Pulses in Rotation Battle Clubroot

EDMONTON - Jan 14/08 - SNS -- Including pulses such as field peas, lentils and chickpeas in crop rotations with canola will help prevent clubroot from spreading rapidly across Alberta.

"When one crop type is repeatedly or frequently grown on the same soil for numerous years, resting spores of pathogens specific to this crop type build up their numbers in the soil," says Neil Whatley, crop specialist with Alberta Agriculture and Food, Stettler. "These spores are then ready to attack its host plant's roots again and again. Without the availability of effective seed treatments or resistant crop varieties to prevent disease damage, these resting spores will soon devastate this frequently grown crop."

As this process continues over the years, soil spore population increases, which causes upwards of 100% plant infection and consequent crop losses. This is the situation that has occurred with clubroot disease of canola, caused by the pathogen Plasmodiophora brassicae. This disease is currently affecting canola growers in numerous counties in the wetter areas of Alberta. In the spring of 2007, clubroot was declared a pest in Alberta's Agricultural Pests Act, which legislates authority for enforcement of control measures.

"In most situations the fields where clubroot has been identified have been planted to either continuous canola, or to a canola/cereal--canola/cereal crop rotation," says Whatley. "Accumulation of the clubroot resting spores in the soil is rapidly occurring in these fields. Clubroot is a particularly serious plant disease because its spores can stay viable in the soil for upwards of 20 years. Without an effective seed treatment or resistant canola variety, a more proactive approach must be taken to prevent this disease from seriously reducing future canola yields."

Regardless of the presence of clubroot disease, canola can continue to be produced as a crop in infected fields if a longer crop rotation is used. Plant researchers are finding that canola can be safely planted once in four years. However, under the Agricultural Pests Act, some counties may legislate longer rotations than one in four years. These longer rotations allow for some build-up of the clubroot resting spores in the soil, but never enough to cause an epidemic. For growers who have not discovered clubroot in their fields, this is an important strategy to use to prevent the field from becoming infected.

"Using a one-in-four year rotation for canola requires that other profitable crops be grown in the other three cropping positions," says Whatley. "Since canola has been a very profitable crop for growers, replacements may not appear to measure up. However, with the potential danger to future canola yields by using tight canola rotations, proactiveness is of paramount importance. With careful foresight, some cropping alternatives may appear more agronomically and economically attractive in the long run than previously thought. This is made more attractive when one considers current high commodity prices and steep nitrogen fertilizer input prices."

Cereal crops are a natural choice to use to lengthen a cropping rotation. However, since some soil borne diseases such as fusarium head blight and other common seedling blight diseases affect wheat, barley, oats and other small cereal grains, repeatedly planting too many cereal crops may also backfire. Therefore, it is wise to use crops from other plant families as well to prevent disease problems within cereal crops. This is where pulse crops from the legume plant family can be valuable, both to prevent a clubroot epidemic and to improve financial and agronomic choices. Viable pulse crops for Alberta are field pea, lentil and chickpea. However, the adaptability of field pea to the wetter soil zones of Alberta makes field pea the best pulse crop choice for growers who are facing a clubroot problem.

Pulse crops have a shallower root system than cereals and oilseeds, penetrating approximately half the soil depth. This means that field pea utilizes less soil water and soil nutrients than a cereal crop or an oilseed crop, leaving more for successive crops. Due to its ability to fix atmospheric nitrogen, a pulse crop contains a higher proportion of nitrogen in its plant parts than oilseeds and cereals. When decomposing, this plant material mineralizes, causing a flush of available nitrogen for subsequent crops.

Dr. Slinkard, professor emeritus at the University of Saskatchewan's Crop Development Centre, explains that in general 50% of the nitrogen in the pulse crop residue (leaves, stems, roots, nodules) is mineralized and available for the next year's cereal crop and about 50% of the remainder is available to the second crop after a pulse crop. Dr. Slinkard says the beauty of it is that this nitrogen is slowly mineralized during the growing season and becomes available during flowering and seed development, when available soil nitrogen is nearing depletion. This results in increased protein concentration of a cereal crop and, if available soil moisture is adequate at this time, increased yield.

"The inclusion of a pulse crop into the cropping rotation assists with nutrition and water conservation, as well as reducing the incidence of clubroot in canola," says Whatley. "Taking these factors into consideration, a beneficial four-year crop rotation is to sequence crops in the following manner: barley/field pea/wheat/canola--barley/field pea/wheat/canola. Sclerotinia fungal disease potentially affects both field pea and canola, however semi-leafless field pea varieties cause less build-up of sclerotinia than do the vining types, so are more acceptable in a rotation with canola."