I>()\V1)KHY SCAH <)l' I'OTATOKS 



i:n 



svniptonis ])ro(liu'i'd may resemble tliosi- of tlio can- 

 ktTous stai;c in the (iild. 



Till- open K'sions may also bo invaded liy womul 

 parasites of tlu' •ti-nera Phonta, Fusarium, lihizoc- 

 tonia, Papulospora, etc.. and tliis initiates the most 

 destnietive type of powdery seal) dry rot. Phomn 

 tiihrrosa, aeeordinir to Melluis, <-t al., is commonly 

 associated witii the early stages of rot and prodnees 

 brownish to gray lesions in the bottom of the old 

 pustules. .\s these lesions progress they become more 

 sunken, darker and often hard and bony. .\t later 

 stages the lesions may vary from 2 to 5 cms. in diam- 

 eter and extend to a depth of 2 to 1- cms. 



.\t this ))oint it may be noted thatShajJOvalov ('23) 

 contended that the skin-spot disease of tubers, which 

 had been attributed to several causal organisms, in- 

 cluding Ooxpora piistiilans, is an early stage of ))0W- 

 derv scab, but this was innnediately refuted by Mil- 

 lard and Burr ('23). They reported that the former 

 disease is caused solely by O. ptistulaii.s and is in no 

 way related to jjowdery scab. Powdery scab is co- 

 extensive with late blight, caused by Phi/tophlhora 

 infrsians, and both diseases are favored by the same 

 climatic conditions. The latter disease may often be 

 greatly increased by tuber and root infection by S. 

 suhtrrranea, according to Beregovoy ('39). 



Cellular Relations Between Host and 

 Pathogen 



Sponc/ospora suhtt-rranea has a marked efifect on 

 the host cells. Young infected cells as well as adja- 

 cent healthy ones are stimulated to divide by the 

 presence of the parasite. The repeated division of 

 healthy cells results in the formation of a new peri- 

 derm around the regions of infection. When this 

 periderm is invaded further cell divisions follow, 

 which lead to the development of a second wound 

 periderm, according to Wild ('29). Kunkel found a 

 marked difference in reaction between the yoimg 

 growing cells of tubers and mature cells in the tis- 

 sues around the old lesions. The former are not 

 killed by invasion of the parasite but are stimulated 

 to expand and divide. The latter, on the other hand, 

 are quickly killed, and their contents are partly or 

 wholly consumed. The increase in cell multijilication 

 noted above is usually accomjjanied by <ell enlarge- 

 ment. .\ceording to Kunkel, the latter )>rocess may 

 begin while the plasmodium is still in the intercellu- 

 lar spaces and before it has entered the cells (fig. 

 17). This reaction suggests that the plasmodium may 

 secrete a stimulating substance which precedes its 

 invasion of the cells. 



Infected cells may become .5 to 10 times their nor- 

 mal size, but enlargement is not equal in all direc- 

 tions. The expanding cells generally elongate out- 

 ward towards the surface of the tuber, which finally 

 results in the lifting and ru))turing of the e])idtrniis 

 and the formation of cushion-like excrescences. In 

 galls on the roots of Solatium 7carce7cicsii and L. 

 esculentum the infected cells occur in groujjs (fig. 



1() ). according to Melhus. ct al., like the "Kranheits- 

 herde " described by Nawascliin for club root of 

 erueifers. These grou|)s origin.ite by continual divi- 

 sion of one or more infected cells whereby the .-imoe- 

 bae and young ))lasnu)dia are jiassively distributed. 

 The nuclei of infected cells may divide mitotieally 

 .•md i)ossibly amitotically also, as in the case of Tri- 

 (flochini.i cells ))ar;isiti/ed by Tt'tranti/jca Triylo- 

 chhiis. When normal mitosis occurs, a cell ))late is 

 formed between the d;iughter inu-lei (fig. It), but 

 in c.ises of amitosis the giant cells become multinucle- 

 ate and l;iter dividi- into numerous smaller cells, ac- 

 cording to Kunkel. However, it is not obvious from 

 his description whether these latter divisions occur 

 by cell plate formation or cleavage after which walls 

 are laid down. The host nucleus may be enveloped by 

 or embedded in the ))l;ismodiuni and become greatly 

 enlarged, lobed, and distorted (fig. 18, 19. 26). Sev- 

 er.al nucleoli may frequently develoj). while the chro- 

 matin strands become abnormal in appearance or 

 disappear entirely. The nuclei are usually destroyed 

 before the parasite is mature, but in exceptional 

 cases it may remain intact until after the spore balls 

 have been formed and lie between them. 



The presence of S. subterranea ajjparently also 

 stimulates an undue production of starch in and 

 around infected cells. At least the starch ap{)ears to 

 be more abundant in the regions of infection in the 

 potato and tomato. In S. u-arscexclczil, however, 

 Melhus, et al., found that numerous infected cells 

 may be found which are totally lacking in starch. 

 The starch grains usually do not disap)jear entirely 

 until after the spore balls are mature, but it is not 

 certain that they are consumed directly by the jiara- 

 site. Osborn claimed that the i)lasmodium feeds on 

 starch, but Melhus, ei al., pointed out that if this 

 were true an abundant supply would not always be 

 present. Kunkel also reported that the starch grains 

 are only slightly changed by the parasite and may 

 remain after the cytojjlasm and nuclei have been 

 destroyed. Other workers, however, have claimed 

 that the su])])ly of starch diminishes as the parasite 

 matures, ^^'ild found that starch disappears below 

 the diseased areas, being utilized in the process of 

 cell division, or for nutrition of the parasite. It may 

 also be noted here that infection with S. suhterranea 

 reduces the ])H value of tubers from .5.70 to l'.3.5, 

 according to Robertson and Smith ('31). 



The physical relations between the protoplasts of 

 host and pathogen a))))ears to be close and intimate 

 in light of Kimkel's .and Melhus' observations. No 

 antagonism is exhibited, and the two blend into each 

 other in such a way that it is often imi)ossible to de- 

 termine clearly where one ends and the other begins. 

 In fixed and stained preiiarations. on the other hand, 

 the parasite st.iins more intensely with Congo red 

 and Orange (i th.an the host |)rotopl;isni (Massee .■md 

 Kunkel). In the initial develo))mental stages there 

 seems to be a marked attraction between the host 

 nucleus and the amoebae, according to Melhus, et al. 

 As is shown in figures 1.5 and 1(5 the latter may be 

 crowded around the nucleus, which suggests that the 



