COMMON SCAB OF POTATOES1
Authors: MILLARD W. A. 
Published: February 2008

SUMMARY OF PARTS I AND II.

Common Scab consists of a number of types of scab, which vary considerably in general appearance. Of these, the two most marked types have been called “Raised and “Pitted respectively. Other types are intermediate between these extremes. The causal organism in all cases examined belongs to the Actinomyces genus. The different strains isolated exhibit considerable differences in culture but, for the present, they may be placed in a single Actinomyces scabies group.
The incidence of scab is closely associated with soil type. It occurs most commonly and with greatest virulence on light, sandy or gravelly soils, especially on those of a “hungry nature. Conversely, it is rarely found on peat soils.
The diseaae may be inhibited by the applieation of sufficiently liberal dressings of green‐manure to the soil. Spent hops have also proved to be of value in this respect and leaf mould has long been used by gardeners with good results.
The action of lime or chalk on scab production depends largely upon the initial reaction of the soil. On neutral soils, it exerts little or no effect. On distinctly acid soils, it tends to produce or aggravate the disease unless the soil contains a large reserve of vegetable organic matter. This effect may be counteracted by green‐manuring.
The hydrogen‐ion concentration of the soil is not the direct factorof control in the occurrence of scab and this statement is borne out by the following facts:

Scab is found to only a slight extent in soils of a high hydrogen‐ ion concentration, yet, it may occur and Actinomyces have been found in large numbers in soils with a pH value as low as 4.4.
In more nearly neutral soils, there is no relation between the hydrogen‐ion exponent of the soil and the occurrence of scab. Thus, for example, one soil having a pH value of 7.0 may yield absolutely clean crops, whilst another with the same exponent may produce very scabby crops. Thia distinction cannot be explained by supposing the former soil to be virgin and uncontaminated with scab organisms.
In the treatment of scab by green‐manuring, any change brought about in the soil reaction appears to be towards a decrease rather than an increase in the hydrogen‐ion concentration.

6. The established facts concerning the common occurrence of scab on light soils, its comparative absence on peat soils, the action of lime and the cure of the disease by green‐manuring may be explained by the Preferential food hypothesis. According to this theory, the scab organisms are primarily saprophytic living on vegetable remains in the soil. They remain 80 until their natural food supply is exhausted and develop their parasitic tendenciee only under the stress of hunger.
7. Scab is more prevalent in dry than in wet seasons. This is no doubt due in some small degree to the higher soil temperature obtaining in dry seasons. The climatic factor of greatest importance, however, would seem to be rainfall which modifies the air content of the soil and thus regulates the development of the scab organisms which are strongly aerobic. The effect is most marked in clay soils where scab is almost entirely inhibited by a wet season.