EXPERIMENT STATION BULLETINS. 491 



HOW TO PREVENT LOSS FROM TOMATO LEAF-SPOT. 



Resistant Varieties. 



lu an experiment with more than 50 of the leading tomato No resistant 

 varieties, no variety was found strikingly resistant or sus- y|['found.^^ 

 ceptible. Many varieties are vigorous growers and continu- 

 ally keep ahead of the leaf-spot by the production of new 

 foliage. Dwarf varieties usually suffer- more damage than 

 the ordinary sorts. 



Before other control measures can be considered, we must 

 recognize the following observations that have been dealt 

 with above: 



1. The sources of infection are at least two: the green- summary ot 

 house or the hotbed and the diseased trash in the field. KdisS. 



2. Infection results from inoculation upon the upper and 

 lower surfaces of the leaf. 



3. The period from the time of inoculation to spore exuda- 

 tion is about 13 days. 



4. Moisture is the primary agent in dissemination of the 

 disease. 



5. The exudate of spores is in the form of a mucilaginous 

 mass. The spores are always transferred by some external 

 agency. 



6. It has been shown that the fungus will not grow at 

 85l° F. and will not revive after 5 days at 98° F. Since such 

 temperatures are frequently reached during the summer 

 months, the heat relation must be taken into consideration 

 as a natural means of checking the disease. 



The most important control measures for this disease are prevention 

 preventive. Clean seedlings in clean soil, if reasonably necessary. 

 isolated remain practically free from the disease. 



In order to be sure that the seedlings do not become dis- clean soil. 

 eased at the start, clean soil should be used in the green- 

 houses devoted to seedling production. The soil should be 

 fresh or sterilized. The seedlings should be sprayed as 

 soon as their height above ground makes it practicable, and 

 again before being transplanted to the field. For this a 

 weak Bordeaux mixture (2-2-50) is advised. 



Since it has been determined that wintered-over, diseased Destroy old 

 vines possess spores which are viable, the old trash must be *^^^^- 

 destroyed as far as possible. Since, however, this is not 

 practical except in greenhouses and gardens, rotation is 

 strongly urged. While there is no experimental evidence to 

 demonstrate the value of rotation as a means of control, 

 numerous instances have been noted in which rotation has 

 been successful in controlling the disease. 



