STUDIES ON BROWNING ROOT ROT OF CEREALS: III. PHOSPHORUS-NITROGEN RELATIONS OF INFESTED FIELDS IV. EFFECTS OF FERTILIZER AMENDMENTS V. PRELIMINARY PLANT ANALYSES

Abstract

These papers deal with the elucidation of the factors which predispose wheat seedlings following summerfallow to Pythium root rot in certain parts of Saskatchewan. The chemical soil analyses on 66 sample pairs reveal that available phosphorus in the soil is lower and nitrate nitrogen higher in browning patches than in normal patches. Greenhouse experiments and field observations show that phosphate amendments at the time of seeding, and farmyard manuring, permit the development of normal plants in browning soil; cereal straw applications inhibit the disease to some extent, while inorganic nitrogenous fertilizers usually have no effect or occasionally may prove detrimental. Preliminary plant analyses indicate that the total phosphorus in diseased plants is lower than in healthy ones, and that there is relatively more water-soluble nitrogen and relatively less protein nitrogen in diseased than in healthy plants.The soil and crop-culture factors, especially the practice of bare summer-fallowing, which may influence the available phosphorus-nitrate nitrogen ratio in the soil, are discussed in relation to disease incidence. It appears that any crop practice which tends to increase the available phosphorus-nitrate ratio in the soil inhibits the disease, and conversely. This is used as a basis for control recommendations.Attention is drawn to the striking similarity of the factors affecting the severity of Pythium root rot of wheat and Pythium root rot of sugar cane in Hawaii.It is considered that an improper balance of available phosphorus and nitrate nitrogen in the soil leads to an unbalanced metabolism of the wheat seedlings at a critical stage in their development, thereby predisposing them to fungal attack. The effect of the unbalanced nutrients on the parasitic vigor of the pathogen still requires elucidation.