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Physiology and Biochemistry

Effect of Nitrogen Fertilization on Free Amino Acid and Soluble Sugar Content of Poa pratensis and on Infection and Disease Severity by Drechslera sorokiniana. Philip W. Robinson, Research and Development Representative, American Hoechst Corporation, Agricultural Division, Somerville, NJ 08876; Clinton F. Hodges, Professor of Horticulture and Botany and Plant Pathology, Department of Horticulture, Iowa State University, Ames, IA 50011. Phytopathology 67:1239-1244. Accepted for publication 25 March 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1239.

Preinfection germination and growth of conidia of Drechslera sorokiniana on leaf surfaces of Poa pratensis and subsequent postinfection lesion and disease development were influenced by soil applications of (NH4)2SO4, NH4NO3, and Ca(NO3)2 to P. pratensis. All nitrogen sources increased bipolar germination, germ-tube length, and germ-tube branches on leaf surfaces; appressoria formation was increased by NH4 NO3, and penetrations without appressoria were not influenced by any of the nitrogen sources. Number of lesions per unit leaf area was not significantly changed by any of the nitrogen sources, but lesion size increased in response to all nitrogen sources, and disease severity increased significantly in response to (NH4)2SO4. Total soluble sugars in leaf tissue were decreased by (NH4)2SO4 and NH4NO3, but did not change significantly in response to Ca(NO3)2. Individual soluble sugars decreased were fructose, glucose, and sucrose. Correlations between individual sugars and preinfection growth characteristics of D. sorokiniana (bipolar germination, germ-tube growth, appressoria formation) on leaf surfaces showed that decreases in fructose, glucose, and sucrose were negatively correlated with increased growth of the pathogen; decreases in sugars showed essentially no correlations with postinfection disease development (lesion number, lesion type, disease severity) suggesting that decreasing sugar levels in response to nitrogen fertilization may induce growth of the pathogen on leaf surfaces, but have little direct influence on disease severity. Total free amino acids in leaf tissue increased in response to all nitrogen sources; individual amino acids, however, increased, decreased, or remained unchanged in response to the nitrogen sources. Increases in aspartic and glutamic acids were negatively correlated with various preinfection characteristics of D. sorokiniana (germ-tube growth, germ-tube branching, appressoria formation) on leaf surfaces; increases in proline were positively correlated with almost all preinfection germination and growth characteristics of D. sorokiniana conidia on leaf surfaces of P. pratensis. Glutamic acid and proline increases were positively correlated with most postinfection disease development characteristics (lesion number, lesion type, disease severity), suggesting that increases in these and other amino acids in response to nitrogen fertilization may contribute more directly to disease severity after infection than the decreases in soluble sugars.

Additional keywords: aspartic acid, Bipolaris sorokiniana, etiology, exosmosis, exudates, fructose, glucose, glutamic acid, guttation, Helminthosporium sorokinianum, H. sativum, proline, sucrose.