By Anne Dorrance

Following the field day at western last week, we received some excellent questions about the pathogen that causes frogeye leaf spot.

1. Where does the inoculum come from?  

Based on surveys of fields over the past few years, the pathogen can overwinter in Ohio, and did overwinter in some fields this past year. We expect that the snow cover helped that survival. We have observed this year that frogeye leaf spot can also spread via storm movement. Fields that had high levels of disease the year before, will have lesions on the lower canopy on susceptible varieties of soybean. Fields that only have lesions on leaves in the upper canopy may have been blown in by one of these storms.

2. Threshold for spraying — whole or part of a field.

In the past 9 years of foliar applications, the best yield response from foliar fungicide application (8 bushels per acre or more) is when frogeye leaf spot is present in the field at 1 lesion per 25 square feet at the R1 to R2 growth stage — evenly distributed through the field. Basically, all of the plots had some level of disease.

This is a polycyclic disease, so the amount of disease is very dependent on the primary or first inoculum that is present in the field. Every dew, rain event will have the favorable environment to promote new infections and allow the fungus to move up the plant and infect the new leaves.

As each new layer of leaves are formed there are more lesions per leaf. Initially due to low inoculum there will be 1 to 2, but by the end on super susceptible varieties it will be 20 to 40. On moderate-resistant varieties, it will be 4 to 5 lesions per new leaf. Think how many new leaves there are on an older plant compared to a young plant. What is cool about this disease is that once a leaf is fully expanded it is resistant. 

3. If disease is concentrated in one pocket of the field, it has not spread to high levels throughout the field.

Trials at Western and in farmer-cooperator fields showed it stayed limited to certain areas. This opens the door for precision agriculture if you need to apply material. Use the CORRECT LABELED RATE for this disease in those areas of the field that have the most disease. At western, half the field has lesions and half did not, so we will have side-by-side comparisons. We blocked the study based on the inoculum pressure. If there is no disease, then we have not observed an economic return.

4. Follow the labeled rate of the correct product.

We will emphasize the fungicide story again. When dealing with a pathogen that is present in the field, it is important to follow the labeled rate of the correct product. Many of the fungicide mixes do not have the correct labeled rate — they used reduced rates of two products. This is a great recipe for the fungus to develop resistance to two products, not just one. Colleagues to the south and west in Ohio, all along the Mississippi river, Kentucky, Western Indiana, southern Illinois have reported that resistance to headline has been found in the fungus populations that cause frogeye leaf spot (Cercospora sojina). So if you do choose to use a strobilurin, monitor the development of the lesions two weeks after application. If in doubt, we would be happy to work with you to evaluate for sensitivity to the fungicides.

5. Not the same as gray leaf spot on corn.

One more note for those of you that know the names of these fungi. Cercospora sojina is not the same fungus that causes gray leaf spot on corn — that fungus is Cercospora zeae-maydis. Both of these fungi have limited host ranges, so gray leaf spot on corn is not the same as frogeye leaf spot on soybean.

6. Economic thresholds for this year

Keep in mind the price of the beans, the yield potential, and how much, if any, disease you have present in the field. At the most we are looking at a 8- to 12-bushel return when frogeye is present. If there is no disease, then it is unlikely you will see the benefit. As always, if in doubt, insist on leaving untreated strips for yield comparisons.

Minimum cost of application required to determine economic thresholds for spraying soybeans.             

Increase Price per bushel
  10 12 14 16 18
+ 2 bu $20 $24 $28 $32 $36
+3 bu $30 $36 $42 $48 $54
+4 bu $40 $48 $56 $64 $72
+ 5 bu $50 $60 $70 $80 $90