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Common scab is known to exist in every in the United States where potatoes are grown. Once the organism becomes established, it is capable of surviving for many years, particularly if the soil reaction is neutral or alkaline.
The disease is characterized by the appearance of rough, corky spots on the skin of infected tubers. Hot, dry weather is most favorable for the development of common scab. It is likely to cause extensive damage when potatoes are grown on alkaline or “sweet” soils, or in soils that are about neutral, but it causes little or no damage in strongly acid soils. An application of fresh manure just before plowing and planting favors the development of common scab. In like manner, application of lime and wood ashes make the soil more alkaline and infection is likely to be more severe. Manure produced by livestock that are fed potatoes infected with scab is an important source of infection after it has been spread on fields. Likewise, diseased tubers that are discarded and spread on land to be used for growing potatoes later on are also an important source of infection.
How to control the common scab?
Practices that aid in the control of common scab include the use of (a) seed disinfection to destroy the organisms on infected seed, (b) proper rotations, (c) good central practices, and (d) resistant varieties.
Seed infection controls the organisms that are present on the seed, but it does not destroy those present in the soil. Thus it is not effective in controlling scab if the soil in which the organisms. Seed disinfection is important and useful, however, in that it is a means of control on slightly infected soils and it prevents the introduction of the disease to soils that are not already infected. Ordinarily, the whole tubers are treated. Several methods of treating the tubers may be used. These include treatment with (a) acidulated mercuric chloride (the acid-mercury dip method, (b) mercuric chloride (corrosive sublimate), (c) formaldehyde. Directions for using these chemicals can be obtained from the state agricultural experiment stations and the United States Department of Agriculture.
Continuous cropping with potatoes, or their frequent appearance in the rotation, is likely to result in a marked increase in the number of the scab organisms in the soil. So can to controlling the common scab by using longer rotations, so common scab in the soil is decreased.
Applications of fresh barnyard manure should be made well in advance of planting time. Likewise, lime should not be applied directly to the potatoes. Light applications can be made to the crop that directly precedes them in the rotation, but heavy applications should be made at least three to five years before the potatoes are to be grown. Diseased tubers that are being discarded should not be spread over fields in which potatoes are to be grown.
The development and use of scab-resistant varieties holds the greatest promise for growing potatoes successfully on soil infected with the scab organism.
Recent trial using broadcast applications of PCNB on scab infested soils have provided good control of this bacterial disease (common scab). Moreover, this chemical to control common scab also holds promise as an insecticide and herbicide. PCNB, although not as effective as aldrin or similar chemicals, substantially reduces tuber damage by flea beetles. In addition, good control of annual weeds can be obtained during the year of application and also the following year. Although PCNB holds considerable promise in the control of common scab, the comparatively high price of the chemical limits its value to potato growers at the present time.