The presence of Rhizoctonia and scab on Irish potatoes is an indication of soil conditions that need to be corrected before you plant your next crop of potatoes. The following explains how to identify these problems and provides the necessary guidance to prevent and correct problems with these diseases in the future.
Rhizoctonia – ‘the Dirt that Won’t Wash Off’
|Photos Courtesy of http://potatodiseases.org/rhizoctonia.html|
Tubers selected for planting can be already infected with Rhizoctonia so check them carefully prior planting. Certified disease-free tubers should be free of this disease. Tubers being sold for table use, i.e. from the grocery store, may be infected with this and other potato pathogens.
The Rhizoctonia fungus survives in soil on infected tubers remaining from the previous year’s crop as well as on decomposing plant residue. Using finished compost free of animal waste is critical to preventing problems with this disease. Since the fungus is less likely to attack potato plants that have formed green leaves, the faster the potato plant emerges from the soil, the less likely the plant is to be infected. Planting when the soil is cool (55 – 60oF.), planting tubers deeper than two inches, and wet soil increase problems with this disease. Warming the soil prior to planting with clear plastic sheeting is suggested. Planting shallow encourages rapid emergence resulting in less infection. Hilling up the plants to ensure there is adequate depth of soil for tuber formation will still be necessary but only after plants emerge.
‘Common Scab’ of potatoes
Infection by scab creates roughly circular, raised, tan to brown, corky lesions on the tuber. Pits up to one-half inch deep on the tubers can result. Tubers infected with this disease organism are safe to consume but have a lower quality in the market place.
Common scab, caused by the bacterium Streptomyces scabies, is the scab organism of soils with a pH above 5.0. Acid scab caused by Streptomyces acidiscabies occurs in soils below a pH of 5.0. A third acid-loving Streptomyces species, S. turgidiscabies, along with S. acidiscabies, have only been reported in the northeastern Unites States and Japan. The scabby appearance of the tubers is due to the plant toxins produced by these bacteria. Other Streptomyces species are known in Europe and elsewhere causing russet or netted scab. Unlike the Streptomyces species of North America, these species are known to cause root rot and yield reduction.
Choosing disease-free tubers for planting, as with Rhizoctonia, is critical with this disease as scab can be introduced into the soil by planting scabby tubers.
Scabby potatoes are more common in areas where high soil organic content is the norm, especially when unfinished compost or animal manure is used as soil amendments. Even in soils where low organic content (the native soils in Western Colorado typically have less than 1% organic matter) is normal, potatoes can suffer from scab when the tubers develop in contact with pockets of organic matter. This scab-forming bacterium is a saprophyte surviving on fresh dead organic matter as well as seedlings. To prevent scab thoroughly work compost into the soil breaking up any clods in the process. Lasagna gardening and the use of straw or hay in which potatoes are grown often results in more problems with scab.
Maintaining adequate moisture (80 – 90%) during tuber development has been shown to reduce scab formation. A three to four year rotation with grasses and cereal crops helps reduce the incidence of scab. Rotations with carrot, beet, spinach, turnip, and radish should be avoided as these crops can increase scab problems with potatoes.
The use of scab resistant cultivars of potatoes is recommended.
Table 1 - Scab Susceptible and Resistant Cultivars
Scab Resistant Cultivars
Table contents in part extracted from Rowe, R.C. 1993. Potato Health Management, APS Press.