326 Journal of Agricultural Research voi.xx,No. 4 



plants affected earlier in their life. In the last stages of the disease, 



when the rot has progressed far enough to cut off completely the water 



supply, the entire plant turns brown and sooner or later, depending on 



weather conditions, falls prey to the attack of saprophytic bacteria and 



fungi. 



MATERIAL AND METHOD OF STUDY 



The field from which the material for study was obtained is located in 

 a clearing of the river bottom land near the Fort Lewis Mesa, Colo. The 

 altitude is 7,500 feet. The soil is a sandy loam containing some organic 

 matter and a water table sufficiently high to insure the growing of a crop 

 without the customary irrigation. The tubers used were of Green 

 Mountain and Rural New Yorker types. They were cut before planting, 

 and because of the apparent soundness of the tubers no surface steriliza- 

 tion was attempted. The season was a normal one. The months of 

 May and June were characterized by excessive dryness. Throughout 

 July and August frequent showers insured a rapid growth of the plants. 

 During the first week of August a severe hailstorm injured the foliage 

 and stems so badly as to make further observations impracticable. 



The first diseased plants appeared early in July. Their number 

 increased during the following two weeks and then showed a decline on 

 account of the death of a number of early infected plants and the reduc- 

 tion in number of new infections. Tubers of the same lot which had 

 been disinfected and grown on irrigated mesa soil remained free from 

 disease. The observations made at Fort Lewis were extended on mater- 

 ial obtained from other parts of the State, especially the San Louis 

 Valley. In every case the symptoms were similar, the only real differ- 

 ence being in the number of diseased plants per acre. 



The plants taken for study were examined while fresh. For the pur- 

 pose of completing microchemical work and checking results, suitable 

 material was killed in Flemming's weaker solution and embedded in 

 paraffin in the usual way. The principal reagents used were Haiden- 

 hein's haematoxylin-safranin stain for histological structures, Devaux's 

 stain for pectic degeneration, phloroglucin-hydrochloric acid for lignifi- 

 cation, and Altmann's acid fuchsin stain for protein crystals. 



While all previous investigations on the blackleg disease deal with the 

 morphology of the causal organism and its pathogenicity, this study 

 has for its object a consideration of the pathological changes concomitant 

 to the presence of the organism. 



PATHOLOGICAL ANATOMY 



In general, the histological changes consist in an increase of strongly 

 lignified vascular tissue and in a transformation of part or most of the 

 parenchyma cells of pith and cortex into sclereids (Pi. 58, B). Cyto- 

 logical abnormalities lie mainly in the occurrence of protein crystals in 

 the parenchyma cells of the leaves, the stems, and the tubers. 



