The Histogeny of Potato Scab
Authors: Jones A. Powell 
Published: February 2008

Summary.

Pitted scabs. It has been shown that the pitted type of scab may originate in a tuber lenticel of which the meristem is then stimulated to division giving rise to an elongated type of cell. The newly divided cells are invaded by the parasite and subsequently collapse, meanwhile their contents and walls become brown tinted, giving to the scab its characteristic colour. As the destruction of the elongated cells proceeds the lenticel widens, and after a time its meristem becomes less active; its last‐formed daughter cells cease to elongate and become suberised, forming a compact tissue, or barrier, of wound cork, cutting off the infected external cells from the healthy tissues below.

Further development is due to the organism being able to grow through incompletely suberised cells that are present in the wound cork barrier and infect the cells below. Here another wound cork arises, following closely the contours of its predecessor and permitting also the further inward penetration of the parasite, thereby enabling a third wound cork to develop beneath the scab. No more than three of these barriers have been found in one scab, but if active tuber growth could be prolonged it is probable that further barriers would develop there.

As a result of the development of the successive cork barriers an increasing amount of tuber tissue is excised and decays, leading to the extension of the scab which, in time, is found to be lined with the last formed cork barrier.

The parasite was observed within the cells of all stages of scab and has been induced to grow within the living tissues of potato rootlets, etiolated potato shoots and plugs bored out of tubers. It has been observed to fructify freely in the tissues of mature scabs and infected plugs, where it displayed characters similar to those displayed when growing upon culture media.

The damage that the organism is capable of inflicting upon the root system of a potato plant is made evident.

The raised type of scab. The structure of two types of raised scab has been investigated. Both types have been shown to be more superficial than the pitted type and to possess only one wound cork barrier. The inability of their causative organisms to penetrate this barrier is suggested in explanation of their superficiality.

The two raised types have been found to differ from one another mainly in the restriction of abundant cell division, following the formation of a wound cork beneath the infected areas, to the naturally produced raised type.

The presence of Actinomyces flavus (Millard and Burr) was noted in artificially produced tumulus scabs where it displayed characteristics that have formerly only been observed on culture media. A species of Actinomyea, closely resembling A. flavus in sporophores and spore characteristics, was found in the naturally infected scabs.

An attempt has been made to explain the prevalence of wound cork beneath all the infected potato tissues examined, excepting those of young scabs, and its adjacency to these tissues on the basis of pH changes that the parasites may induce in the region of infection. It has been suggested that during the early stages of invasion the parasites may give rise to a gradient of pH favourable to cell division and extending from the infected to the healthy tissues. Later, owing to the increased activity of the parasite in the infected regions, conditions favouring suberisation are set up in the newly divided tissues leading to the formation of a wound cork barrier. Subsequent barriers will be formed in a similar manner below this first‐formed barrier, following penetration by the parasite of the barrier preceding them.

It is pointed out that the period of susceptibility of potato tubers to, at least, the pitted form of common scab is probably determined by the degree of suberisation of the tuber lenticels.