130 



PI/ASMODIOPHORALES 



tain soil types and the degree of infection was ob- 

 served bj' Melhus, et al., in Maine, and they were 

 accordingly able to predict the extent of develop- 

 ment of the disease from the type of soil and its 

 drainage. Wherever the Washburn silt-loam type of 

 soil occurred infection was unusually Iieavy. Wild, 

 likewise found that powdery scab flourishes in 

 Switzerland in soils with a large pore space, high 

 humus and methylpentosan content, coarse texture, 

 and high water-holding capacity. 



Symptoms 



Powdery scab may manifest itself as shallow, 

 scabby lesions or deep eroded cankers on the tubers, 

 and galls or warts on the roots and stems. These 

 phases of the disease may be followed by powdery 

 scab dry rot after the tubers have been harvested 

 and stored. The first evidence of infection on the 

 tubers is the appearance of faint brownish-purple 

 spots of pinhead size, which doubtless indicate the 

 point of entry of the parasite. Each spot is usually 

 surrounded by a circular translucent, 1 to 2 mm., 

 area which apparently marks the distance to which 

 the Plasmodium has spread beneatli the epidermis, 

 according to Kunkel ('15). In the course of 6 to 8 

 days the areas may increase to l/o cm. in diameter, 

 lose their brownish color, and protrude as a meta- 

 plastic, jelly-like mass of i)roliferating host cells and 

 fungus spores. According to Home ('12) these pro- 

 trusions may be so prominent that they look like 

 cushions or wart-like excrescences. The diseased 

 areas gradually die, leaving shallow, crateriform 

 depressions filled with a fine powdery mass of spore 

 balls (PI. 10, fig. 1), These are the so-called pow- 

 dery scab symptoms of the disease which may be 

 readily mistaken for those of the common scab. 



Further development of the disease on the tubers 

 depends to a great extent on the relative amount of 

 moisture in the soil or in the storage bins after the 

 potatoes have been harvested. If the infected tubers 

 are growing in fairly dry soil, wound cork is rapidly 

 formed under and around the lesions, so that the 

 diseased areas are delimited. With abundant mois- 

 ture and in poorly drained soil, however, the para- 

 site may continue its depredations. As a result the 

 lesions become deeper, larger and sometimes coalesce 

 to form extensive eroded cavities or cankers as much 

 as 1/2 inch in depth. This is one of the most severe 

 types of the disease and is referred to as the canker- 

 ous stage (fig. 2). This type appears to be common 

 in Ireland, England, and Europe, but is not very 

 prevalent in Maine and Canada. Melhus, et al., at- 

 tributed the latter to the shorter growing period of 

 tlie potato in the northern regions of North America. 

 In addition to causing shallow lesions and deep 

 cankers S. subterranea may also lead to the forma- 

 tion of tuberous outgrowths and extensive warts on 

 the tubers with the result that the latter are often 

 misshapen and deformed, according to Home (12). 

 These outgrowths are apparently formed in tlie 



same manner as the galls on the roots and stems, 

 although Home did not describe their development. 

 They ma3' be more or less uniformly infected and 

 covered with scabs and bear a superficial resem- 

 blance to the tumors caused by Si/nchi/trium endo- 

 bioficum. 



The galls on the roots and stolons of potatoes and 

 other related species vary in size from minute tu- 

 bercles to balls as large as garden peas (fig. 3). 

 They usually precede tuber infection and may be 

 present in abundance before there is any indication 

 of lesions on the tubers, but their presence does not 

 appear to have any great injurious effect on the 

 growth of the host. These galls are simple in struc- 

 ture and consist primarily of enlarged and fre- 

 quently divided undifferentiated cells, so that they 

 are typically kataplasmic in structure. The causal 

 organism is confined largely to the phloem and meri- 

 stematic tissues, as in the case of club root of cruci- 

 fers. Amoebae may be found occasionally in the 

 xylem, but they do not occur in great numbers or 

 cause distortion of the vessels. The presence of the 

 parasite in the phloem stimulates the cells to en- 

 large and divide, and this hyperplastic growth often 

 puslies the xylem out of its normal position. 



Powdery scab dry rot usually sets in after in- 

 fected tubers have been in storage for some time, 

 and in some cases is abetted by numerous other 

 fungi. This rot was first described by Melhus ('l-i) 

 in North America, but it has been found subse- 

 quently on ])otatoes collected in Ireland, Holland, 

 Chile, and other countries. It is accelerated by poor 

 storage conditions, but even in good storage as much 

 as 30 to 73 per cent of the tubers may be partly or 

 wholly decayed and rendered useless for seed or 

 table use, according to INIelhus, et al. Although 

 tubers may be often totally decayed, powdery scab 

 dry rot is usually less severe and occurs in localized 

 spots, 1 to 10 cms. in diameter. These areas may be 

 only slight depressions in the superficial layers or 

 extend to the center of tlie tubers. The extent of in- 

 jury, however, depends to some extent on the time 

 of harvesting, degree of infection, storage condi- 

 tions, and the stage of development of the parasite 

 when the tubers are stored. Dry rot may accordingly 

 exiiibit various types of symptoms. Desiccation or 

 loss of water from the open lesions is a common 

 occurrence when tubers are placed in warm dry stor- 

 age and results in discoloration of the affected areas, 

 wrinkling, shrinkage, and marked loss in weight. 

 However, this type of dry rot is retarded as storage 

 temperatures drop with the advent of the winter sea- 

 son. Another type of dry rot is caused by secondary 

 infection and invasion of tissue around the old pus- 

 tules by the jjlasmodium of S. subterranea. If mois- 

 ture and temperature are favorable, the resting 

 spores in old lesions may germinate and give rise to 

 Plasmodia which invade and kill the surrounding 

 healthy cells. The plasmodium usually feeds on the 

 tissue immediately beneath the epidermis, but occa- 

 sionally it may be found at depths of 6 to 8 mm. in 

 the tuber. In such extreme cases of penetration the 



