ANNUAL REPORT, 1943^4 21 



Study of Diseases of Plants Caused by Soil-Infesting Organisms, with Par- 

 ticular Attention to Control Measures. ( W. L. Doran.) Especial attention has 

 been paid this year to investigations of the use of urea, dichromates, sodium 

 nitrate, formaldehyde, and other materials as soil fungicides for the control of 

 damping-off and for the control of onion smut; also to the control of damping-ofF 

 of vegetable seedlings other than by the use of soil fungicides and to certain seed 

 treatments for use with vegetables. 



The application of a very dilute solution of formaldehyde to soil immediately 

 after seeding is a safe, convenient, and simple method for the prevention of post- 

 emergence damping-off. At the request of the Du Pont Company, an article 

 was written on the subject and published in their Agricultural News Letter.^ Such 

 treatment with formaldehyde is readily combined with subirrigation immediately 

 after seeding, and L. H. Jones and the writer published on that method.^ 



In the absence of any chemical soil treatment against damping-ofT fungi, 

 stands of vegetable seedlings under glass may be improved by postponing the 

 first watering. Stands of eight commonly grown vegetables were better when 

 soil was not watered until three to five days after seeding. 



In work with Thomas Sproston, Jr., oh onion smut, soil treatments with sodium 

 nitrite, Fermate, or urea markedly reduced the severity of the disease; but urea, 

 unless applied to soil long before seeding, may interfere with germination or 

 growth. 



Urea proved to be a safe and effective soil fungicide with peas when used on a 

 limed sandy soil (0.2 milli-equivalents of exchangeable H per 100 gm. soil), but 

 it gave poor results in soils which were unlimed or not, sandy. 



Fermate, applied to soil immediately before seeding, or Elgetol, ammonium 

 dichromate, or potassium chromate, applied to soil immediately after seeding, 

 gave good and safe control of damping-off of certain vegetables. 



As seed treatments for vegetables, ammonium, potassium, or sodium dichro- 

 mate, diluted with graphite, gave good results, comparable to those obtained with 

 new proprietaries. 



Seed treatments of beet with Arasan, of cucumber with Semesan, and of 

 lettuce with Spergon prevented pre-emergence damping-off equally well whether 

 soil was first watered 1, 2, 3, 4, or 5 days after seeding and whether the temp- 

 erature of the water applied was 40° or 100°F. 



Soaking seeds of eight commonly grown vegetables in water for various lengths 

 of time immediately before seeding did not sufficiently hasten germination or 

 emergence to affect either pre- or post-emergence damping-off. 



Effect of Soil Temperature on Leaf Pattern of Tobacco Mosaic Virus. (L. H. 



Jones.) Seedlings of Havana Seed tobacco were established at a soil temperature 

 of 70° F. One fourth of the plants were inoculated with a mosaic virus, and 24 

 hours later the soil temperatures were altered to establish a range from 50° to 

 95° at 5-degree intervals. The inoculated plants at all temperatures showed the 

 typical pattern of mosaic infection by the 13th day; but on the plants at 90° and 

 95°, terminal growth gradually stopped and a rosette of frenched leaves appeared 

 at the top, while at temperatures below 90° the common mosaic pattern continued. 

 These results were confirmed when the pots at 95° were interchanged with those 

 at 50° soil temperature. 



' Doran, W. L. Soil treated with formaldehyde after seeding to control damping-off. Agr. 

 News Letter of E. I. du Pont de Nemours and Co. 11:3:47-49. 1943. 



- Jones, L. H. and Dciran, W. L. A practical method for sterilization and subirrigation of 

 soil in flats. Flor. Exchange 100:22:9, 11. 1943. (Mass. Sta. Contrib. 482.) 



