Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Frogeye Leaf Spot
Frogeye continues to be reported in double crop soybeans. As I have mentioned in the past, significant frogeye present in the mid canopy in the early reproductive stages (R1-R3) may warrant a spray. You should shoot for the R3 stage in terms of timing. Factors that increase your risk levels are: 1) a Frogeye susceptible variety planted; 2) Frogeye is detected at significant levels in the field; and 3) favorable weather in the future (warm, humid, foggy). See last week’s WCU article for more information on Frogeye leaf spot on soybean.
Downy Mildew
Cool temperatures have resulted in greater than normal levels of downy mildew in Delaware soybeans (Figure 1). In fact, there are reports of higher than normal levels of downy mildew in several states this year. In general downy mildew is considered to be more of an annoyance than a major problem, but it can cause some defoliation in severe instances. In some cases seed and pods can become infected. The best way to manage downy mildew is by using clean, certified seed, not planting consecutive soybean crops in the same field, and, if available, using resistant varieties.
Figure 1. Underside of a soybean leaf showing signs of downy mildew
Nematode Damage
Keep your eye out for signs of nematode damaged soybeans in Kent and Sussex counties. Severely infected plants will often be stunted, chlorotic, and have a burned appearance (Figure 2). Other agents such as pH, nutrient deficiency/toxicity, and even chemical burn can give the appearance of nematode damage. If you have confirmed that your field is infested with nematodes you should strongly consider submitting end of the year soil samples to the UD clinic for nematode enumeration. This information will be useful for determining future management practices in the infested field. Often symptoms of nematode activity may not be present, but they can still cause significant reductions in yield. Keep this in mind when assessing yields during harvest.
Figure 2. A soybean field showing symptoms of nematode damage. Other agents can cause similar symptoms. Thus it is important to confirm the causal agent as this will impact management.
Soybean Rust Update
As of August 28th, 99 counties in seven states are reporting soybean rust, and several more counties in South Carolina reported the disease over the last week (Figure 3). The forecast is not calling for any Northward spread of soybean rust over the next 5 days. Visit the soybean rust ipmPIPE site (http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi) for updates during the week.
Figure 3. Soybean Rust observations as of 28-August-2013.