EXPERIMENT STATION BULLETINS. 



373 



the inner and heart leaves, until no salable part is free from the trou- 

 ble. The rotting celery has a characteristic odor. Severe attack means 

 worthless celery, since the bunches will not keep in market or storage. 



THE CAUSE OF CELERY LEAF SPOT. 



Celery leaves show such spots and the stems rot because of the attack 

 of a parasitic fung-us. A parasitic fungus, or mold, is a small plant that 

 steals its Thing from another plant. These parasitic fungi consist of 

 threads so small that a miscroscope is necessary for one to see them. 

 The threads of this fungus twine between the leaf cells and feed upon 

 the cell substances. Just as the larger plants are spread by seeds, these 

 microscopic plants are spread by small bodies which do the same work 

 as seeds. These hodies are called spores. The spores of the celery leaf 

 spot fungus are needle-shaped and are only about 1-250 of an inch long. 

 They cannot be seen with the naked eye. These spores are produced in 

 immense numbers inside the black bodies found on the diseased spots ou 

 the leaves. 



SPORES 





Fig. 3. Section through diseased spot, showing fungus threads penetrating leaf tissue and spore case 

 with exuding spores; enlarged view of spores at right. (Diagrammatic.) 



The spores are sticky and when the diseased leaf is wet they ooze out 

 of the black spore case. These spores float about the leaf in the dew. 

 The chief spreading comes from the washing and splashing by rain. 

 When the leaves are dry the spores dry down. Therefore, the fungus 

 spreads only when the leaves are wet. 



When a spore (compare with a seed) is washed to a leaf and Ihe leaf 

 stays Avet long enough (several hours) the spore sprouts by sending out 

 several tubes. These sprouts grow into the celery leaf and continue to 

 grow inside by sending threads through the tissue. After about nine 

 days (varying with the temperature) a well-marked spot may be seen 

 ou the leaf. In about 15 to 20 days the spots show many black spore 

 cases and a second crop of spores is produced. In this way one spore 

 produces, in less than three weeks, millions of others, each of which is 

 capable of producing millions more. It is little wonder that fields can 

 be completely blighted. 



