MARKET DISEASES OF FRUITS AND VEGETABLES 49 



growth appears to increase the likelihood of black-heart development. 

 Thus, although the form of nitrogen used in the fertilizer has little 

 effect, the greater the amount used the greater will be the amount of 

 black-heart if moisture conditions are favorable for its appearance. 

 The disease is more severe after periods of high temperature and 

 is of much more importance as the celery reaches maturity than earlier 

 in the growing season. 



CONTROL MEASURES 



In celery grown under irrigation black-heart has been successfully 

 controlled by careful regulation of the irrigation waters. Excessive 

 soil moisture immediately prior to harvesting is to be avoided par- 

 ticularly. Only moderate amounts of nitrogen fertilizer should be 

 used. Harvesting the crop promptly as it approaches maturity may 

 reduce losses. Some measure of control has been secured by growing 

 partly resistant types of celery. 



Where any evidence of black-heart has occurred in the field partic- 

 ular care should be taken to avoid packing affected plants, for they 

 are practically certain to develop bacterial soft rot (see p. 46) in 

 transit. 



(See 172, 173, 206, 2U, 224.) 



BROWN STEM 



The symptoms of brown stem are a browning and pitting of the 

 tissues immediately beneath the epidermis of the outer leafstalks. 

 Although the cause is unknown it appears to be nonparasitic. Where 

 brown stem occurs it usually is found on celery that has been allowed 

 to remain in the field for some time after having reached maturity. 

 At such times it frequently is associated with black-heart, 



Brown stem is of importance as a blemish of celery both in the 

 field P and on the market. 13 The affected areas of the leafstalks 

 probably also serve as points of entry for organisms causing decay 

 in transit and on the market. 



(See 173.) 



CRACKED STEM 



Cracked stem has been of major importance in Florida and of 

 occasional importance in a number of other States. It is found from 

 time to time on the market, where it frequently causes serious losses. 



The disease is first recognized by a brownish mottling of the leaves. 

 The roots are also affected, turning brown and dying back from the 

 tips. The leafstalks become brittle, and brown longitudinal streaks 

 appear over the ribs or vascular bundles. Transverse cracks then 

 appear in the epidermis over these streaks and the torn epidermis 

 curls and peels back from the cracks, giving the leafstalks a roughened 

 appearance (pi. 13, A). Although the cracks are shallow and nar- 

 row they usually occur in such abundance as to render the stalk 

 unsightly, hence of little market value. 



12 MacMillan, H. G., and Plunkett, O. A. plant diseases observed in southerx 

 california in 1936. U. S. Bur. Plant Indus., Plant Dis. Rptr. 21 : 76-79. 1937. 

 [Mimeographed. ] 



13 Bratlet, Cyril O. new york market pathology notes. U. S. Bur. Plant Indus., 

 Plant Dis. Rptr. 15 : 50. 1931. [Mimeographed.] 



