52 MISC. PUBLICATION 4 4 0, U. S. DEPARTMENT OF AGRICULTURE 



There are two types of late blight, the large-spot form and the 

 small-pot form (pi. 16, B and C). Although both are widely dis- 

 tributed, the latter is the more common in this country. The first 

 symptom of the small-spot type on the leaflets is the formation of 

 small yellowish (or chlorotic) spots. As the spots enlarge the affected 

 tissues turn brown and then nearly black. The edges of the spots 

 are indefinite. The diameter of the spot varies somewhat, with an 

 average of around one-sixteenth of an inch and an upper limit of 

 about one-eighth of an inch. Where the spots are numerous and 

 therefore close together they may coalesce to form large brownish- 

 black areas. The spots are further characterized by the presence of 

 numerous black granular bodies of pin-point size, which are the 

 fruiting bodies (pycnidia) of the pathogen. These are present even 

 in early stages and are found either singly or in clusters over the 

 entire surface of the spots; they frequently occur in the apparently 

 healthy green tissues bordering the spots. The leaflet petioles and 

 the main leafstalks are also attacked. There the lesions are longi- 

 tudinally elongated and the affected tissues are brown, with abun- 

 dant fruiting bodies (pycnidia) scattered over their surface (pi. 

 13, C). The lesions on the leafstalks are most numerous on the 

 outer surface of the older ones. 



In the large-spot type of late blight the spots are larger, with 

 an average diameter of around one-half inch, more regular in 

 outline, and have more definite boundaries. They are brown in 

 the center and have a darker reddish-brown border. The pycnidia 

 are not abundant, frequently develop late, and are confined to the 

 center of the spot. Where pycnidia have not yet appeared the 

 spots closely resemble those of early blight (see p. 50). The occur- 

 rence on the leafstalks of this type of late blight has not been 

 reported. Possibly it occurs there with symptoms indistinguishable 

 from those of the small-spot type of the disease. 



CAUSAL FACTORS 



The small-spot type of late blight is caused by the fungus Septoria 

 apii-graveolentis and the large-spot type by Septoria apii. Both 

 fungi are capable of living over from season to season in the seed, 

 and in diseased celery refuse in the soil. Celery plants are thus 

 subject to attack from the first stages of growth so that unless 

 proper control measures are taken a high percentage of the seedlings 

 may become diseased in the seedbed. In the field the disease may 

 originate from spores produced either on the diseased seedlings or on 

 plant refuse in the soil. The dissemination of spores by drainage 

 and irrigation waters, by wind, by farm animals and machinery, 

 and by man may cause the further spread (if tin 1 disease throughout 

 the fi< ; !<!. 



Late blight is generally considered a cool-weather disease. This 

 is borne out by the fact that the organism responsible for the small- 

 spot type of the disease, which is the more prevalent of the two 

 types, grows best at a temperature range of approximately 64° to 

 72° F. Above 75° growth of the pathogen is very slow. The fungus 

 responsible for the less prevalent large-spot type of the disease has 

 an optimum temperature of approximately 72° to 75° and makes 



