Protecting plant stands: Peanut industry moves quickly to address emerging disease threat

As with any crop, the first step toward successful production is getting the optimum number of plants growing at the right time of the year. For peanuts in Georgia, that means four uniformly emerging plants per foot of row in either single rows or as the total of two twin rows. This stand has consistently provided the greatest yield potential but also is an important component in the integrated management of tomato spotted wilt virus (TSWV), a devastating disease that nearly ended the peanut industry in Georgia in the 1990s. To achieve this plant population, a seeding rate of 6 seeds/ft is recommended although some growers will plant higher seeding rates to ensure having an adequate stand.

Some issues with obtaining peanut stands are similar to those facing any other crop. Planting at the best time (mid-April through late May) after soils have warmed adequately is critical. Average daily soil temperatures of at least 68°F at the 4-inch depth for three consecutive days are recommended. Preparation of a friable seedbed with excellent seed-to-soil contact is important as is planting depth (normally about 1.5 to 2 inches, shallower in heavier soils). Adequate soil moisture should be present for uniform germination. If irrigation is needed, it is best to apply it to the soil before planting. This allows the soils to reach appropriate temperatures before putting seed in the ground. Herbicides can affect developing peanut plants both pre- and post-emergence, but that is a topic that could be an article by itself. Yet even when all is done right, there can still be problems.

Peanut seedlings are tough and able to conquer adverse conditions (such as soil crusting) that can be devastating to some other crops. Peanut has a very large seed that must be removed from a tough hull (pod) during the shelling process. Damage to the seed during this process can include bruising and death to tissue of the cotyledons, which can then serve as entry points for pathogens. Also, the tip of the radicle (embryonic root) is exposed on a peanut seed and can be easily damaged during shelling and handling. The seed also have a fragile testae (seed coat) that is important in protecting the seed from infection as well as physically holding the cotyledons of the seed together. If the testae is damaged or removed, seed quality is greatly impaired. This can be caused by physical damage or issues at harvest or subsequent handling (such as over-drying) as well as during the shelling process.

Seed Treatment Fungicide

Another important factor is seedling disease. Since peanut pods develop in the soil, they are exposed to a wide range of pathogens as they mature, and many of these can be seedborne. In fact, one of the most overlooked but essential inputs for peanut production is an effective seed treatment fungicide. I have often said that growers get more financial return (by far) from the money they pay for seed treatment than any other input on the crop. I have often seen lower quality (but legal for sale) seed planted without a seed treatment be completely wiped out with not a single surviving plant.

One issue unique to peanuts is the challenge of applying the seed treatment without damaging the fragile testae. Such damage is much more likely to occur if the seed coat gets wet as happens when any type of liquid is applied. This has prevented the use of liquid seed treatments on peanut seed and is the reason dust formulations are still the standard for this crop. The equipment used to apply this dust is not very precise, and there is considerable variability in dosing rates. Liquid and polymer-based seed treatments are used on many other crops where this is not an issue, and the use of polymer seed treatments on peanut is already standard in some areas of the world. Changes in polymer formulations and application equipment are rapidly making this technology more suitable for use on peanuts, and several large-scale commercial seed treaters are now transitioning to polymer-based peanut seed treatments. Such formulations offer huge advantages in terms of reduced worker and pollinator exposure from dust formulations and the ability to apply more components to seed such as growth regulators, insecticides, and even inoculants for nitrogen fixation. There are also benefits in the physical handling of polymer-treated seed such as flowability and maintaining the structural integrity of the seed coat.

However, for a seed treatment to be effective, the fungicidal components must be effective on the target pathogens. There are numerous pathogens that can seriously damage peanut seed and seedlings, and they can be both seed- and soil-borne. Some of the most damaging are fungi, specifically Aspergillus, Rhizoctonia, Rhizopus, and Pythium (which is actually an oomycete, not a fungus). Several species of Rhizopus can cause a rapid rot that literally turns intact seed to mush in a matter of days. This fungus can grow rapidly and move down the furrow, rotting additional seeds, especially with higher seeding rates. With wet soils and stressful conditions, Pythium and Rhizoctonia can rot seed and seedlings. These diseases are worse in heavier soils and with deeper-planted seed that will be slower to emerge. In fact, the outcome of this interaction is a race—the faster a plant develops, the less likely it is to succumb to the effects of the disease. Obviously, the environment plays a big role in this interaction.

Aspergillus Pathogens

One of our most damaging seedling diseases in recent years has been Aspergillus crown rot, caused by the fungus Aspergillus niger. Crown rot is easily identified by the abundant, fuzzy black sporulation on the necrotic, shredded tissue just below the soil surface. Plants usually emerge normally but wilt suddenly and die at about 14 to 35 days after planting. One problem associated with this disease is this delayed development that makes replanting an unlikely option. Seed treatments help control crown rot, and in-furrow sprays of Abound (Syngenta) have been very effective, at least until recent years. Research at the University of Georgia in Tifton, GA has shown a high incidence of A. niger isolates from peanut seed with high levels of resistance to QoI (quinone outside inhibitor) fungicides like azoxystrobin, a key component in Dynasty PD seed treatment from Syngenta (Jordan et al., 2019). The fact that A. niger can be seedborne at a high incidence is troubling as this is a very effective mechanism of long-range distribution.

Yet another very bad pathogen associated with peanut seed is Aspergillus flavus. It has yellow-green spores and usually rots the seed prior to emergence. This pathogen gets a lot more attention as the source of highly carcinogenic aflatoxin and low-value Seg 3 peanuts. It is well known to survive and even thrive in hot, dry conditions. While it is present in peanuts at some level every year, 2019 will long be remembered for the incredible heat and drought experienced across the Peanut Belt from August through September. As the scorching temperatures persisted, even irrigated fields were stressed and wilted. The first effect observed was a spike in Seg 3 peanuts at grading, which also translated into greatly increased aflatoxin levels in peanuts coming out of storage. This alone cost the industry millions of dollars in reduced crop values of contaminated peanuts that had to be regraded and/or crushed for oil stock.

Another effect that was less widely anticipated was much higher than normal amounts of Aspergillus flavus in the peanut seed. This was first noticed at the Georgia State Seed Lab where thousands of commercial peanut seed lots are assessed to certify germination. When the early samples were processed in the winter of 2020, it became apparent that the germination of commercial peanut seed lots was going to be generally low and highly erratic. Also, even though all seed were treated with fungicides, in some lots a yellow-green fungus was found growing rampantly on the germination blotters. Our lab obtained isolates and positively identified the pathogen as A. flavus. We also started assaying seed lots and found that many were nearly 100% contaminated with this deadly pathogen (Brenneman et al., 2020). Additional contributions were made by Dr. Emran Ali, Director of the University of Georgia Plant Molecular Diagnostic Lab in the Plant Pathology Department in Tifton. He extracted DNA from the pathogen and demonstrated clearly that many of the isolates were highly resistant to the QoI fungicides, just as the A. niger isolates. This resistance was previously unknown but helped explain the unexpected explosion of this fungus in peanut seed for planting in the 2020 season (i.e., those produced in the drought of 2019). The combination of unprecedented heat and drought during pod fill, in addition to a surge of resistance to one of our main seed treatment fungicides, created a “perfect storm” for seed quality issues that plagued our spring plantings in 2020.