Sentence:  Calcium sprays of 10 to 20 pounds of Ca(NO<sub>3</sub>)<sub>2</sub> or 5 to 10 pounds of CaCl<sub>2</sub> per 100 gallons of water per acre applied once or twice per week have provided good control in several trials and are considered most beneficial when <WSD>blackheart</WSD> appears before harvest, when plants may outgrow it, and when it is necessary to hold mature celery in the field. 
Meaning of the word marked with <WSD> tags:  Any of various plant diseases that cause darkening of the central tissue.