It’s August, and it’s still a great time of year to be out in your soybean fields! The pods are starting to fill in and yields are getting the finishing touches. Varietal characteristics are showing up at this point. This is the time when pests and pathogens can infest the fields. Late season scouting is a good idea during this time.
This time of year you typically see diseases like White Mold, Brown Spot, Bacterial Blight, Brown Stem Rot and SDS – all but SDS have been found in North Dakota. Not all are major yield robbers, but Sclerotinia Stem Rot or White Mold really can be. Yield loss is normally 2bu/ac for every 10% increase in disease incidence. Moist, humid conditions in the crop foliage are needed for symptoms to show up. Stems of affect plants will appear bleached and may have a white growth called mycelia on them. The leaves will wilt and die, normally occurring in patches and not be across the whole field. Fungicide treatment for White mold has not shown to be very effective as of yet and only early applications around R1 have shown advantageous.
Brown Stem Rot is an emerging issue in the state and needs to be watched. BSR infection normally shows up in mid-August and is most recognized by cutting the plant open and looking at the brown infected pith of the lower stem. The leaves will have brown and yellow discoloration between the veins and can look like SDS. BSR can reduce yields 5-20% in some cases, and like most diseases yield is affected before visual symptoms are noticed.
Of course, we still need to be out checking for our old nemesis the soybean aphid. Keep scouting until R5 (beginning seed) stage. Control in these later stages of soybean development has shown to be economically beneficial in past years. The threshold of 250 aphids/plant is still recommended. This level is not the actual level of yield loss for soybean, but it figures the growth of the aphid into it and allows time to get the field sprayed. Spraying before that level is not economical; it’s really a waste of your time and money.
While scouting your fields for aphids you should also be checking for Bean Leaf Beetle. The damaged leaves are easy to detect, and the BLB is very recognizable. BLBs are green to brown in color. They usually have four black spots on their wing covers, and always a black triangle behind their thorax. In North Dakota, I have found BLB mostly along wooded river edges (Sheyenne and Red Rivers.)
If Bean Leaf Beetle is found in your fields, check population by using a sweep net. NDSU’s recommends that treatment for BLB is necessary if 3-7 beetles per sweep are found. During pod development, use 10% pod feeding or the presence of clipped pods.
The overall message remains, keep track of your fields this time of year and take action to minimize any yield loss.