A common question asked by growers whenever there is a plant disease problem is, "What can I spray to control this disease?" When he hears this question, SDSU Extension Plant Pathologist, Emmanuel Byamukama explains that even though spraying may be the first solution which comes to growers minds, there are several other tools that limit level of disease development.
"There are a number of reasons why spraying is usually the sought after and/or the preferred method of plant disease management. It is convenient, just load up the tank and spray. It can also be effective when applied in the right situation, at the right time, and with the right product. However, chemical control as a management practice has its challenges," Byamukama said.
He emphasized this point by adding that some diseases, such as Fusarium head blight have a very short window of applying the chemical.
"The residual effect of the chemical may last for a few weeks, the cost of pesticides is an added expense and weather conditions may not permit the application," he said. "Some soil-borne diseases like root and stem rots cannot be controlled by spraying plants, and overreliance on pesticides can result in resistance development and environmental pollution."
Preventative Plant Disease Management
Byamukama said there are several preventive plant disease management practices that can be done well ahead of the season that may help to minimize the need to apply fungicides later in the season.
"This is the time to evaluate the preceding season and use the lessons learned to make some of those decisions," he said. "Awareness of field disease history and cropping practices that favor disease development may help in determining the likelihood of diseases that could develop in the next growing season."
Host resistance/tolerance, when available, is the first line of defense in plant disease management Byamukama explained.
"Unlike glyphosate resistance, Bt, and other GMO traits, disease resistance traits do not add to the cost of seed," he said. "Yet host resistance is an effective, sustainable and affordable plant disease management practice."
Several seed companies provide disease ratings for their varieties making it easier to choose the right cultivar, depending on the grower's needs. Growers should consider the history of diseases in their fields, cropping practice (such as corn on corn or no-till) to decide on cultivar selection.
"For instance, corn on corn under irrigation is likely to have Goss's wilt develop; therefore a grower in this case would want to plant a Goss's wilt resistant/tolerant cultivar," he said.
Byamukama encourages growers to keep good records on the performance of the cultivars grown to aid decision making when selecting a cultivar to plant. He added that monitoring performance of cultivars planted may also help to indicate development or change in a pathogen allowing it to overcome the resistance source used.
"Examples here may include Phytophthora root rot or soybean cyst nematode (SCN). A cultivar previously resistant to SCN that yields less than expected and an increased SCN egg load in the soil may indicate the presence of a SCN population that has overcome this resistance source. Change in the cultivar to be planted would therefore be necessary," he said.
Another strategy to reduce the chances of plant disease development is exclusion.
"That is, do not introduce the pathogen to the field - keep it out."
This can be done, Byamukama explained, by using clean seed, clean equipment and starting with a clean seedbed. If the pathogen cannot be avoided, the next tools available would be to minimize its impact on crop production.
"Most of the economically important pathogens that infect corn, soybean, and wheat, apart from rust, nematode, and virus diseases, are residue-borne and therefore cannot be avoided or excluded from the field," he said. "This is when cultural practices like crop rotation, residue management through tillage, and time of planting become effective in limiting the negative impacts of plant diseases."
Integrated Disease Management
The use of an integrated disease management approach is encouraged by SDSU Extension Plant Pathologist Emmanuel Byamukama.
Integrated disease management consists of combining several plant disease management strategies such as the use of resistant cultivars, crop rotation, field residue management, altering time of planting and applying pesticides only when warranted - through scouting.
"Not only is an integrated approach effective and economical, but it is also sustainable," Bymukama said.