356 DISEASES OF CROP-PLANTS 



South Africa. It is destructive in Porto Rico, and, though it 

 does not seem to have been recorded from the Lesser Antilles, 

 may with some confidence be expected to occur. 



Description. 



A recent and full investigation of leaf-spot has been made by 

 G. H. Coons and E. Levin. The disease is mainly one of the 

 leaves, beginning with small water-soaked spots on the underside, 

 which later penetrate the leaf and dry into definite more or less 

 circular spots, firm in texture, dark or greyish-white, with a 

 black or brown margin. Typically the spots are small, not 

 exceeding one-eighth of an inch in diameter, but they may by 

 coalescence approach to half an inch. In each spot the pycnidia 

 of the causative fungus occur to the number of 3 to 10, appearing 

 to the naked eye as black dots, usually situated on the upper 

 surface of the leaf. 



The lower leaves are first attacked, and as the spots become 

 numerous they turn yellow, wither, and fall. In severe infesta- 

 tions the disease extends to successively younger leaves until 

 only terminal tufts are left. The attacks of the fungus on stems 

 and fruits are of minor importance. The quantity and quality 

 of fruit produced are affected in proportion to the injury to the 

 leaves, and total failure of crop may ensue. The disease readily 

 attacks seedlings and young plants, and is often carried by them 

 into the field. 



Causative Fungus. 



The disease has been demonstrated by infection experiments 

 to be caused by the fungus Septoria lycopersici Speg. The 

 mycelium is of two types, a hyaline thin-walled vegetative form, 

 and a dark closely septate thick-walled type associated with 

 reproduction. The former is intercellular with knob-like haus- 

 toria. The spores from the pycnidia are filiform and hyaline, 

 60-120 X 2-4 microns, with 2-8 septa. The thickness of a spore 

 is not uniform throughout and the shape of the ends varies from 

 pointed to globose. The spores are exuded under the influence 

 of moisture in mucilaginous masses which disintegrate in water. 



Conditions of Occurrence. 



The spores are washed and splashed about by rain and are 

 readily transported during the handling of the crop under wet 

 conditions. In dry weather the disease does not spread, and the 

 spores do not appear to be able to tolerate drying for more than a 

 few days. Infection is stomatal, and takes place for the most 

 part and with greater effect on the lower surfaces of the 

 leaves. The source of the infectious material initiating an out- 

 break appears to be mainly the remains of previous crops. It is 

 not known that infection can be introduced with the seed. 



