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Suppression of Fusarium Yellows of Celery with Potassium, Chloride, and Nitrate. R. W. Schneider, Department of Plant Pathology and Crop Physiology, Louisiana State University, Baton Rouge 70803. Formerly, Department of Plant Pathology, University of California, Berkeley 94720; Phytopathology 75:40-48. Accepted for publication 30 May 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-40.

Ammoniacal-N (NH4-N) was more conducive to Fusarium yellows of celery, caused by Fusarium oxysporum f. sp. apii, than nitrate-N (NO3-N) regardless of the accompanying ions. The inclusion of specific concentrations of KCl with NO3 (as Ca [NO3]2) resulted in almost complete control of the disease in the greenhouse and field. The provision of both K and Cl was essential inasmuch as disease suppression was related to a specific tissue concentration ratio of K and Cl rather than absolute concentrations of either ion. Factors that affected the ratio were the predominant form of N, the availability of competing cations (Ca and NH4) and anions (SO4 and NO3), and the absolute concentrations of available K and Cl in the soil. Thus, small amounts of NH4 repressed uptake of both NO3 and K and stimulated Cl uptake, while concentrations of K and Cl were lower in the presence of Ca and SO4, respectively. In addition, excessive amounts of KCl under an NO3 regime resulted in large accumulations of Cl but not K. Quantification of these interactions was accomplished by assessing rates of root infection (colonies per 100 cm of root). These data showed that a propagule under an NH4 regime was about five times as likely to cause a root infection as under a NO3 plus KCl regime. Furthermore, differences in maximum rates of root infection as a function of inoculum level indicated that either there were fewer sites of infection or these sites were more resistant under the latter regime. The process of osmoregulation, ion uptake mechanisms, and charge balance are discussed with respect to the centrality of the K:Cl ratio in the root infection process. Substantial disease control was achieved in the field with sidedress applications of Ca (NO3)2 and KCl.

Additional keywords: mineral nutrition.