Early Disease Control

PG4-2015_Page_01_Image_0001Plus, research briefs on a new Spanish variety and a weed competition study.

Producers have come to depend on disease resistance built into their peanut varieties, but that in no way lessens the attention that should be paid to all the factors that affect peanut yield. Besides variety selection, planting date, plant population, crop rotation, tillage and other factors can have a tremendous impact on the potential for disease in a field. These factors and others are covered in the Peanut Disease Risk Index, portions of which are below, as part of the University of Georgia’s 2015 Peanut Update.

Tomato Spotted Wilt Virus

Losses to tomato spotted wilt across the Southeast have continued to show a slight increase over the past couple of years, though the disease is not nearly as severe as it has been in the late 1990s. It is estimated that losses associated with spotted wilt were approximately three percent in 2014. Though this was an increase since 2013, it is believed that growers were able to achieve good-to-excellent management of this disease in large part through combined use of Peanut Rx and varieties with improved resistance.

When tomato spotted wilt virus (TSWV) infects a host plant, it can cause a disease that severely weakens or kills that plant. This particular virus is capable of infecting an unusually large number of plant species including several that are important crops in the Southeast. In recent years, peanut, tobacco, tomato and pepper crops have been seriously damaged by TSWV. The only known method of TSWV transmission is via certain species of thrips that have previously acquired the virus by feeding on infected plants. The factors leading to the rapid spread of this disease in the Southeast are very complicated and no single treatment or cultural practice has been found to be a consistently effective control measure. However, research continues to identify factors that influence the severity of TSWV in individual peanut fields.

Fungal Disease

Successful peanut production requires that growers use a variety of tactics and strategies to minimize losses to disease. Weather patterns in Georgia and neighboring areas during the growing season, including high temperatures, high humidity and the potential for daily rainfall and thunder storms, create the near-perfect environmental conditions for outbreaks of fungal diseases. Common fungal diseases include early and late leaf spot, rust, Rhizoctonia limb rot, southern stem rot (referred to as “white mold”), Cylindrocladium black rot and a host of other diseases that are common, but of sporadic importance. If peanut growers do not take appropriate measures to manage fungal diseases, crop loss in a field may exceed 50 percent.

Strategies for managing fungal diseases are typically dependent on the use of multiple fungicide applications during the growing season. Fungicide applications are initiated approximately 30 days after planting, as the interaction between the growth of the crop and environmental conditions are likely to support the development of leaf spot diseases.

The length of time in which a fungicide can protect the peanut plant from infection is dependent on the properties of the fungicide and on weather conditions. Weather plays a major role in the potential for disease. Most fungal diseases will be more severe during periods of increased rainfall and of less concern during drier periods.

When weather conditions are very favorable for disease, severe epidemics may occur in fields where disease was not thought to be a problem. When weather conditions are unfavorable for fungal growth, disease severity may be low even in fields where it has been common in the past.

The AU-PNUT leaf spot advisory that has been used to effectively manage diseases in peanut is based on this relationship between disease and weather. Even those growers who do not use AU-PNUT recognize the need to shorten the time between fungicide applications in wet weather.

For more information from 2015 Peanut Update, go to the UGA website at www.caes.uga.edu/commodities/fieldcrops/ peanuts/ and click on 2015 Peanut Update.

Research Briefs:PG4-2015_Page_11_Image_0001

New Spanish Variety

A new Spanish peanut variety called OLé could provide producers with needed disease resistance and consumers with a peanut that has a longer shelf life and heart-healthy qualities. Released by U.S. Department of Agriculture Agricultural Research Service (USDA-ARS) and Oklahoma State University, Olé was developed with resistance to Sclerotinia blight with a high-oleic acid content.

USDA-ARS biologist Kelly Chamberlin, of the Wheat, Peanut and Other Field Crops Research Unit in Stillwater, Okla., says that OLé, with its resistance to the fungal disease Sclerotinia blight, will be important for producers in Oklahoma, Texas and the Virginia- North Carolina region, where the disease is a particular problem. Depending upon severity of field infestation, yield losses due to Sclerotinia can be as much as 50 percent.

In tests at three locations in Oklahoma, Chamberlin and her colleagues found that growing OLé reduced Sclerotinia blight infestation. This can save growers approximately $100 per acre in fungicide costs for Sclerotinia blight alone. The variety also has good pod rot resistance.

The OLé variety, shown below, is now being grown for foundation seed before being made available commercially.

Competition From Palmer amaranth

Palmer amaranth, with its resistance to glyphosate and other herbicides, is the most economically destructive weed in the southeastern United States. A new study published in the journal Weed Science investigated the effect of delaying Palmer amaranth establishment on weed growth and seed production.

In field trials conducted over two consecutive growing seasons in Georgia, Palmer amaranth was planted in cotton fields between zero and 12 weeks after the crop and then compared with Palmer amaranth planted simultaneously without cotton.

Without crop competition, the earliest planting of Palmer amaranth produced 446,000 seeds per plant, with seed production falling 50 percent when plants were established six weeks later. When cotton plants were present and competing with the weeds for resources, the number of seeds per plant was reduced 30 percent to 312,000.

The more time into the growing season the establishment of Palmer amaranth was delayed, the fewer seeds developed. Planting at six weeks after cotton establishment reduced the Palmer amaranth seed count by half, and for weeds established nine and 12 weeks into the growing season, seed production was reduced by 89 percent and 99 percent, respectively, compared to weeds established at the same time as cotton.

Cotton yield was reduced 67 percent when Palmer amaranth was established at the same time as the crop, while delaying weed establishment until six weeks after crop planting reduced yield loss to below 30 percent.

These results show that early season weed control programs that successfully delay Palmer amaranth establishment can have a large effect on crop yield, weed growth and weed seed production. In addition, reducing the soil seedbank is a vital preemptive measure against the next season’s weeds.

Full text of the article can be found in Weed Science, Vol. 63, No. 1, January– March 2015.