ANNUAL REPORT, 1947-48 23 



Urocystis cepulae Frost), on clubroot of cabbage (caused by Plasmodiophora 

 brassicae Wor.), and on growth of plants are here summarized. Rates of applica- 

 tion of the fungicides are in all cases expressed as grams per square foot, 1.0 

 gram per square foot being about 96 pounds per acre. None of the materials 

 were injurious to plants unless it is so stated, and their use was often followed by 

 an increase in average green weights of seedlings. 



The result of the control of pre- and post-emergence damping-off is expressed 

 as the percentage increase in number of living seedlings, the basis of comparison 

 being the numbers of plants which lived in untreated soil. Numbers of living 

 cabbage seedlings were increased 633 percent by Phygon (2, 3-dichloro-l, 4 

 naphthoquinone) 0.65 gm.; less by Arasan similarly used; and 377 percent in 

 one case, 889 percent in another, by Tuads (tetramethyl thiuram disulfide) 0.65 

 gm. Increase in weight of cabbage seedlings was 86 percent with Phygon, 50 

 percent with Tuads. Numbers of tomato seedlings which lived were increased 

 281 percent by Tuads 0.65 gm., 246 by Phygon 0.65 gm. Increases in average 

 green weights of the seedlings were 183 and 233 percent, respectively. The use 

 of Tuads 0.65 gm. was followed by an increase of 35 percent in the number of 

 living beet seedlings and 82 percent in the number of living lettuce seedlings. 



With more than 50 percent of the onion seedlings infected with smut in un- 

 treated soil, Tuads 0.65 gm., Phygon 0.65 gm., or Fermate (ferric dimethyl 

 dithio-carbamate) prevented all smut. Since Tuads and Phygon gave better 

 control of damping-off than Fermate, larger numbers of plants survived, the 

 increase over numbers surviving in untreated soil being more than 1000 percent 

 with both materials. However, Phygon sometimes retarded the growth of 

 onion seedlings slightly, so Tuads or Arasan (another tetramethyl thiuram di- 

 sulfide product) is probably to be preferred. When 50 percent of the onion 

 seedlings in untreated soil were infected with smut, there was no smut with 

 Arasan 0.6.gm. and, as a result of good control of damping-off, numbers of living 

 plants were greater by more than 2000 percent. 



The organic fungicides used gave some degree of protection against clubroot 

 of cabbage when it was not too severe in untreated soil, and the abstract of a 

 paper on this phase of the subject has been published.! But with 100 percent 

 severe infection in untreated and heavily watered soil, there was poor or little 

 control of clubroot by Arasan 0.7 gm., Phygon 0.7 gm., Tuads 0.65 gm., or a 

 zinc dimethyl dithiocarbamate-cyclohexylamine complex 2.0 gm. 



With clubroot of cabbage severe, 100 percent infection in untreated soil, there 

 was also 100 percent infection in soil to which hydrated lime 20 gm. had been 

 applied and in soil to which mercurous chloride 0.2 gm., but no lime, had been 

 applied. But when hydrated lime 20 gm. and mercurous chloride 0.2 gm. were 

 applied together to this soil, there was only 13 percent clubroot, the degree of 

 infection was slight, and the average green weight of the plants was 128 percent 

 greater than in untreated soil. Mercurous chloride 0.2 gm. or mercuric chloride 

 0.1 gm. retarded the early growth of cabbage if applied to imlimed soil immediately 

 before seeding, but not if applied to soil limed as described above. (This was 

 true also of their effect on onion seedlings.) Neither of these mercury salts was 

 injurious to cabbage plants if applied to soil 22 days before seeding. 



The effect of soil-moisture content on the control of clubroot is now under 

 investigation, with soil watered daily to 50, 65, and 80 percent of its water-holding 

 capacity. Thirty-six days after seeding, there was 100 percent clubroot in the 

 wettest soil, 56 percent in the soil 65 percent saturated, 10 percent in the dryest 



'Doran, W. L. Fungicides applied in fertilizers for the control of cabbage clubroot and damping- 

 off. (Abst.) Phytopathology 37:11 :84S. 1947. 



