How effective are crop rotations?

Abstract

Current crop rotation recommendations suggest at least 3 non-susceptible crops be grown between planting of Sclerotinia susceptible crops such as canola and pea. However current pricing of pea and canola make production of these crops more profitable providing their yields are not greatly reduced by disease. Under center pivot irrigation the soil surface and plants are wetted every 2-4 days during periods of high water use; this results in a good environment for spore production and germination. Levels of Sclerotinia infection were generally somewhat lower than predicted by the petal test method. Yields were not improved by the application of iprodione at 250, 500 or 700 g/ha at the 40% bloom stage. The yield levels obtained and visual observation of time and severity of stem damage indicated that the disease probably occurred late enough in the season to limit damage. No lodging occurred and Sclerotinia levels were near zero in 1991 as a result of a very hot dry August. In 1992, 2 of 3 tests showed significant lodging. Sclerotinia levels increased with an increase in lodging (r2 = 0.85* and 0.73*). Lodging reduction can be accomplished by the use of resistant cultivars and by lowering seeding rates to less than 100 seeds/m2. Yields were not reduced at lower seeding rates at wider row spacings. Sclerotinia is an environmentally influenced disease which is not well controlled by crop rotations up to 5 years however its severity can be limited by reducing lodging. When canola prices are at least 1.9 times that of cereal grains, shortening rotations to 2-3 years between crops, such as pea and canola, could be a viable risk.