Soybean South

 - Disease Control -

The ‘Other’ Pathogens

Besides rust, several major soybean diseases can cause economic
damage to southern soybean crops. Look for symptoms, obtain a
correct diagnosis and treat accordingly.

 

By Carroll Smith

As a “new” disease that moved into the United States in 2004, Asian soybean rust has been in the spotlight for the past 18 months. However, other soybean diseases, which also cause decreased yields and loss of revenue, should not be ignored or forgotten.

Dr. Melvin Newman, plant pathologist at the University of Tennessee, says there are “at least four major diseases that Tennessee producers should fear: stem canker, sudden death syndrome, soybean cyst nematode and frogeye leaf spot.”

Newman says stem canker kills soybean plants from mid-season to maturity.

“Dead plants with dried leaves still attached may be the first indication to the producer of the diseases’s presence,” he notes. “Usually a brown, slightly sunken lesion girdles the stem at the base of a lower branch or leaf petiole. Typical leaf symptoms are interveinal chlorosis and necrosis.”

The plant pathologist estimates that soybean yield reductions can range from 50 to 90 percent with susceptible cultivars, depending on the environmental conditions.

The second major disease is sudden death syndrome (SDS), whose symptoms are much like stem canker’s.

“Instead of remaining attached, leaves exhibiting SDS symptoms fall prematurely, leaving the petioles attached,” Newman says. “Lower stems and roots are more discolored near maturity, with the pith remaining normally white. Lateral and tap roots are typically severely rotted at maturity, making it easy to pull the plants from the soil.” One difference to be aware of, Newman says, is that SDS is usually localized in the field, while stem canker usually attacks an entire field or area.

Soybean cyst nematode is another disease that can show up in your soybean crop.

“Infested fields usually show round to oval patterns of stunted and/or yellowed, unthrifty plants,” Newman says. “Infested roots have very small, white or yellow cysts loosely attached. The cyst can easily be seen on the roots of susceptible cultivars 30 to 35 days after germination of the seed.”

Frogeye leaf spot is typically a foliar disease, but it can also infect stems, pods and seeds.

“After mid-season, minute, reddish-brown, circular to angular spots first appear on the upper leaf surface,” Newman says. “As the lesions enlarge with age, the central area becomes olive-gray or ash-gray, surrounded by a narrow, dark, reddish-brown border. Older spots become very thin, often paper-white and translucent.”

Plant sampling
Because the symptoms of these diseases are very similar, a correct diagnosis may require an experienced opinion. If someone can’t come to your field, another option is to collect plant samples yourself.

According to the Southern Soybean Disease Workers (SSDW), collect both healthy and diseased plant tissue for comparison purposes. The SSDW is an organization made up of pathologists, nematologists, Extension scientists, industry personnel and private consultants.

“Suspect plants that are already dead are generally of little value,” they say. “Whole plants are best because leaf symptoms may be caused by damage to the stem or roots. After the sample is collected, protect it from deterioration and take it to the professional as soon as possible.

“Place the sample in a plastic bag and store in an ice chest or refrigerator to prevent drying,” they add. “Do not place the specimen in direct sunlight or near extreme heat.”

The final step in the sampling process, according to SSDW, is to include information about your cultural practices, pesticide applications and cultivar that will be useful in making a correct diagnosis.


Promote Plant Health To Maximize Yield And Quality

By gaining a better understanding of the physiology of soybean and corn plants, BASF researchers have discovered that a few key factors are critical to the overall health of the plant – factors that ensure healthier, more productive plants:
• Foliar disease control.
• Efficient plant growth.
• The ability of the plant to withstand stresses throughout the growing season.

According to BASF, growers who protect their soybean and corn plants from foliar diseases with Headline fungicide also realize additional plant growth and stress tolerance benefits.

These factors contribute to maximizing the yield and quality potential of the crop.

Headline fungicide, which was registered for use on soybeans and corn in December 2004, is in the strobilurin family and is active on the major foliar diseases in soybeans.

BASF says disease control is the foundation of the Plant Health advantage.

According to Greg Stapleton, BASF, “The R3 stage is where the soybean plant needs to be to apply Headline. It’s important to control diseases first, then the benefits from that help increase your yields.”

To achieve the best results with Headline fungicide, Stapleton says growers should remember the acronym ACT – adjuvant + coverage + timing.

BASF says that another key factor in Plant Health is stress tolerance. By controlling disease with Headline, the company notes that plants improve their own natural defenses, reducing leaf loss and minimizing the adverse impact of plant response to drought.

BASF contributed information for this sidebar.