It is remarkable how explosive target spot can be in favorable conditions, causing a 60% to 70% loss of cotton canopy within three weeks, according to Dr. Bob Kemerait, plant pathology professor at the University of Georgia.
Risk Factors in Cotton and Soybeans
From an environmental standpoint, target spot, caused by the fungus Corynespora cassiicola, is typically found on healthy cotton or soybeans with very good fertility when there has been extended periods of leaf wetness. Continuous cotton and irrigated fields are also at a higher risk for a disease outbreak.
Initial leaf infections start five to seven days of high relative humidity (greater than 80% to 85%) or moisture provided by a combination of light rain or heavy dew and warm temperatures. More than three consecutive days of rainfall can intensify disease development.
Target spot is most often seen in the Midsouth and southern states. The fungus can overwinter on crop debris and in the soil, so caution your growers if they’ve had problem fields in the past. To reduce the total inoculum, recommend growers rotate soybeans with corn, grain sorghum or rice. Target spot can be more severe when production practices allow a dense canopy to quickly develop.
Disease Identification in Cotton and Soybeans
When scouting target spot, begin looking for disease symptoms in the lower canopy. The most diagnostic characteristics are the concentric target-shaped lesions and the significant loss of leaves beginning from the interior of the lower canopy moving up the plant. Often, the very topmost leaves remain untouched by the disease. In soybeans, reddish-brown lesions can also be found on the petioles, pods and stems.
“The best thing to recommend to growers is to get out in the field at the approach of first bloom and start looking,” Kemerait says. “If a grower scouts and finds target spot but doesn’t stop it early, the genie is out of the bottle – nothing is going to slow it down. It’s so explosive. Many of our growers in south Georgia have had significant losses from target spot, and if conditions favor the disease, they will spray preventatively.”
Timing of Fungicide Applications
In soybeans, fungicide timing is the key to protection, according to Travis Faske, plant pathology associate professor at the University of Arkansas. Advise soybean growers that suppressing target spot development ahead of its advancement up the canopy and utilizing the water volume to move the fungicide into the canopy will both help in the management of this disease.
On the other hand, if cotton growers apply a fungicide the first week of bloom, they may need another application 14 to 21 days after the first application. However, if conditions allow them to wait and spray near the third week of bloom, they may be able to spray only once, Dr. Kemerait says.
“In cotton today, we’ve got several different fungicides labeled for target spot, and the four I usually discuss with growers are Headline®, Priaxor®, Quadris® and Elatus,” he says. “Priaxor is not only our most effective fungicide but it is also desirable because it allows us to have two modes of action, which reduces the risk for resistance to develop.”
To learn more about the benefits of using Priaxor fungicide to combat target spot, click here.
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