394 ANNUAL REPORT OF THE Off. Doc. 



this is the first season, so far as the speaker is aware, that it has 

 been recognized as a factor in the production of disease of economic 

 importance. The fungus works several types of injury to the potato 

 and the plant. The main type, and the one by which it is best 

 known, is the ''black speck" or "black speck scab" stage. These 

 hard, black specks adhere to the tubers and appear like particles 

 of soil ; but are really made up of threads of the fungus which serve 

 as a means of carrying it over winter. The fungus also causes a 

 scabby or corky condition of the tubers, a rot of the tubers and roots 

 and secondary effects known as little potato and aerial potato. 

 Sufficient of these forms were observed to make it evident that this 

 disease must be considered seriously. It is caused by a soil or- 

 ganism and without doubt climatic conditions have much to do with 

 its development, although it is not known what factors favor the di- 

 verse forms which it assumes. General methods of control can 

 scarcely be suggested at this time. The variety, character of the soil 

 and climatic conditions, together with the different manifestations 

 of disease make the problem a difficult one. If it is desired to dis- 

 infect the tubers, corrosive sublimate must be used since formalde- 

 hyde as used for common scab is not effective. Rotation of crops 

 with as long a time as possible between crops of potatoes is de- 

 sirable. It should be said also that it is not possible to state what 

 the future of the disease may be with the variable nature of our 

 weather conditions. The late blight caused by the fungus known as 

 Phytophthora infestans destroyed perhaps one-half of the potato 

 crop in 1915. Other epidemics are known to have occurred in 1910 

 and 1891. It is well known that the season was marked by exces- 

 sive rainfall and unusually low temperatures. The amount of pre- 

 cipitation and atmospheric temperatures seem not to be sufficient 

 to explain outbreaks of the late blight, and investigation has led 

 Professor Orton, of State College, to the belief that low soil tem- 

 perature combined with high relative humidity at the proper time, 

 late July and August, are probably the most important factors favor- 

 ing the development of the disease. 



An unusual outbreak of late blight also occurred on the tomato, 

 caused by the same fungus which affects the potato. It is not 

 known how general this outbreak may have been but it was observed 

 in the central part of the State and was prevalent especially on the 

 College farm, where the crop was practically destroyed. In America 

 the disease has been known to cause losses to tomatoes in California 

 and Virginia and has been reported also from Maine, Connecticut 

 and Massachusetts, but has not been reported previously in Penn- 

 sylvania. No record of varietal resistance was kept but all of the 

 thirty-nine varieties grown in the experimental plats were affected 

 to some extent and the crop was almost a total failure. The fungus 

 attacks leaves, stems, and fruits with about equal virulence. 



Apple rust was bad in the southeastern sections and is without 

 doubt increasing in importance from year to year. The white pine 

 blister rust is known to persist within the State. Inspections have 

 shown that it has been imported into seven different plantations but 

 the effectiveness of the attempts to eradicate it are unknown. Num- 

 erous other plant diseases might be mentioned, but time does not per- 

 mit of their discussion. 



