a rough appearance. Potato scab is the most economically important disease caused by streptomyces species and the fourth most problematical in potatoes.
Scab is normally introduced into fields on infected seed potatoes. However, once in the soil the microorganism is persistent and survives indefinitely. Scab can survive on soil on plant trash and is favored in fields fertilized with animal manure. It can also be distributed in soil water, wind and on farm equipment. Scab can infect root crops such as carrots, beets, turnips, etc. The pathogen survives best in soils at pH 5.5-7.5. Short rotations between susceptible crops increases the pathogen population and severity of the disease.
Potato tubers are most susceptible to infection during early tuber development. The tubers are infected through stomata and immature lenticels, which have yet to form a protective barrier. Mature tubers with well-developed skin are not susceptible to infection. However, infections established when the tubers are immature expand as the tubers enlarge and lesions increase in severity over the season. The disease favors warm, dry soil at the time of tuber infection. Wet, well-irrigated soils inhibit scab. Sandy, well-drained coarse soils tend to favor scab and therefore it is essential that sandy soils are kept moist especially during early tuber development.
Scab-free seed should be planted in fields known not to have a history of scab. No seed treatments tested at MSU in scab-infested plots provided any significant control of scab incidence or severity on daughter tubers (table 1, below). However, a foliar application of Quadris 80 WDG (Zeneca) applied in 25 gallons of water per acre when the plants were about six inches tall shortly after emergence, tended to decrease infection.
A three-to-four year rotation with non-susceptible crops such as alfalfa, soybeans or cereals can reduce the scab population. Root crops should be avoided in rotations. Some potato varieties are moderately resistant to scab, e.g. Pike, Atlantic, Onaway and Superior, and should be used where the risk of scab infection is high. Soil pH should be maintained around pH 5.5 in fields where common scab is prevalent - for example by using applications of sulfur or acidic fertilizers, e.g. ammonium sulfate.
By far, the most important management technique is maintenance of soil water capacity to a minimum of 80% during early tuber development from about 10 days after full crop emergence for a further eight weeks. Tuber initiation begins early during canopy development and continues for several weeks, therefore immature tubers are present in the soil for a long time after initial tuber set and are susceptible to infection by scab.
During the past several years, scab resistance breeding has been a priority at MSU and some advanced breeding lines are well into development as commercial varieties (see http://potato.msu.edu/breeding.) In addition, some trials have been established at the MSU Potato Research Farm in Entrican and at the MSU Muck Soils Farm to evaluate the effects of some soil and seed treatments alone and in combination [e.g. Maxim (Novartis), Blocker (Amvac), Quadris (Zeneca)] on the incidence and severity of scab on susceptible potato varieties. These trials will be on display at the Montcalm Research Farm Field Day Aug. 17, the Muck Farm Field Day July 26, and the Late Blight Field Day at the Muck Farm on Aug. 24.
