Now is an optimum time to scout your corn and soybean fields. This time of year you can begin to gain knowledge on the progress and condition of your crop.
Some key items to assess in the corn crop can be, but aren’t limited to: pollination, kernel development, and amount and/or type of foliar disease present in corn fields. Some soybean key items to assess can be: pod set, flower development, pod fill and amount and/or type of foliar disease. However, there seems to be fairly common objections from growers when it comes to scouting fields during this time frame. Whether it has to do with the heat, humidity, pollen shed or wet soybean canopies from morning dews, whatever your objection might be, let’s take a look at the disease triangle and how it can help guide our scouting trips to make them more efficient and productive. Three things must be present for diseases to occur, also known as the three legs of the disease triangle. These include: disease inoculums, susceptible host and favorable conditions.
The first leg of the disease triangle to consider is that the disease inoculums must be present. Each plant disease has differing modes for diseases to be introduced. Some diseases over winter on residue, others move in on storm fronts, others reside in the soil, and or they can come from multiple points. For example, frogeye leaf spot can over winter on residue or it can be deposited via storm fronts from southern regions of the country. Non-rotated fields and no-till fields favor higher residue levels and will have more residue present to overwinter disease inoculums. Field history can determine which fields have the highest probability of having disease inoculums present. By taking management practices and field history into account, you can start narrowing down which fields you need to scout for certain diseases.
The second leg of the disease triangle is a susceptible host. These fields can also be narrowed down in the office before you start your scouting trips. Corn hybrids and soybean varieties have differing susceptibilities to foliar diseases due to differences in genetic background. There are three sources to use to determine which hybrids or varieties are more susceptible than others. The first is your local seed dealer. The second is literature or seed guides from the respective seed company. The third is your own experience with a hybrid or variety.
Seed companies, publish ratings for the common foliar diseases on a rating scale or they may rate a product’s response to foliar fungicides. It is important to reference the legend for the ratings, as each company may use a different scale for ratings. For example, a rating of 1 from company A represents the best and a rating of 1 from company B represents the poorest. Also, ratings are only relative to that company’s product ratings and cannot be used in comparison to another company’s product. The third source of information is your own experience with a product, which can be the most influential source. Past experiences with a product may be your best indictor of whether or not it is susceptible since it was in your fields and under your management practices. However, remember that the other two legs will influence the amount of disease that is present, especially the third leg.
The third leg of the disease triangle is favorable conditions — the most variable and dynamic influencer on disease presence is the environmental conditions. This is due to the fact that each disease favors slightly different conditions for acceleration and that environmental conditions change so rapidly across a field, across time, and throughout the day. For example, gray leaf spot thrives in warm wet periods with temperatures between 75degrees F to 85degrees F and 90% relative humidity. In contrast, northern corn leaf blight favors more moderate temperatures 64degrees F to 81degrees F and high relative humidity. Conditions that favor disease development may be present in bottom fields, fields bordered by woods, irrigated fields, or environments that foster higher relative humidity.
Consideration of the three legs of the disease triangle: disease present, susceptible host and favorable conditions can help prioritize your scouting trips to make them more efficient and productive. Then you can use the information you have gathered from product ratings, field history, environmental conditions and your in-field observations to determine whether or not further management practices need to be implemented. Additionally, Extension publications can be helpful for disease identification, fungicide timing or trigger levels or future management tools.