EXPERIMENT STATION BULLETINS. 637 



SIGNS. 



The general sigDs of this tomato disease are conveyed by the popular 

 term, ''blight." The growers notice a general unthriftiness in their 

 plants, along with a dying of the leaves beginning first with the lower 

 ones and gradually advancing upward on the plant. (Plate 1). The 

 fruit fails to ripen evenly or rapidly, and with severe attack, the entire 

 crop may fail. Such fruit as is produced from diseased plants, lacks 

 sweetness and solid matter. This last condition is often complained of 

 by canners, although they ascribe "watery" tomatoes to wet season, rather 

 than to a fungous disease made severe by the season. In short, all the 

 general signs which accompany interference with the photosynthetic 

 function are to be expected witli this disease. The great reduction in 

 leaf surface is accompanied by a reduction of carbohydrate formation. 



Along with these general signs of the disease there are distinct lesions 

 on various parts of the plant which allow sure diagnosis. 



The fungus causes lesions upon the leaf, the stem, the fruit and the 

 calyx. 



On the Leaves: 



The earliest indication of the disease is a water-soaked spot which can 

 be distinguished with a hand lens on the underside of the leaf. There 

 is no noticeable discoloration of the tissue at the outset. As the spot 

 grows larger it becomes more or less circular in outline and shows a 

 definite margin. The affected tisssue darkens, becomes shrunken and 

 later appears hard and dry. The color of this spot may vary from 

 black to grayish-white. The spots may vary in shape and size from a 

 small circular spot of pin-head size to a large irregular spot of about 2 

 cm. in diameter. Not infrequently the spots coalesce. While the tissue 

 is shrinking 3-10 small, black, glistening pycnidia appear in the spot. 

 Finally, yellowish, mucilaginous masses can be seen exuding from the 

 pycnidia. Upon microscopic examination these are found to be masses 

 of long filiform spores. The number of pycnidia in the larger spots may 

 be as high as 30. The pycnidia are well defined, visible with the naked 

 eye and separate (Plate 2.). Pycnidia may occur on the under side of 

 the leaf; usually, however, they occur on the upper side. At this point 

 it must be noted that not all spots contain pycnidia when the leaf dies. 

 This point will be taken up in detail later. (Page 24). 



About the time of spore exudation, the green tissue of the leaflet con- 

 tiguous to the fungous spot begins to turn yellow. This yellowing in- 

 creases, eventually involving the entire leaflet. Then the fungous spots 

 which have so far been pliable become dry and brittle. The leaflets grad- 

 ually droop and dry on the stalk which later also shrivels up but re- 

 mains attached to the stem until broken off by a slight jar. On tomatoes 

 which are staked the disease is sometimes confined to the lower leaves 

 (Plate 1). Where the plants are allowed to trail at will, the disease 

 may cause almost complete defoliation of the plant, the small tufts of 

 young, terminal leaves alone escaping. 



On the Stems: 



The disease is commonly found on the stems. It is manifested by 

 small, slightly elongated, dark spots containing pycnidia. These spots 



