Seed & Chemical
Fighting fungal foes
By Alberta Agriculture and Rural Development
July 18, 2013, Edmonton, AB - Warm, moist conditions in spring over the last few years have caused a huge increase in the acres of crop sprayed with fungicide. As with most pesticides, the product has to be sprayed at the right time and correct rate to address the identified problem.
According to Harry Brook, crop specialist with Alberta Agriculture and Rural Development (AARD), fungicides do not translocate as well as some herbicides, and sufficient water volumes are needed to get good plant coverage. "To get the protection you are paying for, time of spraying is also crucial. Each particular crop disease needs to be treated at the appropriate time."
Blackleg of canola, once a debilitating canola disease, is making a comeback in Central Alberta. The crop has to be sprayed at the 2 to 6 leaf stage to be protected. Producers should look for spots on the leaves. For most canola crops, it is already too late to spray due to crop development.
"If you are looking to protect a canola crop against sclerotinia, you need to wait until the canola is between 20 and 50 per cent full bloom," says Brook. "For this disease, it is most effective to spray when you can maximize the number of flower petals that will be covered with the fungicide, thus preventing the sclerotinia organism from beginning growth. This occurs well before the crop shows any symptoms and the decision to spray has to be evaluated on a number of factors to try and assess the potential risk. If there are enough risk factors present, the decision to spray is made. As for timing, spraying will be done within the next week or 10 days."
Pulses need to be sprayed for disease when they show initial symptoms such as leaf spotting. Under the present weather conditions, pea diseases can rapidly spread and cut crop yields. On the cereals side, a lot of producers are being told to include a half rate of fungicide in with the herbicide when weed spraying. This would only be a good idea if the crop is already showing signs of disease at the 4 to 6 leaf stage.
"There is no benefit in spraying fungicide if the crop shows no symptoms, and it might even be detrimental," says Brook. "Using a pesticide when the pest is not visibly present may hasten the onset of resistance in the pest. Also, a half rate of pesticide can encourage moderately resistant organisms to thrive and overcome the particular mode of action of the pesticide."
The majority of diseases in cereals can be divided into those that affect the roots, usually seed or soil borne diseases, and those that affect the leaves and the filling of the head. Seed treatment will give protection for seed borne diseases and some root rots for a couple of weeks. Fungicide application on cereals to protect crop yield involves protecting the top two or three leaves from disease by spraying from flag leaf emergence to head emergence. The spray decision is based again on risk factors and also on presence of disease in the lower canopy.
"The decision to spray with fungicides should never be a matter of routine," says Brook. "You have to assess risk in terms of crop rotations, varietal disease resistance, weather conditions, potential damage and costs versus benefits. Alberta's current humid, warm conditions are perfect for the development of disease, but some field walking and thoughtful evaluation are still required to make the best decision for your crop. As with all pesticides, it is important to keep in mind the pre-harvest intervals when deciding to spray. With enough information, you can make the right decision and don't forget that a good crop rotation can prevent a lot of disease problems from occurring.