Frogeye Leaf Spot a New Soybean Disease
Howdy!
No rainfall was recorded again this week. Temperatures were way above normal for most of this week. The lack of rain is causing severe plant stress.
Wheat harvest is nearly completed and yields were better than expected. Corn and soybeans are advancing quickly with the heat and no rain.
A new soybean disease called Frogeye leaf spot appeared late in the 2020 growing season in North Dakota. That I know of none was found in Griggs County in 2020 and none has been observed yet in 2021, however we need to be on the look out as it has been found again in North Dakota.
The pathogen causing the disease is a fungus called Cercospora sojina. The pathogen is very diverse genetically. There are between 12 and 20 different races of the pathogen. A race are genetically similar individuals of the pathogen having the same combination of genes making them able to cause the disease on a particular variety of soybean.
Symptoms usually just appear on leaves, but can appear on pods and stems in years when the disease is severe. Symptoms usually appear shortly after flowering. The youngest leaves are the most susceptible to the pathogen. Lesions (spots) appear as small gray spots with reddish-brown to purple borders. On the underside of the leaf, the lesion appears brown to gray with tiny dark “hairs”. The hairs are the long conidia or infective spores of the fungus. Severe leaf drop can occur when the disease is severe. Pods and stems can become infected if frequent rainfall and high humidity persists. The lesions (spots) on pods are reddish brown, sunken, and circular to elongate in shape. Older lesions on pods become brown to dark gray, usually with a narrow, dark brown border.
It is unknown if the disease over winters in North Dakota at this time. If not it must be blown into our area from other areas. Spores are also carried by wind and rain from other areas. Rain splashes spores from soybean residue up onto young leaves in the canopy. The pathogen prefers warm (77 degrees F to 86 degrees F) and wet (rain, heavy dew or >90% relative humidity) conditions to cause infection and disease development. Symptoms develop in 7 to 12 days after infection, depending upon temperature. The disease will complete multiple life cycles during the season, so the disease will get worse on individual plants and throughout the field.
If frogeye leaf spot shows up prior to or at flowering then substantial amounts of the disease can develop and will have a negative impact upon yield. Frogeye leaf spot can reduce yields by 10 to 60%. If the disease is first found at or after R4 (full pods on one of upper four nodes) to R5 (beginning seed (1/8 inch seed) in one of the upper four nodes) then very little impact to yield is observed. At this time many soybeans are at R5, so the disease will have little impact in 2021.
One of the biggest problems with frogeye leaf spot is that the pathogen is likely resistant to strobilurin fungicides such as Headline, Quadris, Aftershock, Evito, and Aproach. The resistant pathogen was found in lesions in 2020 in North Dakota.
The best way to manage frogeye leaf spot is to plant varieties resistant to the pathogen. Purchase varieties with the Race (RCS) 3 gene. The next method to managing frogeye leaf spot is to completely bury the soybean residue and/or rotate to corn, wheat, or alfalfa, which are non-host crops to the disease. Planting continuous soybeans will only increase the problem. The only other management strategy is to apply fungicides. For fungicides to be effective, timely scouting is required. Fungicides should be applied when one or two lesions (spots) can be found in every 25 foot of row when soybeans are at the R2 (full flower) growth stage. Effective single active ingredient fungicides include Topguard, Proline, Domark, and Topsin-M. Remember that strobilurin fungicides are no longer recommended for managing frogeye leaf spot due to the likelihood of resistance.
Get out and scout soybean now to see if frogeye leaf spot is in Griggs County.
