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



Cracked stem is undoubtedly a nutritional disorder. In the acid 

 muck soils of Florida the disease has been controlled by soil appli- 

 cations of commercial borax. On heavily limed 14 or otherwise alka- 

 line soils in several Northern States beneficial results were obtained 

 by applications of soil-acidifying agents, such as sulfate of ammonia, 

 manganese sulfate, or sulfur; boric acid and borax were successfully 

 used there on both acid and alkaline soils. 



(See 173, 192, 200, 201.) 



EARLY BLIGHT 



(Cercospora apii Fresen.) 



OCCURRENCE, SYMPTOMS. AND EFFECTS 



Early blight is of common occurrence in all celery-growing districts 

 of the country and is frequently of great importance in California 

 and Florida. Losses vary considerably within a given area from 

 season to season. They may range in severity from a blemishing of 

 the leaves to almost total destruction of the crop. 15 On the market 

 the disease is primarily of importance because of the less attractive 

 appearance of infected celery and because the presence of the lesions 

 facilitates infection by soft rot bacteria. 



The first symptom of early blight is the appearance on the leaves 

 of small pale-yellow spots which enlarge and become roughly circu- 

 lar. Where the spots are numerous they coalesce to form large 

 irregularly shaped lesions. The color of the spots changes from 

 yellow to brown, and finally, by the growth of the pathogen over 

 the affected surface, to ash gray. The lesions are most abundant 

 on the leaflets although they are found frequently on the leaflet 

 petioles and occasionally on the main leafstalks, where they appear 

 as brown longitudinal streaks that later turn gray and then nearly 

 black (pi. 16. A). In most cases the lesions can be distinguished 

 from those of late blight by the absence of the small black, fruiting 

 bodies (pycnidia) so characteristic of the latter disease. 



CAUSAL FACTORS 



Early blight is caused by the fungus Cercospora apii. The 

 pathogen is capable of living over from crop to crop in diseased plant 

 refuse in and on the soil. Under favorable conditions the spores are 

 produced there in great abundance and are carried by air currents 

 to the leaves of growing healthy plants. The most favorable con- 

 ditions for spore germination are the presence of moisture on the leaf 

 surface and a temperature of 81° to 84° F. The optimum tem- 

 perature for growth of the fungus is 77° to 8()°, although it grows well 

 between 73° and 86°. During periods of high temperature with 

 abundant moisture the disease develops rapidly throughout the field. 

 Periods of continued warm days followed by cool nights, with the 

 formation of heavy dews that remain long on the plant the following 

 day, are particularly conducive to severe losses from the disease. 



11 Boyd, 0. C. vegetable diseases i\ Massachusetts in iwe. r. s. Bur. Plant Indus., 

 Plant Dls. Rptr. 20: 333-337. \'X'a\. [Mimeographed.] 



ib Wbllman, f. L. an unusual outbreak op celeri barm blight, r. S. itiir. Plant. 

 In. Ins.. Plant. Dis. up''". 1<*> : 18-45. 1932. [Mimeographed.] 



