Plan Now
For Disease Problems

Extension experts offer updates for this production season.

By Amanda Huber
 

Growing conditions for 2008 are already starting out much better than 2007. Last year’s complete lack of rain events and, therefore, soil moisture delayed planting, at best. Some growers were not able to plant peanuts at all.

Obviously, rain is needed to achieve a respectable yield. But with rain comes the increase of disease. However, most every grower would say, “That’s okay. Rain is better than no rain, and disease problems we can manage.”

Timeliness is a key component of disease management. Planting in the optimum-planting window has become an important part of reducing the likelihood of Tomato Spotted Wilt Virus.

For foliar diseases, being on time with the initial fungicide application and then continuing at regular intervals offers the best possibility for control.

For soilborne disease, growers are usually able to wait until about 60 days after planting to begin a control program. However, that time frame can be moved up if the weather is unusually warm and ample rainfall is received.

The following are management guidelines and updates from Extension plant pathologists for the different disease problems growers are likely to encounter.

Leaf Spot
The effectiveness of your leaf spot spray program should be periodically evaluated. If disease control is inadequate, adjustments can be made to prevent serious yield losses. If leaf spot control is fair-to-poor, shorten the interval between fungicide applications or increase the application rate to the highest amount on the label. Disease development will continue for 14 days or more before any improvement in control will be seen following this change in your spray program.

2008 AU-Pnut Rules For Peanut Leaf Spot Control
In order to use this method for controlling leaf spot on peanuts, you need to know the following:

1. A “rain event” is any day (a 24-hour period) with more than 0.1 inch of rain and/or irrigation or, it is fog that begins before 8:00 p.m.

2. The AU-Pnut Weather Forecast provides you: a) the 5-day average forecast for rain; b) the rain forecast (percent chance of rain) for each day within that 5-day average. You will use the 5-day average forecast until you plan to irrigate. Then, you will use the forecast for each day.

3. The day you irrigate, the forecast automatically becomes 100 percent, and it becomes your fifth day. So, to figure your 5-day forecast, substitute 100 percent for the forecast on the planned irrigation day. Then, add the forecasts for each day and divide by five.

Timing For The First Spray
From true cracking, count the number of rain events. Spray if you have counted four rain events since cracking and the 5-day forecast calls for a 50-percent or greater chance of rain; or you have counted five rain events since cracking, and the 5-day forecast calls for a 40-percent or greater chance of rain; or there have been six rain events, spray immediately. If leaf spot is seen (two or more spots per plant) in the lower leaves of the plant, spray immediately.

Timing For All Other Sprays
Ten days after your last leaf spot spray, begin counting rain events and check the 5-day average forecast daily. To accurately determine days since application, count the day you sprayed as Day 0; the day after will be Day 1 and so on. When you reach Day 10, start counting rain events again and checking the 5-day average forecast.

Spray if no rain event has been recorded and the average chance of rain for the next five days is 50 percent or greater; or one rain event has been recorded and the average chance of rain for the next five days is 40 percent or greater; or two rain events have been recorded, and the average chance of rain for the next five days is 20 percent or greater; or if there have been three rain events, spray immediately. If the crop is within 14 days of harvest, stop fungicide applications.

Weather Forecast
The AU weather forecast for each day is available on the Internet at the Agricultural Weather Information Service (AWIS) Web site www.awis.com. To access this information, click on peanut weather, then Alabama and, finally, AU-Pnut Leaf Spot. On-line information includes both the precipitation forecasts for each of the next five days and also the five-day average precipitation forecast. Check the forecast each morning as you plan that day’s activities.

On-line Registration
To get the on-line rain events needed to run AU-Pnut in each of your peanut fields, you must register each of them with AWIS, using the on-line registration form. To locate your field(s) within the Doppler Radar output grid, the longitude and latitude for each peanut field must be provided to AWIS. A hand-held GPS unit can be used to generate the necessary coordinates.

To get the full benefit from AU-Pnut, be sure to register before true ground cracking occurs. Beginning on that day, start totaling up the number of rain events needed to trigger the first fungicide application.

You may also use the AU-Pnut advisory without the Doppler Radar-generated precipitation data. Place a tapered rain guage, which should read to 0.10 inch, in the middle or end of a minimum of one field within a continuous 640-acre block of land. Since true ground cracking is used to start the AU-Pnut advisory, you will have to separately monitor rainfall totals where peanuts have been planted on different days within each 640-acre block. This situation is most likely to occur where peanut planting is delayed or separated by four or more days.

If numerous showers occur after true ground cracking, the AU-Pnut advisory may trigger the first fungicide spray earlier than the standard 14-day calendar program. If the peanuts are relatively young, the first and, sometimes, the second fungicide application may be banded directly over the middle of the peanut canopy.

See the Web site www.aces edu/dept/IPM/ for peanut IPM publications and www.awis.com for access to the AU-Pnut Leaf Spot Advisory.

Source: IPM Peanut. Prepared by Austin Hagan, Extension Plant Pathologist, and Ron Weeks, former Extension Entomologist, Auburn University, with recommendations based on research from Rodrigo Rodriguez-Kabana and Kira Bowen, professors, Department of Plant Pathology, AU.

Sclerotinia Blight
Sclerotinia blight, caused by Sclerotinia minor, has been found in all traditional peanut counties in North Carolina.

Because the disease starts by killing individual limbs, careful scouting is required to see symptoms when they first appear. Vines must be pulled back to reveal the cottony growth of Sclerotinia on bleached stems. Signs and symptoms are most visible on humid mornings. The end portion of infected limbs may remain green and look healthy for several days before wilting is evident. The small black sclerotia resemble mouse or insect droppings and may be seen on and in infected tissues.

Disease is favored by cool, wet conditions and is more severe on injured vines. The fungicides fluazinam (Omega) and boscalid (Endura) are effective against Sclerotinia blight when applied preventatively. Fields with a history of serious problems should be scouted carefully, beginning when vines are close to touching or about July 4. Treat when Sclerotinia blight is first observed or 60 to 70 days after planting or according to a Sclerotinia blight advisory. If the disease continues to spread, one or two more applications may be made at 3- to 4-week intervals. A weather-based Sclerotinia blight advisory can be used to time applications and prevent unnecessary fungicide applications. County agents in production areas receive e-mail Sclerotinia advisories every day. For custom Sclerotinia advisories, go to www.nc-climate.ncsu.edu.

Source: 2008 Peanut Information, Peanut Disease Management by Barbara Shew, Extension Plant Pathologist, North Carolina State University.

White Mold, Limb Rot And Web Blotch
The key to disease control is preventing diseases from getting started. This is true for soil diseases such as white mold and Rhizoctonia limb rot. Alternating different fungicide chemistries reduces the potential for developing resistant strains of leaf spot and soil diseases. Alternating fungicides also gives some insurance against the failure of one product alone.

General guidelines:

• In fields with high risk for white mold, apply a soil fungicide (Folicur, Abound, Moncut, Artisan, Provost or Headline) no later than 60 days after planting. White mold must be prevented, and hot weather accelerates white mold growth.

• Soil fungicides must be washed into the soil to be effective, but wait 24 hours before irrigating to also help control leaf spot.

• Do not apply more than a combined total of two strobilurin applications (Abound, Headline, Evito or Stratego) in any growing season to reduce risk of resistance.

• Spot check fields for leaf spot and white mold, particularly from 60 days after planting to harvest.

Source: Peanut Moneymaker Production Guide 2008, Jay Chapin, Extension specialist, Clemson University. For information, go to www.clemson.edu/peanuts.

PG


5 New Or Updated Management Tools

1. The 2008 Peanut Rx Disease Risk Index is now available and has been updated and revised by researchers, breeders and Extension specialists from the University of Georgia, the University of Florida and Auburn University.

Changes in the 2008 version include: 1) an update on the peanut varieties and the disease risk points assessed for tomato spotted wilt, leaf spot, white mold and Rhizoctonia limb rot; 2) a slight increase in risk points for white mold when peanuts are planted prior to May 1st; 3) a slight increase in risk points for white mold when the final stand in a field is four or more plants per foot of row.

All other considerations for the disease index remain the same as from 2007.

2. “Prescription Programs,” which are specific disease management programs with an increase or decrease in fungicide applications based upon the 2008 Peanut Rx, continue to gain support from the agrichemical industry.

In 2007, Syngenta launched an innovative and exciting program where peanut producers were offered reduced-input fungicide programs when their fields could be documented as either “moderate” or “low” risk based upon Peanut Rx. Syngenta will again support this program in 2008, and it is likely that they will be joined by at least one other company this season in their endorsement of Peanut Rx.

3. Dr. Corley Holbrook, a peanut breeder with the USDA-ARS in Tifton, Ga., has released the new variety Tifguard for 2008.

In research trials, Tifguard has demonstrated near-perfect resistance to peanut root-knot nematode and has performed well in fields where white mold and leaf spot are present. It is likely that Tifguard, with this new level of nematode resistance, will become a standard for peanut producers who struggle to control this tough pest in their fields.

4. Although these products were released in 2007, the nearly new fungicides Provost (triazole fungicide mixture) and Evito 480SC (strobilurin fungicide) will continue to become more familiar to growers this season, joining Abound, Folicur, Headline, Artisan, Moncut and a number of generic tebuconazole products for management of leaf spot and soilborne diseases.

Although there is still much to be learned about Evito, it is clear from fungicide trials that this product can be effectively used for management of leaf spot and white mold in Georgia. More trial data is needed to determine whether it will work comparably to Abound or other soilborne fungicides.

5. Bayer CropScience has aggressively addressed concerns regarding leaf speckling associated with tankmixes of Provost and other materials and has developed guidelines for best-use of this important fungicide in 2008. Provost is an excellent fungicide that offers strong efficacy against leaf spot, white mold and limb rot.

Source: 2008 Peanut Disease Update, by Bob Kemerait, Extension plant pathologist, Tim Brenneman, plant pathologist, Albert Culbreath, plant pathologist, University of Georgia.