AN^NUAL REPORT, 1929 341 



soil conditions on the efficiency of acetic acid and formaldehyde as soil 

 disinfectants. 



Study was made to determine whether the inhibiting effect of acid 

 soils on Thielavia basicola may be directly due to some other factor than 

 hydrogen-ion concentration, more especially active aluminum. Results 

 indicate that aluminum sulfate is about as effective as sulfur and sulfuric 

 acid in acidifying soil and in suppressing the black root-rot fungus. 



Earlier experiments indicated that phosphoric acid is markedly favor- 

 able to the parasitisni of Thielavia basicola and that its influence may be 

 as great as that of soil reaction, or greater. The effect of this chemical 

 en black root-rot, growth of tobacco, and soil reaction is now being 

 studied in a new series of pot experiments. 



Brown Root-Rot. Field studies have continued on the plots originally 

 planned for investigation of black root-rot. In general, there was prac- 

 tically no brown root-rot on plots in continuous tobacco since 1921; some 

 brown root-rot occurred wherever tobacco followed either timothy or 

 alfalfa, but was somewhat more severe in limed plots following timothy. 

 The presence of brown root-rot was generally accompanied by reduction 

 of yield, but a consistant relation between disease and yield was not 

 always found. 



On the theory that timothy sod is injurious to tobacco because of the 

 large amount of cellulose and the low amount of available nitrogen pres- 

 ent, Johnson and coworkers of the Wisconsin Station recommend ap- 

 plications of nitrogen where tobacco follows timothy. In order to deter- 

 mine the effect of nitrate (calcium nitrate), and the time of its application 

 to timothy sodi, on tobacco and on brown root-rot, a series of pot ex- 

 periments was conducted. From these it may be concluded that heavy 

 applications of calcium nitrate to timothy sod reduce the severity of brown 

 root-rot; and that the nitrate increases growth much more when applied 

 to brown root-rot soil than when applied to soil which does not contain 

 the cause of brown root-rot. 



Assuming a hypothetical relation between brown root-rot and activities 

 of soil organisms resulting in a lack of nitrogen or in the formation of 

 substances (undetermined) toxic to tobacco, it would seem possible that 

 certain inorganic salts applied to the soil might affect the disease through 

 their toxicity to the organisms or by chemically inactivating certain toxic 

 substances. Experiments based on this theory are under way, but it is 

 too early to report definite results. 



Control of Diseases of Greenhouse Vegetables. (E. F. Guba, Waltham). 



Leaf-Mold of Tomatoes. Work on this disease during the last year has 

 been concerned mainly with house management as a means of contr'ol. 

 The conidia of the causal fungus germinate at a relative humidity of 100 

 per cent, which in the greenhouse is attained on the leaf surfaces when the 

 atmospheric humidity is above 8-5 per cent. Proper adjustment of air 

 circulation, water and heat is necessary to maintain conditions unfavorable 

 to conidial germination, and this ordinarily involves much labor and at- 

 tention by the operator. In this investigation equipment for the auto- 

 matic control of temperature and humidity has been employed. In the 

 light of knowledge previously gained in laboratory study of the causal 

 fungus and observations in the greenhouse, a set of house management 



