Apr. 29. i9i8 Anthracnose of Lettuce 265 



In the last stages the spots show dead areas at the center, and these may 

 fray or drop out, giving the leaf a characteristic shot-hole appearance. 

 Such a leaf appears as if gnawed by insects. Affected leaves wither 

 quickly, and the lettuce is liable to rot under storage conditions. Evi- 

 dence of spore production is seldom seen with the unaided eye even in 

 cases of severe infection, although spores can always be obtained from 

 such spots. If, however, the diseased leaves are placed under moist con- 

 ditions, a film of pinkish-white spores appears at the centers of the diseased 

 spots. 



On the; midrib. — The lesion on the midrib is of the type characteristic 

 of the anthracnoses. Sunken elliptical spots commonly appear in great 

 numbers (PI. 20, B). These spots vary with the age and the condition 

 of the leaf. They are elongated in the direction of the long axis of the 

 leaf, 4 to 5 by 2 to 2.5 mm., and are not noticeably sunken. This elonga- 

 tion of the spots on the nerve is undoubtedly due to the shape of the cells 

 in that situation, which are very long and narrow, and offer less resistance 

 to the progress of the parasite in a longitudinal than in a transverse direc- 

 tion, on account of the fewer cross walls. The depression is more apparent 

 in these elongated nerve spots than in the round spots on the blade of the 

 leaf, because the tissue of the nerve is thicker and more fleshy, and the 

 collapse of a greater number of cells is possible. 



These spots begin as barely noticeable water-soaked areas. After a 

 few days the spots become a straw-yellow, while in age they are reddish 

 yellow. Badly affected midribs present a very irregular contour. With 

 the progress of the disease the functions of the entire leaf are disturbed. 

 Occasionally the attack is severe enough and the lesions deep enough for 

 the weight of the leaf to cause the breaking or bending at the affected 

 point. Badly affected leaves often wither, owing, no doubt, to the loss 

 of water from the broken epidermis. 



ETIOLOGY OF THE DISEASE 



Since Berlese's first report (2), the etiological relation of Marssonina 

 panattoniana (Marssonia perforans in American literature) to the disease 

 has been assumed. Berlese, however, performed no experiments to 

 prove the causal relation. The work of Selby was of similar character. 

 Appel and Laibach cultured the organism, but gave no evidence in their 

 article of having used pure cultures in inoculation experiments. The suc- 

 cessful inoculation experiments which they described were those in which 

 spores from diseased plants were transferred to healthy plants. 



Proof, therefore, of the etiology of the anthracnose of lettuce depends 

 entirely upon constant association of the host and organism along with 

 evidence of the infectious nature of the disease. Although this is no 

 doubt sufficient in the case of the organisms of such known pathogenic 

 habit as the Melanconiales, it is now possible to complete the proof. 



