Fertilizer Effects on Common Scab of Potato and the Relation of Calcium and Phosphate-Phosphorus

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Abstract
Results showed significant linear correlations between common scab (caused by Streptomyces scabies) of potato and the Ca, PO4, Mn and Fe content of tuber peelings. Treatments with nitrate and ammonium fertilizers and the N stabilizer, N-Serve [2-chloro-6-(trichloromethyl)-pyridine] at 0.6, 1.1 and 1.7 kg/ha with ammonium sulfate (200 kg N/ha) did not significantly affect scab severity. Treatment with N-Serve reduced Mn, Mg, Cu, Zn and K in potato tissue, and B was increased. Treatments with triple-superphosphate (84-336 kg P2O5/ha) significantly reduced the extent of scab and significantly changed scab lesion morphology by reducing lesion height and depth. The effect of triple-superphosphate on potato scab appeared to be indirect, since the pathogen (S. scabies) was not inhibited by triple-superphosphate. Levels of P in potato petioles and peelings were linearly correlated with common scab. The effect of phosphate-P was related to Ca levels and results with phosphate-P suggest a phosphate-Ca interaction. Foliar sprays with Mg (0.2 kg/ha) also produced a slight, but significant scab reduction.

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