Effects of ammonium and nitrate nutrition or take-all disease of wheat in pots

Plant debris, naturaiiy infested with the take-all fungus (Ophiobolus graminis), was washed from soil and added to a leached sandy loam, deficient in nitrate nitrogen (NO₃-N) and magnesium. Nutrient solutions containing potassium and phosphorus, with and without magnesium, were added to the amended soil unsupplemented, or with either NO₃-N, ammonium nitrogen (NH₄+-N), or both. Nitrification of NH₄+-N was inhibited by 2–chloro-6–(trichloromethy1)-pyridine (N-Serve). After 38 days at 19°C, fewer plants had take-all with N (75 or 100 mg/kg soil) than without and root systems were most discoloured and had most diseased axes when nutrients were not added. Plants given NH₄+-N developed less take-all when magnesium was present. A comparison of forms of N in the presence of added magnesium showed that take-all was least with a mixture of both forms of N, intermediate with NO₃-N alone and worst with NH₄+-N alone. The most extensive lesions on individual root axes occurred on plants given NH₄+-N. It is suggested that take-all will be least when the amounts and ratio of NH₄+-N and NO₃-N are optimum for the growth of the host.