BUREAU OF PLANT INDUSTRY. 200 



fested. Both the immunity and soil-sterilization tests are being con- 

 tinued this season. 



ONION DISEASES. 



The completion of a series of experiments to determine the most 

 effective and economical rate of application of formaldehyde for the 

 control of onion smut is one of the accomplishments of the year. 

 These experiments have shown that 200 gallons of formaldehyde 

 solution per acre will give the most satisfactory results and that there 

 is no advantage in using a stronger solution than 1:128 (1 pint of 

 formaldehyde to 16 gallons of water). Where there is serious objec- 

 tion to using this amount of liquid because of the difficulty of appli- 

 cation the quantity may be reduced to 120 to 150 gallons, but in this 

 case a stronger solution (2 pints of formaldehyde to 16 gallons of 

 water) should be used. Raising the strength of the solution beyond 

 this point entails danger of reduction of the stand, due to seedling 

 injury. Experiments on a small scale have indicated that the arti- 

 ficial curing of onion sets is effective in controlling onion neck-rot. 

 A study of the nature of resistance in the onion to the disease known 

 as " smudge," which develops very readily on w'hite varieties but to 

 only a very slight extent on the colored sorts, has shown that this 

 resistance is associated with the presence of the coloring matter in 

 the outer scales. Further knowledge of the method and time of in- 

 fection by this fungus and the injury resulting from its attacks has 

 been gained. 



PEA ROOT-ROT. 



The widespread occurrence of pea root-rot and the heavy losses 

 caused by this disease, particularly in the principal pea-canning sec- 

 tions in Maryland, Delaware. New Jersey, Wisconsin. Michigan, 

 Indiana, and Ohio, led to recent active investigations of this trouble^ 

 which have shown that the losses may be greatly reduced by the 

 rotation of crops. It has been found that the disease lives in the soil 

 and increases rapidly in severity when peas are grown for seA'eral 

 years in succession on the same land, as is the practice in many com- 

 mercial pea-growing sections. It is distributed by the custom which 

 has grown up in some places of transferring soil from old fields to 

 new ones in order to carry the bacterial nodule organism from one 

 field to another. Pea growers have been advised to keep a careful 

 watch of their fields, especially in the old pea-growing sections, to 

 discover the first signs of the disease and to begin rotation before 

 extensive damage has been done. After a four-year rotation peas 

 can again be grown, except in the Middle AVest, where a longer rota- 

 tion has not always been successful. Progress is being made in the 

 breeding of varieties of peas resistant to root-rot, but some time must 

 elapse before these are ready for commercial use. 



SWEET-POTATO DISEASES. 



Considerable progress has been made during the year in the study 

 of the physiology of the fungi which produce the decay of sweet 

 potatoes in storage; also of the changes produced in the potato as a 

 result of decay. These studies will lead to a better understanding of 

 the factors involved in the susceptibility or resistance of sweet pota- 

 toes to decay, which, in turn, will suggest methods for their control. 



