NITROGEN FORMS AFFECT CITRUS BLIGHT AND ALTER SOIL FACTORS AFFECTING ROOT SYSTEMS

Beginning in 1974, four citrus groves at Babson Park, FL, and six grov es at Lake Garfield, FL. each began receiving Ca(NO3)(2) as the sole s ource of nitrogen (N) fertilization on one portion and 50% Ca(NO3)(2) plus 50% NH4NO3 on the equivalent remaining portion. Nitrogen was appl ied annually as split applications, once each in the spring and summer . Four groves consisted of grapefruit grafted on rough lemon rootstock ; the remaining groves were sweet orange cultivars grafted on rough le mon. Data on tree loss to blight were recorded at Babson Park annually between 1975, 1977, and 1980; and between 1975 and 1981 at Lake Garfi eld all years except 1978 and 1979. Annual tree loss, averaged over al l years, at Babson Park was 9.5% for the combination N treatment and 7 .8% for the Ca(NO3)(2) treatment. The data were significant at a P lev el of 0.1186. Tree loss at Lake Garfield averaged 5.6% in the combinat ion treatment and 4.3% in the Ca(NO3)(2) treatment, with a P level of 0.1125. Between 1977 and 1981 soil pH in the Ca(NO3)(2) treatments of all six groves increased to between 6.5 and 7.1, but did not increase in the combination N treatments, remaining between 6.3 and 6.6. Soil p H measured to a depth of 60 cm also differed significantly between tre atments. For a grove ata third location near Ocoee, FL. total fungi an d Fusaria propagules were measured in soil treatments that received NH 4NO3 (7.9 kg N tree(-1)), urea (17.0 kg N tree(-1)), and a water-only control. Fusaria were affected by N treatments and, in the NH4NO3 trea tment, increased significantly compared to the water-only control as s oon as one week after fertilization. These experiments suggest that th e ammonium form of N may be contributing to development of citrus blig ht. Its involvement with Fusaria, the cause of root rot symptoms on bl ight-diseased trees, may be as a source of N for toxin production.