CAISATIOX OF I'l.Wr l)ISi;.\SKS 



275 



ils own I ypc ill iii()ciil;il ion cNpciiiuciils. This 

 cullui'c also ;itt;u'k(Hl \\\v loots ;iii(l rootlets 

 ot' the iiiociilntcHl in;iii,H('l plants, on which it 

 pro(hic('(l nunicroiis characteristic dai'k 

 hi'owii, nothilar out.iiTow ths. It was identical 

 witli N. scahi( s. 



Swvvl l*<>tat<> Pox or Sofl I?ol 



An actiiioniycete, (iesi»;iiate(i I)}' Tauheii- 

 haiis as .1. poohnsis, was t'ound to l)e a con- 

 t riliiitinu; factor to the causation of soft fot 

 of sweet potato(\s. It was considered as a 

 suporfic'ial wound parasite, usually follow- 

 ing the pox spots produced bj'^ a fungus. 



Other cultures of actinomycetes were la- 

 ter isolated from sweet potato(\s. One of 

 them, designated as .4. pox, was believed 

 to be the cause of the pox disease (Adams). 



Still another organism causing sweet po- 

 tato rot was described as S. ipomoea. It pro- 

 duced aerial mycelium and was thus a mem- 

 ber of the genus Streptomijces. Sweet potato 

 rot does not develop in soils of pH below 5.2; 

 above that reaction, the disease develops 

 readily. According to Person and Martin, 

 sweet potato rot is more serious in dry soils 

 and in wet seasons than under normal mois- 

 ture conditions. The disease has been pro- 

 duced in the greenhouse and in field inocula- 

 tion experiments with pure cultures of S. 

 ipomoea. The optimum reaction for growth 

 is pH 5.6 or above, and the optimum tem- 

 perature 32°C. 



Other Plant Diseases Caused by Actin- 

 omycetes 



A mmiber of other plant diseases have 

 been reported to be caused by actinomy- 

 cetes. Banga described a strawberry disease. 

 Godfrey listed a form causing citrus gum- 

 mosis. 



Hooker reported that seedling plants rep- 

 resenting eight families developed root ne- 

 crosis when grown in soil-extract agar arti- 

 ficially infested with pure cultures of S. 



scdhtVs. Ten ciiltiires of act inoin.\'cetes wei'e 

 tested on seedlings of wheat, garden pea, 

 soybean, corn, radish, and (aicunibei', and on 

 potato sprouts. Six of these cultures caused 

 neither appreciable neci-osis of potato stems 

 nor injury to seedling roots. Four cultures 

 caused sex'ere necrosis of roots as well as a 

 reduction in root weight of wheat, pea, soy- 

 bean, and radish; necrosis was most pro- 

 nounced on loot tips, and the development 

 of secondary roots was almost inhilMted. 

 Corn roots were only slightly necrotic;, but 

 their weight was markedly reduced. Three 

 of the four cultures also caused scab of po- 

 tato tubers and necrosis of potato stems. 



Palm described the occurrence in South 

 Sweden of actinomycotic infections of Beta 

 vulgaris, Brassica sp., Raphanus sativus, and 

 Daucus carota. He concluded that the infec- 

 tion of these plants may be caused by the 

 same pathogen. The disease appears as 

 sunken spots, delimited by the more promi- 

 nent veins of the bulb scales, and are of a 

 greenish mother-of-pearl-like color. The 

 pathogen lives intracellularly, filling the cells 

 completely with its very thin (1-1.2 ii in 

 diameter) mycelium. The mycelium is non- 

 septate, of a strong yellow color, and spore 

 formation in or on the host is not seen. The 

 organism undoubtedly belongs to the chro- 

 mogenic actinomycetes, of the genus Strep- 

 tomyces. 



IVIycorrhiza Formations 



The associations of certain actinomycetes 

 with the root systems of certain plants are 

 of particular interest. These associations are 

 believed to be comparable to mycorrhiza for- 

 mations by true fungi. Peklo (1910) made a 

 detailed study of the endophytes of the alder 

 bush, Almis glutinosa, and of sweet gale, 

 JMijrica gale. Two species of actinomycetes, 

 A. alni and A. myricae, were isolated. These 

 organisms produced, in culture, swellings 

 comparal)le to those formed by animal path- 

 ogens. The significance of these associations 



