Dr. Bob Kemerait gives a row crop disease update
- Soybeans: Asian soybean rust is still lightly scattered from Decatur County to Appling County, but has only been found so far in KUDZU. Soybean rust is certainly not a major problem at the moment; however it could become so. Management options are to protect the crop with a fungicide sometime between the R1 (early bloom) and R3 (early pod set) stages. Such timings may correspond well with other disease and insect control measures. Some “Cercospora leaf blight” is being reported; this disease causes much of the upper foliage to take on a “bronzed” cast and then leaves drop prematurely leaving the “bony” petioles like skeleton fingers to the sky. Cercospora leaf blight also causes purple seed stain. Fungicide applications at pod set (R3) can help manage this disease.
- Late-planted corn: Southern corn rust is now commonly observed on older corn across the Coastal Plain, corn that is too late for it to matter. However, southern corn rust does pose a threat to younger corn and preventative protection with a fungicide is something to consider, especially as the crop approaches the tassel growth stage. Also, I am receiving numerous reports of young corn affected by northern corn leaf spot (Bipolaris zeicola) which produces numerous, small-to-medium sized red/brown spots, sometimes with appearance of concentric rings. Typically, corn is most severely affected by northern corn leaf spot early in the season and then grows out of it; however I cannot be sure that this will always be the case. I have no data on fungicides for management of northern corn leaf spot, but as it is closely related to northern and southern corn leaf blights, I am confident that mixed mode of action products we already use will be helpful for the “spot” disease. If a grower does spray, applications as early as V6-V8 would be appropriate. But again, I just don’t know if it matters.
- Cotton: I hear you. And I feel your frustration. We have three diseases of significant importance in the field right now. Boll rot. The rain and heavy vegetative growth we have seen this year has created perfect conditions for fungal boll rot. We are seeing a lot of it. We are not seeing a lot of bacterial boll rot, though some is certainly there. Fungal boll rot is most severe in lower bolls deep in the canopy or where insect damage also occurs. Fungicides are not an effective treatment; only opening the canopy up to increase airflow and reduce humidity can help reduce boll rot. 2. Areolate Mildew. First, Andrew S. and others, I didn’t make the name up. Second, I know that there is great concern and I have heard growers complaining that there fungicide applications did not stop the disease. Here are some thoughts. For the second year in a row, Areolate mildew is early and widespread. Areolate mildew can cause significant premature defoliation. I do know that fungicides like Headline and Quadris and certainly Priaxor can slow the spread of the disease, though not necessarily stop it, especially when it is well established in a field. It is not clear how much yield is at risk or that can be protected; but it is believed that significant premature defoliation is not a good thing, unless one is trying to open the canopy up to slow boll rot. Here are my recommendations, though they have not been proven with any hard data. If a grower is within 4 weeks of defoliating the crop anyway, save the money and don’t spray. If the grower is more than 4 weeks of defoliating and the areolate mildew is not too severe (i.e. already causing significant leaf drop) then there may be a benefit to treating with a fungicide. This may not stop the disease but will slow its development. 3. Target Spot. Target spot has been severe and widespread in this rainy season. I believe well-timed fungicides have been helpful this year. I don’t believe there is any benefit to a fungicide application after the 6th week of bloom. Either there is too much disease already to stop it or there is not enough time for disease to develop. In this season, a second fungicide application 2-3 weeks after the first application is something to consider.
- PEANUTS: Getting lots of questions these days about late-season peanut disease problems. Just a few thoughts. Three weeks to go until you dig the peanuts and little-or-no disease in the field? I wouldn’t put out any more fungicides unless there is threat of a hurricane or tropical storm. If three of more weeks out and on your last spray and you are seeing some leaf spot develop, applying a pint of chlorothalonil tank-mixed with 7.2 fl oz of tebuconazole or 5.5 fl oz of Alto or 5 fl oz of Topsin or 2.5 fl oz of Domark. If time for your last spray and very little leaf spot is present, then 1.5 pints of chlorothalonil may be all you need.
If white mold is popping up in your field late in the season and is confined to individual plants scattered across the field, then you may want to mix tebuconazole with your last leaf spot spray. If the disease is more severe, or you are really worried about it, then you might consider using 16 fl oz or Convoy rather than tebuconazole.
It is generally advisable to wait to dig the peanuts until they are “ready” based upon the hull-scrape test. This is true even if there is significant tomato spotted wilt in the field or some white mold. HOWEVER: if there is significant defoliation from leaf spot or significant white mold in the field, it often best to dig the peanuts earlier than planned to avoid excessive digging losses.
Dr. Bob Kemerait gives row crop disease update:
DISEASES of PEANUT: White mold and leaf spot aren’t breaking lose in every peanut field in Georgia, BUT hot temperatures, high humidity and frequent rains have created near-perfect conditions for the development, spread and, sometimes, explosion of these diseases. Growers need to stay on a good fungicide program, tightening spray intervals where disease is becoming problematic and/or where there is concern if the crops have received enough drying time after a fungicide was applied.
FOUR COTTON DISEASES: Target spot, areolate mildew, Stemphylium leaf spot, and bacterial blight.
What a season it has become when target spot and areolate mildew are causing greater angst than bacterial blight.
- Bacterial blight is present and is affecting yield in some fields. Susceptible varieties will get this disease and losses may occur. THERE IS NOTHING TO BE DONE ABOUT IT NOW. However, growers should note the varieties where they find it and determine at the end of the season if the disease became severe enough to avoid planting those varieties again.
- Stemphylium leaf spot is present as well. Stemphylium leaf spot is caused by a deficiency in potassium in the plant. Dr. Glen Harris is our soil fertility expert; I believe he would agree that excessive rains could have leached potassium this year. Fungicides ARE NOT a solution for Stemphylium leaf spot; Dr. Harris has the best information about managing potassium.
- Target spot and areolate mildew are present in a number of fields this year.
- At times, target spot and areaolate mildew appear late enough in the season that the defoliation resulting from these diseases does not affect yield and use of fungicides is not needed.
- The question for both diseases is not, “Can we protect the cotton in this field with a fungicides?” but rather, “Should we protect the cotton in this field with a fungicide?”
- We have very very little data on areolate mildew, but from what I do have, I am confident that we can easily control this disease using strobilurin products like Headline or Quadris, or mixed products like Priaxor or Elatus. Proline may work as well but I don’t have data.
- Though we can control areaolate mildew, does it make us any more yield than if we didn’t control it? When conditions are favorable, areolate mildew can rapidly defoliate a cotton crop. If a grower is withing 3-to-4 weeks of defoliating anyway, I would NOT use a fungicide. If the crop still had 4 or more weeks to go, I would consider weather, yield potential, how much disease is in the field and then decide to spray or not.
- If areolate mildew, or target spot, is already well-established in the field (i.e. causing significant defoliation, then there is little hope that a fungicide will help.
- Target spot is a significant concern this year and is widespread. Not every cotton grower in the state needed to spray a fungicide for target spot, but I encourage growers to carefully consider their options.
- Target spot is of particular concern this year because a) the wet and warm conditions are perfect for an explosion of the disease, b) the disease has been found early in many fields, and c) the price of cotton makes protection 100-250 lbs lint/acre attractive.
- I believe the best window of opportunity for managing target spot is from the first week of bloom to the sixth week of bloom.
- A little target spot in a crop (meaning scattered spots on lower leaves and no defoliation) during the first week of bloom and favorable weather IS a concern, as it would be at the third week as well. A “little target spot” at the 4th-6th week of bloom is much less of a concern.
- When a fungicide program begins as early as the first week of bloom because of the disease situation; a second application may be beneficial two-to three weeks later. I don’t envision an application, follow-up or otherwise, after the 6th week.
- It will be quite difficult to control (impossible?) target spot if there is already significant defoliation in the field before an application is made. If 25-30% of the leaves are already gone, a fungicide likely won’t work.
- THE THREE MOST IMPORTANT POINTS TO MANAGE TARGET SPOT, in order of importance, are 1. TIMING 2. COVERAGE 3. SELECTION of FUNGICIDE.
Five row crop and vegetable production meetings are currently scheduled in Lowndes County. Commercial (cat. 21), and private pesticide credit will be given with each meeting to all license holders who attend and sign in. Please call the office (333-5185) a few days ahead if you plan to attend so that plans can be made for the meal. I look forward to working with you this year and please contact me if you need anything.
Don’t let your license expire put these meetings on your calendar now.
- January 10, 2017 Peanut Production andPeanut Insects
12:00 noon Lowndes Extension Office
Dr. Scott Monfort
Dr. Mark Abney
- January 13, 2017 Vegetable Production
9:30 a.m. 4-H Center-Lake Park
Dr. Stormy Sparks
Dr. Tim Coolong
Dr. Bhabesh Dutta
- February 6, 2017 Row Crop Disease and Fertility
12:00 noon Lowndes Extension Office
Dr. Glen Harris
Dr. Bob Kemerait
- March 2, 2017 Cotton Production Cotton Insect
6:00 pm Lowndes Extension Office
Dr. Jarod Whitaker
Dr. Phillip Roberts
- March 8, 2017 Row Crop Weed Control
12:00 noon Lowndes Extension Office
Dr. Eric Prostko
Stink bugs are the main pest to be scouting for right now. Also be looking out for spider mites and whiteflies. These insects are important when deciding on an insecticide to spray. A good phone app to help with insecticide choice is the GA Cotton Insect Advisor App. Bacterial blight has been reported in some counties.
The main diseases to be looking out for are white mold and peanut rust.
Below is a guide to soybean scouting that has ways to identify caterpillars in soybeans and also thresholds for foliage and pod feeders.
Please feel free to call the office with any questions.