1916] DISEASES OF PLANTS. 751 



Experiments with clean seed and potatoes on new land in southern Idaho, 

 O. A. Pratt {U. S. Dept. Agr., Jour. Agr. Research, 6 {1916), 'So. 15, pp. 57S- 

 575).— In couuectioD with investigations of potato diseases in southern Idaho, 

 where the crop is grown under irrigation, the author had a chance to tost the 

 belief that newly reclaimed lands offer an opportunity for the production «f 

 disease-free potatoes. In this region the diseases most prevalent are wilt, due 

 to Fusarium oxysporum; black rot, due to F. raOieicola; jelly end rot, due to 

 Fusarium sp. ; Rhizoctonia disease ; a powdery dry rot, due to F. tricliothecir 

 oides; and common scab. 



A preliminary report is given of experiments conducted in the spring of 1915 

 to determine whether, by planting disease-free seed on new land, a disease-free 

 crop would be obtained. Plats on virgin soil as well as on land where alfalfa 

 or grain had been grown for several years were planted, and at harvest time 

 a number of tubers were examined. 



The presence of disease in the plats in which grain or alfalfa had been 

 previously grown was considerably less than the percentage of diseased tubers 

 obtained on plats of recently reclaimed desert land. Planting clean seed pota- 

 toes on new land did not guarantee a disease-free product. 



The disease of potatoes known as " leak," L. A. Hawkins (V. S. Dept. Agr., 

 Jour. Agr. Research, 6 {1916), No. 17, pp. 627-6^0, pi. 1, fig. 1).—An account is 

 given of an investigation, carried on in the delta region in the San Joaquin 

 Valley of California, of the tuber rot of potato known as potato leak, which 

 was formerly reported as due to Rhizopus nigricans (E. S. R., 20, p. 948). The 

 investigations of the author corroborate the work previously described in part. 

 However, another fungus was obtained 49 times out of 61 attempts, and a study 

 made of this proved it to be Pythium deharynnum. Inoculation experiments 

 with this organism produced a rot identical to all appearances with potato leak, 

 and it is considered probable that this disease is produced by both R. nigricans 

 and P. debaryanum, the latter being apparently more frequently the causal 

 organism. 



Infection is believed to take place in the field from infected soil getting into 

 wounds made in digging, as no infection was observed in the field or in the 

 laboratory where the skin of the tuber was unbroken. From the results of these 

 experiments, it is believed that the disease may be controlled by more care in 

 harvesting and handling the potatoes and the careful sorting out of wounded 

 tubers. 



Some properties of the virus of the mosaic disease of tobacco, H. A, 

 AxLAKD {U. S. Dept. Agr., Jour. Agr. Research, 6 {1916), No. 17. pp. 640-674, 

 pi. 1). — In continuation of previous work (E. S. R., 30, p. 450), the author has 

 described additional investigations on the virus of the mosaic disease of tobacco. 



Evidence is presented to show that the infective principle can not be identified 

 with peroxidase. The author claims that neither peroxidase nor catalase in the 

 sap of diseased plants can be responsible for the mosaic disease. These enzyras 

 are normally present in healthy plants, while the sap of such plants is without 

 infectious properties. By evaporation of the enzyms present in healthy sap 

 they may be brought to a high concentration, but such solutions never acquire 

 infectious properties. On the other hand, the peroxidase content of mosaic sap 

 may be diminished to such an extent that peroxidase reactions are no longer 

 discernable, yet such solutions may remain highly infectious. 



The author claims that neither enzyms nor any other normal constituent 

 present in healthy sap is responsible for this disease, but that it is due to a 

 particulate substance that is not a constituent of healthy plants. This patho- 

 genic agent is highly infectious and is capable of increasing indefinitely within 



