24 MASS. EXPERIMENT STATION BULLETIN 449 



soil, and no clubroot at any soil-moisture content in soil treated with both hy- 

 drated lime 20 gm. and mercurous chloride 0.2 gm. 



Tobacco Frenching. (L. H. Jones.) Research this past year has been con- 

 cerned with the effect of partial sterilization of the soil on the elimination of 

 the frenching factor; with the control of frenching by adding iron to the soil and 

 its relation to iron content of the foliage; and with the symptoms of frenching 

 induced by nitrogen deficiency and soil temperature, separately or combined. 



A compost soil was divided into four fractions for treatment by autoclaving, 

 air-drying, or formaldehyde, with one portion left untreated as a check. Both 

 soil temperatures, high 95° F. and low 70° F., were used. Nitrogen was frequently 

 added to the soil and the diphen^'lamine test on the foliage always showed ni- 

 trates present in the plants. No frenching occurred at the low soil temperature 

 of 70° F. or on the plants in the autoclaved soil at 95°F. Air-drying of the soil 

 did not prevent frenching in this experiment although it had been effective in 

 previous experiments. Formaldehyde was not a preventive, although the soil 

 had been treated with four times the amount usually recommended. 



That the organism, or frenching factor, does not enter the soil through fresh 

 cow manure is indicated by the results of an experiment in which manure was 

 mixed with autoclaved soil and the plants grown at 95° F. No frenching oc- 

 curred. Frenching did occur when the manure was mixed with non-autoclaved 

 soil and in the check soil receiving no manure. The nitrogen content of the plants 

 was high in all tests. 



Freezing the soil does not eliminate the frenching agent. Immediately after 

 thawing, soil from a pile sub;ected to freezing all winter produced frenched 

 plants in 7 days, whereas soil protected against freezing required 10 to 13 days. 



Chemical analysis for iron in the above-ground portion of Havana Seed to- 

 bacco plants indicated a relationship between iron content of the plants and 

 frenching. The iron content was much lower in plants grown at a soil tempera- 

 ture of 95° F., which frenched, than in plants grown at a soil temperature of 70°, 

 which did not french. However, when the soil was autoclaved before the plants 

 were set, no frenching occurred at 95°, and the iron content of the plants was even 

 higher than that of plants in autoclaved soil at 70°. 



The addition of a ferrous sulfate-peat mixture prevented frenching and gave 

 the plants a very high iron content. However, when the iron was supplied 

 from the less available iron of ferric phosphate mixed with peat, frenching was 

 not prevented by three lessening amounts and the iron content of the plants was 

 no greater than that of plants grown in soil to which no iron was added. The 

 heaviest application, however, did prevent frenching and the plants were higher 

 in iron content, containing an amount similar to that found in plants growing in 

 autoclaved soil where frenching did not take place. 



It has been reported frequently that frenching is due to nitrogen deficiency. 

 During the year it has been established that the early symptoms of frenching, 

 pinhead mottling, induced either by high soil-temperature or by nitrogen de- 

 ficiency are so nearly alike that the one cannot be distinguished from the other 

 by general observation. Study of the various steps in the onset of the diseaase 

 has shown that the pinhead mottling induced by high soil-temperature appears 

 at the tip of a young leaf, earlier leaves being a dark healthy green; while pinhead 

 mottling due to nitrogen deficiency appears first on the side near the margin of 

 a yellow-green leaf, stunted by lack of nitrogen, the earlier leaves being quite 

 yellow and even fired. Succeeding leaves in both instances are so nearly alike 

 that, unless on the plant, the cause would be sheer guesswork. Sometimes it is 

 possible to use the diphenylamine test to distinguish the cause. 



